WIP FPC-III support

[linux/fpc-iii.git] / fs / btrfs / relocation.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2009 Oracle.  All rights reserved.
 */

#include <linux/sched.h>
#include <linux/pagemap.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/rbtree.h>
#include <linux/slab.h>
#include <linux/error-injection.h>
#include "ctree.h"
#include "disk-io.h"
#include "transaction.h"
#include "volumes.h"
#include "locking.h"
#include "btrfs_inode.h"
#include "async-thread.h"
#include "free-space-cache.h"
#include "qgroup.h"
#include "print-tree.h"
#include "delalloc-space.h"
#include "block-group.h"
#include "backref.h"
#include "misc.h"

/*
 * Relocation overview
 *
 * [What does relocation do]
 *
 * The objective of relocation is to relocate all extents of the target block
 * group to other block groups.
 * This is utilized by resize (shrink only), profile converting, compacting
 * space, or balance routine to spread chunks over devices.
 *
 *              Before          |               After
 * ------------------------------------------------------------------
 *  BG A: 10 data extents       | BG A: deleted
 *  BG B:  2 data extents       | BG B: 10 data extents (2 old + 8 relocated)
 *  BG C:  1 extents            | BG C:  3 data extents (1 old + 2 relocated)
 *
 * [How does relocation work]
 *
 * 1.   Mark the target block group read-only
 *      New extents won't be allocated from the target block group.
 *
 * 2.1  Record each extent in the target block group
 *      To build a proper map of extents to be relocated.
 *
 * 2.2  Build data reloc tree and reloc trees
 *      Data reloc tree will contain an inode, recording all newly relocated
 *      data extents.
 *      There will be only one data reloc tree for one data block group.
 *
 *      Reloc tree will be a special snapshot of its source tree, containing
 *      relocated tree blocks.
 *      Each tree referring to a tree block in target block group will get its
 *      reloc tree built.
 *
 * 2.3  Swap source tree with its corresponding reloc tree
 *      Each involved tree only refers to new extents after swap.
 *
 * 3.   Cleanup reloc trees and data reloc tree.
 *      As old extents in the target block group are still referenced by reloc
 *      trees, we need to clean them up before really freeing the target block
 *      group.
 *
 * The main complexity is in steps 2.2 and 2.3.
 *
 * The entry point of relocation is relocate_block_group() function.
 */

#define RELOCATION_RESERVED_NODES       256
/*
 * map address of tree root to tree
 */
struct mapping_node {
        struct {
                struct rb_node rb_node;
                u64 bytenr;
        }; /* Use rb_simle_node for search/insert */
        void *data;
};

struct mapping_tree {
        struct rb_root rb_root;
        spinlock_t lock;
};

/*
 * present a tree block to process
 */
struct tree_block {
        struct {
                struct rb_node rb_node;
                u64 bytenr;
        }; /* Use rb_simple_node for search/insert */
        struct btrfs_key key;
        unsigned int level:8;
        unsigned int key_ready:1;
};

#define MAX_EXTENTS 128

struct file_extent_cluster {
        u64 start;
        u64 end;
        u64 boundary[MAX_EXTENTS];
        unsigned int nr;
};

struct reloc_control {
        /* block group to relocate */
        struct btrfs_block_group *block_group;
        /* extent tree */
        struct btrfs_root *extent_root;
        /* inode for moving data */
        struct inode *data_inode;

        struct btrfs_block_rsv *block_rsv;

        struct btrfs_backref_cache backref_cache;

        struct file_extent_cluster cluster;
        /* tree blocks have been processed */
        struct extent_io_tree processed_blocks;
        /* map start of tree root to corresponding reloc tree */
        struct mapping_tree reloc_root_tree;
        /* list of reloc trees */
        struct list_head reloc_roots;
        /* list of subvolume trees that get relocated */
        struct list_head dirty_subvol_roots;
        /* size of metadata reservation for merging reloc trees */
        u64 merging_rsv_size;
        /* size of relocated tree nodes */
        u64 nodes_relocated;
        /* reserved size for block group relocation*/
        u64 reserved_bytes;

        u64 search_start;
        u64 extents_found;

        unsigned int stage:8;
        unsigned int create_reloc_tree:1;
        unsigned int merge_reloc_tree:1;
        unsigned int found_file_extent:1;
};

/* stages of data relocation */
#define MOVE_DATA_EXTENTS       0
#define UPDATE_DATA_PTRS        1

static void mark_block_processed(struct reloc_control *rc,
                                 struct btrfs_backref_node *node)
{
        u32 blocksize;

        if (node->level == 0 ||
            in_range(node->bytenr, rc->block_group->start,
                     rc->block_group->length)) {
                blocksize = rc->extent_root->fs_info->nodesize;
                set_extent_bits(&rc->processed_blocks, node->bytenr,
                                node->bytenr + blocksize - 1, EXTENT_DIRTY);
        }
        node->processed = 1;
}


static void mapping_tree_init(struct mapping_tree *tree)
{
        tree->rb_root = RB_ROOT;
        spin_lock_init(&tree->lock);
}

/*
 * walk up backref nodes until reach node presents tree root
 */
static struct btrfs_backref_node *walk_up_backref(
                struct btrfs_backref_node *node,
                struct btrfs_backref_edge *edges[], int *index)
{
        struct btrfs_backref_edge *edge;
        int idx = *index;

        while (!list_empty(&node->upper)) {
                edge = list_entry(node->upper.next,
                                  struct btrfs_backref_edge, list[LOWER]);
                edges[idx++] = edge;
                node = edge->node[UPPER];
        }
        BUG_ON(node->detached);
        *index = idx;
        return node;
}

/*
 * walk down backref nodes to find start of next reference path
 */
static struct btrfs_backref_node *walk_down_backref(
                struct btrfs_backref_edge *edges[], int *index)
{
        struct btrfs_backref_edge *edge;
        struct btrfs_backref_node *lower;
        int idx = *index;

        while (idx > 0) {
                edge = edges[idx - 1];
                lower = edge->node[LOWER];
                if (list_is_last(&edge->list[LOWER], &lower->upper)) {
                        idx--;
                        continue;
                }
                edge = list_entry(edge->list[LOWER].next,
                                  struct btrfs_backref_edge, list[LOWER]);
                edges[idx - 1] = edge;
                *index = idx;
                return edge->node[UPPER];
        }
        *index = 0;
        return NULL;
}

static void update_backref_node(struct btrfs_backref_cache *cache,
                                struct btrfs_backref_node *node, u64 bytenr)
{
        struct rb_node *rb_node;
        rb_erase(&node->rb_node, &cache->rb_root);
        node->bytenr = bytenr;
        rb_node = rb_simple_insert(&cache->rb_root, node->bytenr, &node->rb_node);
        if (rb_node)
                btrfs_backref_panic(cache->fs_info, bytenr, -EEXIST);
}

/*
 * update backref cache after a transaction commit
 */
static int update_backref_cache(struct btrfs_trans_handle *trans,
                                struct btrfs_backref_cache *cache)
{
        struct btrfs_backref_node *node;
        int level = 0;

        if (cache->last_trans == 0) {
                cache->last_trans = trans->transid;
                return 0;
        }

        if (cache->last_trans == trans->transid)
                return 0;

        /*
         * detached nodes are used to avoid unnecessary backref
         * lookup. transaction commit changes the extent tree.
         * so the detached nodes are no longer useful.
         */
        while (!list_empty(&cache->detached)) {
                node = list_entry(cache->detached.next,
                                  struct btrfs_backref_node, list);
                btrfs_backref_cleanup_node(cache, node);
        }

        while (!list_empty(&cache->changed)) {
                node = list_entry(cache->changed.next,
                                  struct btrfs_backref_node, list);
                list_del_init(&node->list);
                BUG_ON(node->pending);
                update_backref_node(cache, node, node->new_bytenr);
        }

        /*
         * some nodes can be left in the pending list if there were
         * errors during processing the pending nodes.
         */
        for (level = 0; level < BTRFS_MAX_LEVEL; level++) {
                list_for_each_entry(node, &cache->pending[level], list) {
                        BUG_ON(!node->pending);
                        if (node->bytenr == node->new_bytenr)
                                continue;
                        update_backref_node(cache, node, node->new_bytenr);
                }
        }

        cache->last_trans = 0;
        return 1;
}

static bool reloc_root_is_dead(struct btrfs_root *root)
{
        /*
         * Pair with set_bit/clear_bit in clean_dirty_subvols and
         * btrfs_update_reloc_root. We need to see the updated bit before
         * trying to access reloc_root
         */
        smp_rmb();
        if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
                return true;
        return false;
}

/*
 * Check if this subvolume tree has valid reloc tree.
 *
 * Reloc tree after swap is considered dead, thus not considered as valid.
 * This is enough for most callers, as they don't distinguish dead reloc root
 * from no reloc root.  But btrfs_should_ignore_reloc_root() below is a
 * special case.
 */
static bool have_reloc_root(struct btrfs_root *root)
{
        if (reloc_root_is_dead(root))
                return false;
        if (!root->reloc_root)
                return false;
        return true;
}

int btrfs_should_ignore_reloc_root(struct btrfs_root *root)
{
        struct btrfs_root *reloc_root;

        if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
                return 0;

        /* This root has been merged with its reloc tree, we can ignore it */
        if (reloc_root_is_dead(root))
                return 1;

        reloc_root = root->reloc_root;
        if (!reloc_root)
                return 0;

        if (btrfs_header_generation(reloc_root->commit_root) ==
            root->fs_info->running_transaction->transid)
                return 0;
        /*
         * if there is reloc tree and it was created in previous
         * transaction backref lookup can find the reloc tree,
         * so backref node for the fs tree root is useless for
         * relocation.
         */
        return 1;
}

/*
 * find reloc tree by address of tree root
 */
struct btrfs_root *find_reloc_root(struct btrfs_fs_info *fs_info, u64 bytenr)
{
        struct reloc_control *rc = fs_info->reloc_ctl;
        struct rb_node *rb_node;
        struct mapping_node *node;
        struct btrfs_root *root = NULL;

        ASSERT(rc);
        spin_lock(&rc->reloc_root_tree.lock);
        rb_node = rb_simple_search(&rc->reloc_root_tree.rb_root, bytenr);
        if (rb_node) {
                node = rb_entry(rb_node, struct mapping_node, rb_node);
                root = (struct btrfs_root *)node->data;
        }
        spin_unlock(&rc->reloc_root_tree.lock);
        return btrfs_grab_root(root);
}

/*
 * For useless nodes, do two major clean ups:
 *
 * - Cleanup the children edges and nodes
 *   If child node is also orphan (no parent) during cleanup, then the child
 *   node will also be cleaned up.
 *
 * - Freeing up leaves (level 0), keeps nodes detached
 *   For nodes, the node is still cached as "detached"
 *
 * Return false if @node is not in the @useless_nodes list.
 * Return true if @node is in the @useless_nodes list.
 */
static bool handle_useless_nodes(struct reloc_control *rc,
                                 struct btrfs_backref_node *node)
{
        struct btrfs_backref_cache *cache = &rc->backref_cache;
        struct list_head *useless_node = &cache->useless_node;
        bool ret = false;

        while (!list_empty(useless_node)) {
                struct btrfs_backref_node *cur;

                cur = list_first_entry(useless_node, struct btrfs_backref_node,
                                 list);
                list_del_init(&cur->list);

                /* Only tree root nodes can be added to @useless_nodes */
                ASSERT(list_empty(&cur->upper));

                if (cur == node)
                        ret = true;

                /* The node is the lowest node */
                if (cur->lowest) {
                        list_del_init(&cur->lower);
                        cur->lowest = 0;
                }

                /* Cleanup the lower edges */
                while (!list_empty(&cur->lower)) {
                        struct btrfs_backref_edge *edge;
                        struct btrfs_backref_node *lower;

                        edge = list_entry(cur->lower.next,
                                        struct btrfs_backref_edge, list[UPPER]);
                        list_del(&edge->list[UPPER]);
                        list_del(&edge->list[LOWER]);
                        lower = edge->node[LOWER];
                        btrfs_backref_free_edge(cache, edge);

                        /* Child node is also orphan, queue for cleanup */
                        if (list_empty(&lower->upper))
                                list_add(&lower->list, useless_node);
                }
                /* Mark this block processed for relocation */
                mark_block_processed(rc, cur);

                /*
                 * Backref nodes for tree leaves are deleted from the cache.
                 * Backref nodes for upper level tree blocks are left in the
                 * cache to avoid unnecessary backref lookup.
                 */
                if (cur->level > 0) {
                        list_add(&cur->list, &cache->detached);
                        cur->detached = 1;
                } else {
                        rb_erase(&cur->rb_node, &cache->rb_root);
                        btrfs_backref_free_node(cache, cur);
                }
        }
        return ret;
}

/*
 * Build backref tree for a given tree block. Root of the backref tree
 * corresponds the tree block, leaves of the backref tree correspond roots of
 * b-trees that reference the tree block.
 *
 * The basic idea of this function is check backrefs of a given block to find
 * upper level blocks that reference the block, and then check backrefs of
 * these upper level blocks recursively. The recursion stops when tree root is
 * reached or backrefs for the block is cached.
 *
 * NOTE: if we find that backrefs for a block are cached, we know backrefs for
 * all upper level blocks that directly/indirectly reference the block are also
 * cached.
 */
static noinline_for_stack struct btrfs_backref_node *build_backref_tree(
                        struct reloc_control *rc, struct btrfs_key *node_key,
                        int level, u64 bytenr)
{
        struct btrfs_backref_iter *iter;
        struct btrfs_backref_cache *cache = &rc->backref_cache;
        /* For searching parent of TREE_BLOCK_REF */
        struct btrfs_path *path;
        struct btrfs_backref_node *cur;
        struct btrfs_backref_node *node = NULL;
        struct btrfs_backref_edge *edge;
        int ret;
        int err = 0;

        iter = btrfs_backref_iter_alloc(rc->extent_root->fs_info, GFP_NOFS);
        if (!iter)
                return ERR_PTR(-ENOMEM);
        path = btrfs_alloc_path();
        if (!path) {
                err = -ENOMEM;
                goto out;
        }

        node = btrfs_backref_alloc_node(cache, bytenr, level);
        if (!node) {
                err = -ENOMEM;
                goto out;
        }

        node->lowest = 1;
        cur = node;

        /* Breadth-first search to build backref cache */
        do {
                ret = btrfs_backref_add_tree_node(cache, path, iter, node_key,
                                                  cur);
                if (ret < 0) {
                        err = ret;
                        goto out;
                }
                edge = list_first_entry_or_null(&cache->pending_edge,
                                struct btrfs_backref_edge, list[UPPER]);
                /*
                 * The pending list isn't empty, take the first block to
                 * process
                 */
                if (edge) {
                        list_del_init(&edge->list[UPPER]);
                        cur = edge->node[UPPER];
                }
        } while (edge);

        /* Finish the upper linkage of newly added edges/nodes */
        ret = btrfs_backref_finish_upper_links(cache, node);
        if (ret < 0) {
                err = ret;
                goto out;
        }

        if (handle_useless_nodes(rc, node))
                node = NULL;
out:
        btrfs_backref_iter_free(iter);
        btrfs_free_path(path);
        if (err) {
                btrfs_backref_error_cleanup(cache, node);
                return ERR_PTR(err);
        }
        ASSERT(!node || !node->detached);
        ASSERT(list_empty(&cache->useless_node) &&
               list_empty(&cache->pending_edge));
        return node;
}

/*
 * helper to add backref node for the newly created snapshot.
 * the backref node is created by cloning backref node that
 * corresponds to root of source tree
 */
static int clone_backref_node(struct btrfs_trans_handle *trans,
                              struct reloc_control *rc,
                              struct btrfs_root *src,
                              struct btrfs_root *dest)
{
        struct btrfs_root *reloc_root = src->reloc_root;
        struct btrfs_backref_cache *cache = &rc->backref_cache;
        struct btrfs_backref_node *node = NULL;
        struct btrfs_backref_node *new_node;
        struct btrfs_backref_edge *edge;
        struct btrfs_backref_edge *new_edge;
        struct rb_node *rb_node;

        if (cache->last_trans > 0)
                update_backref_cache(trans, cache);

        rb_node = rb_simple_search(&cache->rb_root, src->commit_root->start);
        if (rb_node) {
                node = rb_entry(rb_node, struct btrfs_backref_node, rb_node);
                if (node->detached)
                        node = NULL;
                else
                        BUG_ON(node->new_bytenr != reloc_root->node->start);
        }

        if (!node) {
                rb_node = rb_simple_search(&cache->rb_root,
                                           reloc_root->commit_root->start);
                if (rb_node) {
                        node = rb_entry(rb_node, struct btrfs_backref_node,
                                        rb_node);
                        BUG_ON(node->detached);
                }
        }

        if (!node)
                return 0;

        new_node = btrfs_backref_alloc_node(cache, dest->node->start,
                                            node->level);
        if (!new_node)
                return -ENOMEM;

        new_node->lowest = node->lowest;
        new_node->checked = 1;
        new_node->root = btrfs_grab_root(dest);
        ASSERT(new_node->root);

        if (!node->lowest) {
                list_for_each_entry(edge, &node->lower, list[UPPER]) {
                        new_edge = btrfs_backref_alloc_edge(cache);
                        if (!new_edge)
                                goto fail;

                        btrfs_backref_link_edge(new_edge, edge->node[LOWER],
                                                new_node, LINK_UPPER);
                }
        } else {
                list_add_tail(&new_node->lower, &cache->leaves);
        }

        rb_node = rb_simple_insert(&cache->rb_root, new_node->bytenr,
                                   &new_node->rb_node);
        if (rb_node)
                btrfs_backref_panic(trans->fs_info, new_node->bytenr, -EEXIST);

        if (!new_node->lowest) {
                list_for_each_entry(new_edge, &new_node->lower, list[UPPER]) {
                        list_add_tail(&new_edge->list[LOWER],
                                      &new_edge->node[LOWER]->upper);
                }
        }
        return 0;
fail:
        while (!list_empty(&new_node->lower)) {
                new_edge = list_entry(new_node->lower.next,
                                      struct btrfs_backref_edge, list[UPPER]);
                list_del(&new_edge->list[UPPER]);
                btrfs_backref_free_edge(cache, new_edge);
        }
        btrfs_backref_free_node(cache, new_node);
        return -ENOMEM;
}

/*
 * helper to add 'address of tree root -> reloc tree' mapping
 */
static int __must_check __add_reloc_root(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct rb_node *rb_node;
        struct mapping_node *node;
        struct reloc_control *rc = fs_info->reloc_ctl;

        node = kmalloc(sizeof(*node), GFP_NOFS);
        if (!node)
                return -ENOMEM;

        node->bytenr = root->commit_root->start;
        node->data = root;

        spin_lock(&rc->reloc_root_tree.lock);
        rb_node = rb_simple_insert(&rc->reloc_root_tree.rb_root,
                                   node->bytenr, &node->rb_node);
        spin_unlock(&rc->reloc_root_tree.lock);
        if (rb_node) {
                btrfs_panic(fs_info, -EEXIST,
                            "Duplicate root found for start=%llu while inserting into relocation tree",
                            node->bytenr);
        }

        list_add_tail(&root->root_list, &rc->reloc_roots);
        return 0;
}

/*
 * helper to delete the 'address of tree root -> reloc tree'
 * mapping
 */
static void __del_reloc_root(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct rb_node *rb_node;
        struct mapping_node *node = NULL;
        struct reloc_control *rc = fs_info->reloc_ctl;
        bool put_ref = false;

        if (rc && root->node) {
                spin_lock(&rc->reloc_root_tree.lock);
                rb_node = rb_simple_search(&rc->reloc_root_tree.rb_root,
                                           root->commit_root->start);
                if (rb_node) {
                        node = rb_entry(rb_node, struct mapping_node, rb_node);
                        rb_erase(&node->rb_node, &rc->reloc_root_tree.rb_root);
                        RB_CLEAR_NODE(&node->rb_node);
                }
                spin_unlock(&rc->reloc_root_tree.lock);
                if (!node)
                        return;
                BUG_ON((struct btrfs_root *)node->data != root);
        }

        /*
         * We only put the reloc root here if it's on the list.  There's a lot
         * of places where the pattern is to splice the rc->reloc_roots, process
         * the reloc roots, and then add the reloc root back onto
         * rc->reloc_roots.  If we call __del_reloc_root while it's off of the
         * list we don't want the reference being dropped, because the guy
         * messing with the list is in charge of the reference.
         */
        spin_lock(&fs_info->trans_lock);
        if (!list_empty(&root->root_list)) {
                put_ref = true;
                list_del_init(&root->root_list);
        }
        spin_unlock(&fs_info->trans_lock);
        if (put_ref)
                btrfs_put_root(root);
        kfree(node);
}

/*
 * helper to update the 'address of tree root -> reloc tree'
 * mapping
 */
static int __update_reloc_root(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct rb_node *rb_node;
        struct mapping_node *node = NULL;
        struct reloc_control *rc = fs_info->reloc_ctl;

        spin_lock(&rc->reloc_root_tree.lock);
        rb_node = rb_simple_search(&rc->reloc_root_tree.rb_root,
                                   root->commit_root->start);
        if (rb_node) {
                node = rb_entry(rb_node, struct mapping_node, rb_node);
                rb_erase(&node->rb_node, &rc->reloc_root_tree.rb_root);
        }
        spin_unlock(&rc->reloc_root_tree.lock);

        if (!node)
                return 0;
        BUG_ON((struct btrfs_root *)node->data != root);

        spin_lock(&rc->reloc_root_tree.lock);
        node->bytenr = root->node->start;
        rb_node = rb_simple_insert(&rc->reloc_root_tree.rb_root,
                                   node->bytenr, &node->rb_node);
        spin_unlock(&rc->reloc_root_tree.lock);
        if (rb_node)
                btrfs_backref_panic(fs_info, node->bytenr, -EEXIST);
        return 0;
}

static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
                                        struct btrfs_root *root, u64 objectid)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_root *reloc_root;
        struct extent_buffer *eb;
        struct btrfs_root_item *root_item;
        struct btrfs_key root_key;
        int ret;

        root_item = kmalloc(sizeof(*root_item), GFP_NOFS);
        BUG_ON(!root_item);

        root_key.objectid = BTRFS_TREE_RELOC_OBJECTID;
        root_key.type = BTRFS_ROOT_ITEM_KEY;
        root_key.offset = objectid;

        if (root->root_key.objectid == objectid) {
                u64 commit_root_gen;

                /* called by btrfs_init_reloc_root */
                ret = btrfs_copy_root(trans, root, root->commit_root, &eb,
                                      BTRFS_TREE_RELOC_OBJECTID);
                BUG_ON(ret);
                /*
                 * Set the last_snapshot field to the generation of the commit
                 * root - like this ctree.c:btrfs_block_can_be_shared() behaves
                 * correctly (returns true) when the relocation root is created
                 * either inside the critical section of a transaction commit
                 * (through transaction.c:qgroup_account_snapshot()) and when
                 * it's created before the transaction commit is started.
                 */
                commit_root_gen = btrfs_header_generation(root->commit_root);
                btrfs_set_root_last_snapshot(&root->root_item, commit_root_gen);
        } else {
                /*
                 * called by btrfs_reloc_post_snapshot_hook.
                 * the source tree is a reloc tree, all tree blocks
                 * modified after it was created have RELOC flag
                 * set in their headers. so it's OK to not update
                 * the 'last_snapshot'.
                 */
                ret = btrfs_copy_root(trans, root, root->node, &eb,
                                      BTRFS_TREE_RELOC_OBJECTID);
                BUG_ON(ret);
        }

        memcpy(root_item, &root->root_item, sizeof(*root_item));
        btrfs_set_root_bytenr(root_item, eb->start);
        btrfs_set_root_level(root_item, btrfs_header_level(eb));
        btrfs_set_root_generation(root_item, trans->transid);

        if (root->root_key.objectid == objectid) {
                btrfs_set_root_refs(root_item, 0);
                memset(&root_item->drop_progress, 0,
                       sizeof(struct btrfs_disk_key));
                btrfs_set_root_drop_level(root_item, 0);
        }

        btrfs_tree_unlock(eb);
        free_extent_buffer(eb);

        ret = btrfs_insert_root(trans, fs_info->tree_root,
                                &root_key, root_item);
        BUG_ON(ret);
        kfree(root_item);

        reloc_root = btrfs_read_tree_root(fs_info->tree_root, &root_key);
        BUG_ON(IS_ERR(reloc_root));
        set_bit(BTRFS_ROOT_SHAREABLE, &reloc_root->state);
        reloc_root->last_trans = trans->transid;
        return reloc_root;
}

/*
 * create reloc tree for a given fs tree. reloc tree is just a
 * snapshot of the fs tree with special root objectid.
 *
 * The reloc_root comes out of here with two references, one for
 * root->reloc_root, and another for being on the rc->reloc_roots list.
 */
int btrfs_init_reloc_root(struct btrfs_trans_handle *trans,
                          struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_root *reloc_root;
        struct reloc_control *rc = fs_info->reloc_ctl;
        struct btrfs_block_rsv *rsv;
        int clear_rsv = 0;
        int ret;

        if (!rc)
                return 0;

        /*
         * The subvolume has reloc tree but the swap is finished, no need to
         * create/update the dead reloc tree
         */
        if (reloc_root_is_dead(root))
                return 0;

        /*
         * This is subtle but important.  We do not do
         * record_root_in_transaction for reloc roots, instead we record their
         * corresponding fs root, and then here we update the last trans for the
         * reloc root.  This means that we have to do this for the entire life
         * of the reloc root, regardless of which stage of the relocation we are
         * in.
         */
        if (root->reloc_root) {
                reloc_root = root->reloc_root;
                reloc_root->last_trans = trans->transid;
                return 0;
        }

        /*
         * We are merging reloc roots, we do not need new reloc trees.  Also
         * reloc trees never need their own reloc tree.
         */
        if (!rc->create_reloc_tree ||
            root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID)
                return 0;

        if (!trans->reloc_reserved) {
                rsv = trans->block_rsv;
                trans->block_rsv = rc->block_rsv;
                clear_rsv = 1;
        }
        reloc_root = create_reloc_root(trans, root, root->root_key.objectid);
        if (clear_rsv)
                trans->block_rsv = rsv;

        ret = __add_reloc_root(reloc_root);
        BUG_ON(ret < 0);
        root->reloc_root = btrfs_grab_root(reloc_root);
        return 0;
}

/*
 * update root item of reloc tree
 */
int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
                            struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_root *reloc_root;
        struct btrfs_root_item *root_item;
        int ret;

        if (!have_reloc_root(root))
                goto out;

        reloc_root = root->reloc_root;
        root_item = &reloc_root->root_item;

        /*
         * We are probably ok here, but __del_reloc_root() will drop its ref of
         * the root.  We have the ref for root->reloc_root, but just in case
         * hold it while we update the reloc root.
         */
        btrfs_grab_root(reloc_root);

        /* root->reloc_root will stay until current relocation finished */
        if (fs_info->reloc_ctl->merge_reloc_tree &&
            btrfs_root_refs(root_item) == 0) {
                set_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
                /*
                 * Mark the tree as dead before we change reloc_root so
                 * have_reloc_root will not touch it from now on.
                 */
                smp_wmb();
                __del_reloc_root(reloc_root);
        }

        if (reloc_root->commit_root != reloc_root->node) {
                __update_reloc_root(reloc_root);
                btrfs_set_root_node(root_item, reloc_root->node);
                free_extent_buffer(reloc_root->commit_root);
                reloc_root->commit_root = btrfs_root_node(reloc_root);
        }

        ret = btrfs_update_root(trans, fs_info->tree_root,
                                &reloc_root->root_key, root_item);
        BUG_ON(ret);
        btrfs_put_root(reloc_root);
out:
        return 0;
}

/*
 * helper to find first cached inode with inode number >= objectid
 * in a subvolume
 */
static struct inode *find_next_inode(struct btrfs_root *root, u64 objectid)
{
        struct rb_node *node;
        struct rb_node *prev;
        struct btrfs_inode *entry;
        struct inode *inode;

        spin_lock(&root->inode_lock);
again:
        node = root->inode_tree.rb_node;
        prev = NULL;
        while (node) {
                prev = node;
                entry = rb_entry(node, struct btrfs_inode, rb_node);

                if (objectid < btrfs_ino(entry))
                        node = node->rb_left;
                else if (objectid > btrfs_ino(entry))
                        node = node->rb_right;
                else
                        break;
        }
        if (!node) {
                while (prev) {
                        entry = rb_entry(prev, struct btrfs_inode, rb_node);
                        if (objectid <= btrfs_ino(entry)) {
                                node = prev;
                                break;
                        }
                        prev = rb_next(prev);
                }
        }
        while (node) {
                entry = rb_entry(node, struct btrfs_inode, rb_node);
                inode = igrab(&entry->vfs_inode);
                if (inode) {
                        spin_unlock(&root->inode_lock);
                        return inode;
                }

                objectid = btrfs_ino(entry) + 1;
                if (cond_resched_lock(&root->inode_lock))
                        goto again;

                node = rb_next(node);
        }
        spin_unlock(&root->inode_lock);
        return NULL;
}

/*
 * get new location of data
 */
static int get_new_location(struct inode *reloc_inode, u64 *new_bytenr,
                            u64 bytenr, u64 num_bytes)
{
        struct btrfs_root *root = BTRFS_I(reloc_inode)->root;
        struct btrfs_path *path;
        struct btrfs_file_extent_item *fi;
        struct extent_buffer *leaf;
        int ret;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;

        bytenr -= BTRFS_I(reloc_inode)->index_cnt;
        ret = btrfs_lookup_file_extent(NULL, root, path,
                        btrfs_ino(BTRFS_I(reloc_inode)), bytenr, 0);
        if (ret < 0)
                goto out;
        if (ret > 0) {
                ret = -ENOENT;
                goto out;
        }

        leaf = path->nodes[0];
        fi = btrfs_item_ptr(leaf, path->slots[0],
                            struct btrfs_file_extent_item);

        BUG_ON(btrfs_file_extent_offset(leaf, fi) ||
               btrfs_file_extent_compression(leaf, fi) ||
               btrfs_file_extent_encryption(leaf, fi) ||
               btrfs_file_extent_other_encoding(leaf, fi));

        if (num_bytes != btrfs_file_extent_disk_num_bytes(leaf, fi)) {
                ret = -EINVAL;
                goto out;
        }

        *new_bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
        ret = 0;
out:
        btrfs_free_path(path);
        return ret;
}

/*
 * update file extent items in the tree leaf to point to
 * the new locations.
 */
static noinline_for_stack
int replace_file_extents(struct btrfs_trans_handle *trans,
                         struct reloc_control *rc,
                         struct btrfs_root *root,
                         struct extent_buffer *leaf)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_key key;
        struct btrfs_file_extent_item *fi;
        struct inode *inode = NULL;
        u64 parent;
        u64 bytenr;
        u64 new_bytenr = 0;
        u64 num_bytes;
        u64 end;
        u32 nritems;
        u32 i;
        int ret = 0;
        int first = 1;
        int dirty = 0;

        if (rc->stage != UPDATE_DATA_PTRS)
                return 0;

        /* reloc trees always use full backref */
        if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID)
                parent = leaf->start;
        else
                parent = 0;

        nritems = btrfs_header_nritems(leaf);
        for (i = 0; i < nritems; i++) {
                struct btrfs_ref ref = { 0 };

                cond_resched();
                btrfs_item_key_to_cpu(leaf, &key, i);
                if (key.type != BTRFS_EXTENT_DATA_KEY)
                        continue;
                fi = btrfs_item_ptr(leaf, i, struct btrfs_file_extent_item);
                if (btrfs_file_extent_type(leaf, fi) ==
                    BTRFS_FILE_EXTENT_INLINE)
                        continue;
                bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
                num_bytes = btrfs_file_extent_disk_num_bytes(leaf, fi);
                if (bytenr == 0)
                        continue;
                if (!in_range(bytenr, rc->block_group->start,
                              rc->block_group->length))
                        continue;

                /*
                 * if we are modifying block in fs tree, wait for readpage
                 * to complete and drop the extent cache
                 */
                if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID) {
                        if (first) {
                                inode = find_next_inode(root, key.objectid);
                                first = 0;
                        } else if (inode && btrfs_ino(BTRFS_I(inode)) < key.objectid) {
                                btrfs_add_delayed_iput(inode);
                                inode = find_next_inode(root, key.objectid);
                        }
                        if (inode && btrfs_ino(BTRFS_I(inode)) == key.objectid) {
                                end = key.offset +
                                      btrfs_file_extent_num_bytes(leaf, fi);
                                WARN_ON(!IS_ALIGNED(key.offset,
                                                    fs_info->sectorsize));
                                WARN_ON(!IS_ALIGNED(end, fs_info->sectorsize));
                                end--;
                                ret = try_lock_extent(&BTRFS_I(inode)->io_tree,
                                                      key.offset, end);
                                if (!ret)
                                        continue;

                                btrfs_drop_extent_cache(BTRFS_I(inode),
                                                key.offset,     end, 1);
                                unlock_extent(&BTRFS_I(inode)->io_tree,
                                              key.offset, end);
                        }
                }

                ret = get_new_location(rc->data_inode, &new_bytenr,
                                       bytenr, num_bytes);
                if (ret) {
                        /*
                         * Don't have to abort since we've not changed anything
                         * in the file extent yet.
                         */
                        break;
                }

                btrfs_set_file_extent_disk_bytenr(leaf, fi, new_bytenr);
                dirty = 1;

                key.offset -= btrfs_file_extent_offset(leaf, fi);
                btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF, new_bytenr,
                                       num_bytes, parent);
                ref.real_root = root->root_key.objectid;
                btrfs_init_data_ref(&ref, btrfs_header_owner(leaf),
                                    key.objectid, key.offset);
                ret = btrfs_inc_extent_ref(trans, &ref);
                if (ret) {
                        btrfs_abort_transaction(trans, ret);
                        break;
                }

                btrfs_init_generic_ref(&ref, BTRFS_DROP_DELAYED_REF, bytenr,
                                       num_bytes, parent);
                ref.real_root = root->root_key.objectid;
                btrfs_init_data_ref(&ref, btrfs_header_owner(leaf),
                                    key.objectid, key.offset);
                ret = btrfs_free_extent(trans, &ref);
                if (ret) {
                        btrfs_abort_transaction(trans, ret);
                        break;
                }
        }
        if (dirty)
                btrfs_mark_buffer_dirty(leaf);
        if (inode)
                btrfs_add_delayed_iput(inode);
        return ret;
}

static noinline_for_stack
int memcmp_node_keys(struct extent_buffer *eb, int slot,
                     struct btrfs_path *path, int level)
{
        struct btrfs_disk_key key1;
        struct btrfs_disk_key key2;
        btrfs_node_key(eb, &key1, slot);
        btrfs_node_key(path->nodes[level], &key2, path->slots[level]);
        return memcmp(&key1, &key2, sizeof(key1));
}

/*
 * try to replace tree blocks in fs tree with the new blocks
 * in reloc tree. tree blocks haven't been modified since the
 * reloc tree was create can be replaced.
 *
 * if a block was replaced, level of the block + 1 is returned.
 * if no block got replaced, 0 is returned. if there are other
 * errors, a negative error number is returned.
 */
static noinline_for_stack
int replace_path(struct btrfs_trans_handle *trans, struct reloc_control *rc,
                 struct btrfs_root *dest, struct btrfs_root *src,
                 struct btrfs_path *path, struct btrfs_key *next_key,
                 int lowest_level, int max_level)
{
        struct btrfs_fs_info *fs_info = dest->fs_info;
        struct extent_buffer *eb;
        struct extent_buffer *parent;
        struct btrfs_ref ref = { 0 };
        struct btrfs_key key;
        u64 old_bytenr;
        u64 new_bytenr;
        u64 old_ptr_gen;
        u64 new_ptr_gen;
        u64 last_snapshot;
        u32 blocksize;
        int cow = 0;
        int level;
        int ret;
        int slot;

        BUG_ON(src->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID);
        BUG_ON(dest->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID);

        last_snapshot = btrfs_root_last_snapshot(&src->root_item);
again:
        slot = path->slots[lowest_level];
        btrfs_node_key_to_cpu(path->nodes[lowest_level], &key, slot);

        eb = btrfs_lock_root_node(dest);
        level = btrfs_header_level(eb);

        if (level < lowest_level) {
                btrfs_tree_unlock(eb);
                free_extent_buffer(eb);
                return 0;
        }

        if (cow) {
                ret = btrfs_cow_block(trans, dest, eb, NULL, 0, &eb,
                                      BTRFS_NESTING_COW);
                BUG_ON(ret);
        }

        if (next_key) {
                next_key->objectid = (u64)-1;
                next_key->type = (u8)-1;
                next_key->offset = (u64)-1;
        }

        parent = eb;
        while (1) {
                level = btrfs_header_level(parent);
                BUG_ON(level < lowest_level);

                ret = btrfs_bin_search(parent, &key, &slot);
                if (ret < 0)
                        break;
                if (ret && slot > 0)
                        slot--;

                if (next_key && slot + 1 < btrfs_header_nritems(parent))
                        btrfs_node_key_to_cpu(parent, next_key, slot + 1);

                old_bytenr = btrfs_node_blockptr(parent, slot);
                blocksize = fs_info->nodesize;
                old_ptr_gen = btrfs_node_ptr_generation(parent, slot);

                if (level <= max_level) {
                        eb = path->nodes[level];
                        new_bytenr = btrfs_node_blockptr(eb,
                                                        path->slots[level]);
                        new_ptr_gen = btrfs_node_ptr_generation(eb,
                                                        path->slots[level]);
                } else {
                        new_bytenr = 0;
                        new_ptr_gen = 0;
                }

                if (WARN_ON(new_bytenr > 0 && new_bytenr == old_bytenr)) {
                        ret = level;
                        break;
                }

                if (new_bytenr == 0 || old_ptr_gen > last_snapshot ||
                    memcmp_node_keys(parent, slot, path, level)) {
                        if (level <= lowest_level) {
                                ret = 0;
                                break;
                        }

                        eb = btrfs_read_node_slot(parent, slot);
                        if (IS_ERR(eb)) {
                                ret = PTR_ERR(eb);
                                break;
                        }
                        btrfs_tree_lock(eb);
                        if (cow) {
                                ret = btrfs_cow_block(trans, dest, eb, parent,
                                                      slot, &eb,
                                                      BTRFS_NESTING_COW);
                                BUG_ON(ret);
                        }

                        btrfs_tree_unlock(parent);
                        free_extent_buffer(parent);

                        parent = eb;
                        continue;
                }

                if (!cow) {
                        btrfs_tree_unlock(parent);
                        free_extent_buffer(parent);
                        cow = 1;
                        goto again;
                }

                btrfs_node_key_to_cpu(path->nodes[level], &key,
                                      path->slots[level]);
                btrfs_release_path(path);

                path->lowest_level = level;
                ret = btrfs_search_slot(trans, src, &key, path, 0, 1);
                path->lowest_level = 0;
                BUG_ON(ret);

                /*
                 * Info qgroup to trace both subtrees.
                 *
                 * We must trace both trees.
                 * 1) Tree reloc subtree
                 *    If not traced, we will leak data numbers
                 * 2) Fs subtree
                 *    If not traced, we will double count old data
                 *
                 * We don't scan the subtree right now, but only record
                 * the swapped tree blocks.
                 * The real subtree rescan is delayed until we have new
                 * CoW on the subtree root node before transaction commit.
                 */
                ret = btrfs_qgroup_add_swapped_blocks(trans, dest,
                                rc->block_group, parent, slot,
                                path->nodes[level], path->slots[level],
                                last_snapshot);
                if (ret < 0)
                        break;
                /*
                 * swap blocks in fs tree and reloc tree.
                 */
                btrfs_set_node_blockptr(parent, slot, new_bytenr);
                btrfs_set_node_ptr_generation(parent, slot, new_ptr_gen);
                btrfs_mark_buffer_dirty(parent);

                btrfs_set_node_blockptr(path->nodes[level],
                                        path->slots[level], old_bytenr);
                btrfs_set_node_ptr_generation(path->nodes[level],
                                              path->slots[level], old_ptr_gen);
                btrfs_mark_buffer_dirty(path->nodes[level]);

                btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF, old_bytenr,
                                       blocksize, path->nodes[level]->start);
                ref.skip_qgroup = true;
                btrfs_init_tree_ref(&ref, level - 1, src->root_key.objectid);
                ret = btrfs_inc_extent_ref(trans, &ref);
                BUG_ON(ret);
                btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF, new_bytenr,
                                       blocksize, 0);
                ref.skip_qgroup = true;
                btrfs_init_tree_ref(&ref, level - 1, dest->root_key.objectid);
                ret = btrfs_inc_extent_ref(trans, &ref);
                BUG_ON(ret);

                btrfs_init_generic_ref(&ref, BTRFS_DROP_DELAYED_REF, new_bytenr,
                                       blocksize, path->nodes[level]->start);
                btrfs_init_tree_ref(&ref, level - 1, src->root_key.objectid);
                ref.skip_qgroup = true;
                ret = btrfs_free_extent(trans, &ref);
                BUG_ON(ret);

                btrfs_init_generic_ref(&ref, BTRFS_DROP_DELAYED_REF, old_bytenr,
                                       blocksize, 0);
                btrfs_init_tree_ref(&ref, level - 1, dest->root_key.objectid);
                ref.skip_qgroup = true;
                ret = btrfs_free_extent(trans, &ref);
                BUG_ON(ret);

                btrfs_unlock_up_safe(path, 0);

                ret = level;
                break;
        }
        btrfs_tree_unlock(parent);
        free_extent_buffer(parent);
        return ret;
}

/*
 * helper to find next relocated block in reloc tree
 */
static noinline_for_stack
int walk_up_reloc_tree(struct btrfs_root *root, struct btrfs_path *path,
                       int *level)
{
        struct extent_buffer *eb;
        int i;
        u64 last_snapshot;
        u32 nritems;

        last_snapshot = btrfs_root_last_snapshot(&root->root_item);

        for (i = 0; i < *level; i++) {
                free_extent_buffer(path->nodes[i]);
                path->nodes[i] = NULL;
        }

        for (i = *level; i < BTRFS_MAX_LEVEL && path->nodes[i]; i++) {
                eb = path->nodes[i];
                nritems = btrfs_header_nritems(eb);
                while (path->slots[i] + 1 < nritems) {
                        path->slots[i]++;
                        if (btrfs_node_ptr_generation(eb, path->slots[i]) <=
                            last_snapshot)
                                continue;

                        *level = i;
                        return 0;
                }
                free_extent_buffer(path->nodes[i]);
                path->nodes[i] = NULL;
        }
        return 1;
}

/*
 * walk down reloc tree to find relocated block of lowest level
 */
static noinline_for_stack
int walk_down_reloc_tree(struct btrfs_root *root, struct btrfs_path *path,
                         int *level)
{
        struct extent_buffer *eb = NULL;
        int i;
        u64 ptr_gen = 0;
        u64 last_snapshot;
        u32 nritems;

        last_snapshot = btrfs_root_last_snapshot(&root->root_item);

        for (i = *level; i > 0; i--) {
                eb = path->nodes[i];
                nritems = btrfs_header_nritems(eb);
                while (path->slots[i] < nritems) {
                        ptr_gen = btrfs_node_ptr_generation(eb, path->slots[i]);
                        if (ptr_gen > last_snapshot)
                                break;
                        path->slots[i]++;
                }
                if (path->slots[i] >= nritems) {
                        if (i == *level)
                                break;
                        *level = i + 1;
                        return 0;
                }
                if (i == 1) {
                        *level = i;
                        return 0;
                }

                eb = btrfs_read_node_slot(eb, path->slots[i]);
                if (IS_ERR(eb))
                        return PTR_ERR(eb);
                BUG_ON(btrfs_header_level(eb) != i - 1);
                path->nodes[i - 1] = eb;
                path->slots[i - 1] = 0;
        }
        return 1;
}

/*
 * invalidate extent cache for file extents whose key in range of
 * [min_key, max_key)
 */
static int invalidate_extent_cache(struct btrfs_root *root,
                                   struct btrfs_key *min_key,
                                   struct btrfs_key *max_key)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct inode *inode = NULL;
        u64 objectid;
        u64 start, end;
        u64 ino;

        objectid = min_key->objectid;
        while (1) {
                cond_resched();
                iput(inode);

                if (objectid > max_key->objectid)
                        break;

                inode = find_next_inode(root, objectid);
                if (!inode)
                        break;
                ino = btrfs_ino(BTRFS_I(inode));

                if (ino > max_key->objectid) {
                        iput(inode);
                        break;
                }

                objectid = ino + 1;
                if (!S_ISREG(inode->i_mode))
                        continue;

                if (unlikely(min_key->objectid == ino)) {
                        if (min_key->type > BTRFS_EXTENT_DATA_KEY)
                                continue;
                        if (min_key->type < BTRFS_EXTENT_DATA_KEY)
                                start = 0;
                        else {
                                start = min_key->offset;
                                WARN_ON(!IS_ALIGNED(start, fs_info->sectorsize));
                        }
                } else {
                        start = 0;
                }

                if (unlikely(max_key->objectid == ino)) {
                        if (max_key->type < BTRFS_EXTENT_DATA_KEY)
                                continue;
                        if (max_key->type > BTRFS_EXTENT_DATA_KEY) {
                                end = (u64)-1;
                        } else {
                                if (max_key->offset == 0)
                                        continue;
                                end = max_key->offset;
                                WARN_ON(!IS_ALIGNED(end, fs_info->sectorsize));
                                end--;
                        }
                } else {
                        end = (u64)-1;
                }

                /* the lock_extent waits for readpage to complete */
                lock_extent(&BTRFS_I(inode)->io_tree, start, end);
                btrfs_drop_extent_cache(BTRFS_I(inode), start, end, 1);
                unlock_extent(&BTRFS_I(inode)->io_tree, start, end);
        }
        return 0;
}

static int find_next_key(struct btrfs_path *path, int level,
                         struct btrfs_key *key)

{
        while (level < BTRFS_MAX_LEVEL) {
                if (!path->nodes[level])
                        break;
                if (path->slots[level] + 1 <
                    btrfs_header_nritems(path->nodes[level])) {
                        btrfs_node_key_to_cpu(path->nodes[level], key,
                                              path->slots[level] + 1);
                        return 0;
                }
                level++;
        }
        return 1;
}

/*
 * Insert current subvolume into reloc_control::dirty_subvol_roots
 */
static void insert_dirty_subvol(struct btrfs_trans_handle *trans,
                                struct reloc_control *rc,
                                struct btrfs_root *root)
{
        struct btrfs_root *reloc_root = root->reloc_root;
        struct btrfs_root_item *reloc_root_item;

        /* @root must be a subvolume tree root with a valid reloc tree */
        ASSERT(root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID);
        ASSERT(reloc_root);

        reloc_root_item = &reloc_root->root_item;
        memset(&reloc_root_item->drop_progress, 0,
                sizeof(reloc_root_item->drop_progress));
        btrfs_set_root_drop_level(reloc_root_item, 0);
        btrfs_set_root_refs(reloc_root_item, 0);
        btrfs_update_reloc_root(trans, root);

        if (list_empty(&root->reloc_dirty_list)) {
                btrfs_grab_root(root);
                list_add_tail(&root->reloc_dirty_list, &rc->dirty_subvol_roots);
        }
}

static int clean_dirty_subvols(struct reloc_control *rc)
{
        struct btrfs_root *root;
        struct btrfs_root *next;
        int ret = 0;
        int ret2;

        list_for_each_entry_safe(root, next, &rc->dirty_subvol_roots,
                                 reloc_dirty_list) {
                if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID) {
                        /* Merged subvolume, cleanup its reloc root */
                        struct btrfs_root *reloc_root = root->reloc_root;

                        list_del_init(&root->reloc_dirty_list);
                        root->reloc_root = NULL;
                        /*
                         * Need barrier to ensure clear_bit() only happens after
                         * root->reloc_root = NULL. Pairs with have_reloc_root.
                         */
                        smp_wmb();
                        clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
                        if (reloc_root) {
                                /*
                                 * btrfs_drop_snapshot drops our ref we hold for
                                 * ->reloc_root.  If it fails however we must
                                 * drop the ref ourselves.
                                 */
                                ret2 = btrfs_drop_snapshot(reloc_root, 0, 1);
                                if (ret2 < 0) {
                                        btrfs_put_root(reloc_root);
                                        if (!ret)
                                                ret = ret2;
                                }
                        }
                        btrfs_put_root(root);
                } else {
                        /* Orphan reloc tree, just clean it up */
                        ret2 = btrfs_drop_snapshot(root, 0, 1);
                        if (ret2 < 0) {
                                btrfs_put_root(root);
                                if (!ret)
                                        ret = ret2;
                        }
                }
        }
        return ret;
}

/*
 * merge the relocated tree blocks in reloc tree with corresponding
 * fs tree.
 */
static noinline_for_stack int merge_reloc_root(struct reloc_control *rc,
                                               struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct btrfs_key key;
        struct btrfs_key next_key;
        struct btrfs_trans_handle *trans = NULL;
        struct btrfs_root *reloc_root;
        struct btrfs_root_item *root_item;
        struct btrfs_path *path;
        struct extent_buffer *leaf;
        int reserve_level;
        int level;
        int max_level;
        int replaced = 0;
        int ret = 0;
        u32 min_reserved;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;
        path->reada = READA_FORWARD;

        reloc_root = root->reloc_root;
        root_item = &reloc_root->root_item;

        if (btrfs_disk_key_objectid(&root_item->drop_progress) == 0) {
                level = btrfs_root_level(root_item);
                atomic_inc(&reloc_root->node->refs);
                path->nodes[level] = reloc_root->node;
                path->slots[level] = 0;
        } else {
                btrfs_disk_key_to_cpu(&key, &root_item->drop_progress);

                level = btrfs_root_drop_level(root_item);
                BUG_ON(level == 0);
                path->lowest_level = level;
                ret = btrfs_search_slot(NULL, reloc_root, &key, path, 0, 0);
                path->lowest_level = 0;
                if (ret < 0) {
                        btrfs_free_path(path);
                        return ret;
                }

                btrfs_node_key_to_cpu(path->nodes[level], &next_key,
                                      path->slots[level]);
                WARN_ON(memcmp(&key, &next_key, sizeof(key)));

                btrfs_unlock_up_safe(path, 0);
        }

        /*
         * In merge_reloc_root(), we modify the upper level pointer to swap the
         * tree blocks between reloc tree and subvolume tree.  Thus for tree
         * block COW, we COW at most from level 1 to root level for each tree.
         *
         * Thus the needed metadata size is at most root_level * nodesize,
         * and * 2 since we have two trees to COW.
         */
        reserve_level = max_t(int, 1, btrfs_root_level(root_item));
        min_reserved = fs_info->nodesize * reserve_level * 2;
        memset(&next_key, 0, sizeof(next_key));

        while (1) {
                ret = btrfs_block_rsv_refill(root, rc->block_rsv, min_reserved,
                                             BTRFS_RESERVE_FLUSH_LIMIT);
                if (ret)
                        goto out;
                trans = btrfs_start_transaction(root, 0);
                if (IS_ERR(trans)) {
                        ret = PTR_ERR(trans);
                        trans = NULL;
                        goto out;
                }

                /*
                 * At this point we no longer have a reloc_control, so we can't
                 * depend on btrfs_init_reloc_root to update our last_trans.
                 *
                 * But that's ok, we started the trans handle on our
                 * corresponding fs_root, which means it's been added to the
                 * dirty list.  At commit time we'll still call
                 * btrfs_update_reloc_root() and update our root item
                 * appropriately.
                 */
                reloc_root->last_trans = trans->transid;
                trans->block_rsv = rc->block_rsv;

                replaced = 0;
                max_level = level;

                ret = walk_down_reloc_tree(reloc_root, path, &level);
                if (ret < 0)
                        goto out;
                if (ret > 0)
                        break;

                if (!find_next_key(path, level, &key) &&
                    btrfs_comp_cpu_keys(&next_key, &key) >= 0) {
                        ret = 0;
                } else {
                        ret = replace_path(trans, rc, root, reloc_root, path,
                                           &next_key, level, max_level);
                }
                if (ret < 0)
                        goto out;
                if (ret > 0) {
                        level = ret;
                        btrfs_node_key_to_cpu(path->nodes[level], &key,
                                              path->slots[level]);
                        replaced = 1;
                }

                ret = walk_up_reloc_tree(reloc_root, path, &level);
                if (ret > 0)
                        break;

                BUG_ON(level == 0);
                /*
                 * save the merging progress in the drop_progress.
                 * this is OK since root refs == 1 in this case.
                 */
                btrfs_node_key(path->nodes[level], &root_item->drop_progress,
                               path->slots[level]);
                btrfs_set_root_drop_level(root_item, level);

                btrfs_end_transaction_throttle(trans);
                trans = NULL;

                btrfs_btree_balance_dirty(fs_info);

                if (replaced && rc->stage == UPDATE_DATA_PTRS)
                        invalidate_extent_cache(root, &key, &next_key);
        }

        /*
         * handle the case only one block in the fs tree need to be
         * relocated and the block is tree root.
         */
        leaf = btrfs_lock_root_node(root);
        ret = btrfs_cow_block(trans, root, leaf, NULL, 0, &leaf,
                              BTRFS_NESTING_COW);
        btrfs_tree_unlock(leaf);
        free_extent_buffer(leaf);
out:
        btrfs_free_path(path);

        if (ret == 0)
                insert_dirty_subvol(trans, rc, root);

        if (trans)
                btrfs_end_transaction_throttle(trans);

        btrfs_btree_balance_dirty(fs_info);

        if (replaced && rc->stage == UPDATE_DATA_PTRS)
                invalidate_extent_cache(root, &key, &next_key);

        return ret;
}

static noinline_for_stack
int prepare_to_merge(struct reloc_control *rc, int err)
{
        struct btrfs_root *root = rc->extent_root;
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_root *reloc_root;
        struct btrfs_trans_handle *trans;
        LIST_HEAD(reloc_roots);
        u64 num_bytes = 0;
        int ret;

        mutex_lock(&fs_info->reloc_mutex);
        rc->merging_rsv_size += fs_info->nodesize * (BTRFS_MAX_LEVEL - 1) * 2;
        rc->merging_rsv_size += rc->nodes_relocated * 2;
        mutex_unlock(&fs_info->reloc_mutex);

again:
        if (!err) {
                num_bytes = rc->merging_rsv_size;
                ret = btrfs_block_rsv_add(root, rc->block_rsv, num_bytes,
                                          BTRFS_RESERVE_FLUSH_ALL);
                if (ret)
                        err = ret;
        }

        trans = btrfs_join_transaction(rc->extent_root);
        if (IS_ERR(trans)) {
                if (!err)
                        btrfs_block_rsv_release(fs_info, rc->block_rsv,
                                                num_bytes, NULL);
                return PTR_ERR(trans);
        }

        if (!err) {
                if (num_bytes != rc->merging_rsv_size) {
                        btrfs_end_transaction(trans);
                        btrfs_block_rsv_release(fs_info, rc->block_rsv,
                                                num_bytes, NULL);
                        goto again;
                }
        }

        rc->merge_reloc_tree = 1;

        while (!list_empty(&rc->reloc_roots)) {
                reloc_root = list_entry(rc->reloc_roots.next,
                                        struct btrfs_root, root_list);
                list_del_init(&reloc_root->root_list);

                root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
                                false);
                BUG_ON(IS_ERR(root));
                BUG_ON(root->reloc_root != reloc_root);

                /*
                 * set reference count to 1, so btrfs_recover_relocation
                 * knows it should resumes merging
                 */
                if (!err)
                        btrfs_set_root_refs(&reloc_root->root_item, 1);
                btrfs_update_reloc_root(trans, root);

                list_add(&reloc_root->root_list, &reloc_roots);
                btrfs_put_root(root);
        }

        list_splice(&reloc_roots, &rc->reloc_roots);

        if (!err)
                btrfs_commit_transaction(trans);
        else
                btrfs_end_transaction(trans);
        return err;
}

static noinline_for_stack
void free_reloc_roots(struct list_head *list)
{
        struct btrfs_root *reloc_root, *tmp;

        list_for_each_entry_safe(reloc_root, tmp, list, root_list)
                __del_reloc_root(reloc_root);
}

static noinline_for_stack
void merge_reloc_roots(struct reloc_control *rc)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct btrfs_root *root;
        struct btrfs_root *reloc_root;
        LIST_HEAD(reloc_roots);
        int found = 0;
        int ret = 0;
again:
        root = rc->extent_root;

        /*
         * this serializes us with btrfs_record_root_in_transaction,
         * we have to make sure nobody is in the middle of
         * adding their roots to the list while we are
         * doing this splice
         */
        mutex_lock(&fs_info->reloc_mutex);
        list_splice_init(&rc->reloc_roots, &reloc_roots);
        mutex_unlock(&fs_info->reloc_mutex);

        while (!list_empty(&reloc_roots)) {
                found = 1;
                reloc_root = list_entry(reloc_roots.next,
                                        struct btrfs_root, root_list);

                root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
                                         false);
                if (btrfs_root_refs(&reloc_root->root_item) > 0) {
                        BUG_ON(IS_ERR(root));
                        BUG_ON(root->reloc_root != reloc_root);
                        ret = merge_reloc_root(rc, root);
                        btrfs_put_root(root);
                        if (ret) {
                                if (list_empty(&reloc_root->root_list))
                                        list_add_tail(&reloc_root->root_list,
                                                      &reloc_roots);
                                goto out;
                        }
                } else {
                        if (!IS_ERR(root)) {
                                if (root->reloc_root == reloc_root) {
                                        root->reloc_root = NULL;
                                        btrfs_put_root(reloc_root);
                                }
                                clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE,
                                          &root->state);
                                btrfs_put_root(root);
                        }

                        list_del_init(&reloc_root->root_list);
                        /* Don't forget to queue this reloc root for cleanup */
                        list_add_tail(&reloc_root->reloc_dirty_list,
                                      &rc->dirty_subvol_roots);
                }
        }

        if (found) {
                found = 0;
                goto again;
        }
out:
        if (ret) {
                btrfs_handle_fs_error(fs_info, ret, NULL);
                free_reloc_roots(&reloc_roots);

                /* new reloc root may be added */
                mutex_lock(&fs_info->reloc_mutex);
                list_splice_init(&rc->reloc_roots, &reloc_roots);
                mutex_unlock(&fs_info->reloc_mutex);
                free_reloc_roots(&reloc_roots);
        }

        /*
         * We used to have
         *
         * BUG_ON(!RB_EMPTY_ROOT(&rc->reloc_root_tree.rb_root));
         *
         * here, but it's wrong.  If we fail to start the transaction in
         * prepare_to_merge() we will have only 0 ref reloc roots, none of which
         * have actually been removed from the reloc_root_tree rb tree.  This is
         * fine because we're bailing here, and we hold a reference on the root
         * for the list that holds it, so these roots will be cleaned up when we
         * do the reloc_dirty_list afterwards.  Meanwhile the root->reloc_root
         * will be cleaned up on unmount.
         *
         * The remaining nodes will be cleaned up by free_reloc_control.
         */
}

static void free_block_list(struct rb_root *blocks)
{
        struct tree_block *block;
        struct rb_node *rb_node;
        while ((rb_node = rb_first(blocks))) {
                block = rb_entry(rb_node, struct tree_block, rb_node);
                rb_erase(rb_node, blocks);
                kfree(block);
        }
}

static int record_reloc_root_in_trans(struct btrfs_trans_handle *trans,
                                      struct btrfs_root *reloc_root)
{
        struct btrfs_fs_info *fs_info = reloc_root->fs_info;
        struct btrfs_root *root;
        int ret;

        if (reloc_root->last_trans == trans->transid)
                return 0;

        root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset, false);
        BUG_ON(IS_ERR(root));
        BUG_ON(root->reloc_root != reloc_root);
        ret = btrfs_record_root_in_trans(trans, root);
        btrfs_put_root(root);

        return ret;
}

static noinline_for_stack
struct btrfs_root *select_reloc_root(struct btrfs_trans_handle *trans,
                                     struct reloc_control *rc,
                                     struct btrfs_backref_node *node,
                                     struct btrfs_backref_edge *edges[])
{
        struct btrfs_backref_node *next;
        struct btrfs_root *root;
        int index = 0;

        next = node;
        while (1) {
                cond_resched();
                next = walk_up_backref(next, edges, &index);
                root = next->root;
                BUG_ON(!root);
                BUG_ON(!test_bit(BTRFS_ROOT_SHAREABLE, &root->state));

                if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID) {
                        record_reloc_root_in_trans(trans, root);
                        break;
                }

                btrfs_record_root_in_trans(trans, root);
                root = root->reloc_root;

                if (next->new_bytenr != root->node->start) {
                        BUG_ON(next->new_bytenr);
                        BUG_ON(!list_empty(&next->list));
                        next->new_bytenr = root->node->start;
                        btrfs_put_root(next->root);
                        next->root = btrfs_grab_root(root);
                        ASSERT(next->root);
                        list_add_tail(&next->list,
                                      &rc->backref_cache.changed);
                        mark_block_processed(rc, next);
                        break;
                }

                WARN_ON(1);
                root = NULL;
                next = walk_down_backref(edges, &index);
                if (!next || next->level <= node->level)
                        break;
        }
        if (!root)
                return NULL;

        next = node;
        /* setup backref node path for btrfs_reloc_cow_block */
        while (1) {
                rc->backref_cache.path[next->level] = next;
                if (--index < 0)
                        break;
                next = edges[index]->node[UPPER];
        }
        return root;
}

/*
 * Select a tree root for relocation.
 *
 * Return NULL if the block is not shareable. We should use do_relocation() in
 * this case.
 *
 * Return a tree root pointer if the block is shareable.
 * Return -ENOENT if the block is root of reloc tree.
 */
static noinline_for_stack
struct btrfs_root *select_one_root(struct btrfs_backref_node *node)
{
        struct btrfs_backref_node *next;
        struct btrfs_root *root;
        struct btrfs_root *fs_root = NULL;
        struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
        int index = 0;

        next = node;
        while (1) {
                cond_resched();
                next = walk_up_backref(next, edges, &index);
                root = next->root;
                BUG_ON(!root);

                /* No other choice for non-shareable tree */
                if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
                        return root;

                if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID)
                        fs_root = root;

                if (next != node)
                        return NULL;

                next = walk_down_backref(edges, &index);
                if (!next || next->level <= node->level)
                        break;
        }

        if (!fs_root)
                return ERR_PTR(-ENOENT);
        return fs_root;
}

static noinline_for_stack
u64 calcu_metadata_size(struct reloc_control *rc,
                        struct btrfs_backref_node *node, int reserve)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct btrfs_backref_node *next = node;
        struct btrfs_backref_edge *edge;
        struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
        u64 num_bytes = 0;
        int index = 0;

        BUG_ON(reserve && node->processed);

        while (next) {
                cond_resched();
                while (1) {
                        if (next->processed && (reserve || next != node))
                                break;

                        num_bytes += fs_info->nodesize;

                        if (list_empty(&next->upper))
                                break;

                        edge = list_entry(next->upper.next,
                                        struct btrfs_backref_edge, list[LOWER]);
                        edges[index++] = edge;
                        next = edge->node[UPPER];
                }
                next = walk_down_backref(edges, &index);
        }
        return num_bytes;
}

static int reserve_metadata_space(struct btrfs_trans_handle *trans,
                                  struct reloc_control *rc,
                                  struct btrfs_backref_node *node)
{
        struct btrfs_root *root = rc->extent_root;
        struct btrfs_fs_info *fs_info = root->fs_info;
        u64 num_bytes;
        int ret;
        u64 tmp;

        num_bytes = calcu_metadata_size(rc, node, 1) * 2;

        trans->block_rsv = rc->block_rsv;
        rc->reserved_bytes += num_bytes;

        /*
         * We are under a transaction here so we can only do limited flushing.
         * If we get an enospc just kick back -EAGAIN so we know to drop the
         * transaction and try to refill when we can flush all the things.
         */
        ret = btrfs_block_rsv_refill(root, rc->block_rsv, num_bytes,
                                BTRFS_RESERVE_FLUSH_LIMIT);
        if (ret) {
                tmp = fs_info->nodesize * RELOCATION_RESERVED_NODES;
                while (tmp <= rc->reserved_bytes)
                        tmp <<= 1;
                /*
                 * only one thread can access block_rsv at this point,
                 * so we don't need hold lock to protect block_rsv.
                 * we expand more reservation size here to allow enough
                 * space for relocation and we will return earlier in
                 * enospc case.
                 */
                rc->block_rsv->size = tmp + fs_info->nodesize *
                                      RELOCATION_RESERVED_NODES;
                return -EAGAIN;
        }

        return 0;
}

/*
 * relocate a block tree, and then update pointers in upper level
 * blocks that reference the block to point to the new location.
 *
 * if called by link_to_upper, the block has already been relocated.
 * in that case this function just updates pointers.
 */
static int do_relocation(struct btrfs_trans_handle *trans,
                         struct reloc_control *rc,
                         struct btrfs_backref_node *node,
                         struct btrfs_key *key,
                         struct btrfs_path *path, int lowest)
{
        struct btrfs_backref_node *upper;
        struct btrfs_backref_edge *edge;
        struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
        struct btrfs_root *root;
        struct extent_buffer *eb;
        u32 blocksize;
        u64 bytenr;
        int slot;
        int ret = 0;

        BUG_ON(lowest && node->eb);

        path->lowest_level = node->level + 1;
        rc->backref_cache.path[node->level] = node;
        list_for_each_entry(edge, &node->upper, list[LOWER]) {
                struct btrfs_ref ref = { 0 };

                cond_resched();

                upper = edge->node[UPPER];
                root = select_reloc_root(trans, rc, upper, edges);
                BUG_ON(!root);

                if (upper->eb && !upper->locked) {
                        if (!lowest) {
                                ret = btrfs_bin_search(upper->eb, key, &slot);
                                if (ret < 0)
                                        goto next;
                                BUG_ON(ret);
                                bytenr = btrfs_node_blockptr(upper->eb, slot);
                                if (node->eb->start == bytenr)
                                        goto next;
                        }
                        btrfs_backref_drop_node_buffer(upper);
                }

                if (!upper->eb) {
                        ret = btrfs_search_slot(trans, root, key, path, 0, 1);
                        if (ret) {
                                if (ret > 0)
                                        ret = -ENOENT;

                                btrfs_release_path(path);
                                break;
                        }

                        if (!upper->eb) {
                                upper->eb = path->nodes[upper->level];
                                path->nodes[upper->level] = NULL;
                        } else {
                                BUG_ON(upper->eb != path->nodes[upper->level]);
                        }

                        upper->locked = 1;
                        path->locks[upper->level] = 0;

                        slot = path->slots[upper->level];
                        btrfs_release_path(path);
                } else {
                        ret = btrfs_bin_search(upper->eb, key, &slot);
                        if (ret < 0)
                                goto next;
                        BUG_ON(ret);
                }

                bytenr = btrfs_node_blockptr(upper->eb, slot);
                if (lowest) {
                        if (bytenr != node->bytenr) {
                                btrfs_err(root->fs_info,
                "lowest leaf/node mismatch: bytenr %llu node->bytenr %llu slot %d upper %llu",
                                          bytenr, node->bytenr, slot,
                                          upper->eb->start);
                                ret = -EIO;
                                goto next;
                        }
                } else {
                        if (node->eb->start == bytenr)
                                goto next;
                }

                blocksize = root->fs_info->nodesize;
                eb = btrfs_read_node_slot(upper->eb, slot);
                if (IS_ERR(eb)) {
                        ret = PTR_ERR(eb);
                        goto next;
                }
                btrfs_tree_lock(eb);

                if (!node->eb) {
                        ret = btrfs_cow_block(trans, root, eb, upper->eb,
                                              slot, &eb, BTRFS_NESTING_COW);
                        btrfs_tree_unlock(eb);
                        free_extent_buffer(eb);
                        if (ret < 0)
                                goto next;
                        BUG_ON(node->eb != eb);
                } else {
                        btrfs_set_node_blockptr(upper->eb, slot,
                                                node->eb->start);
                        btrfs_set_node_ptr_generation(upper->eb, slot,
                                                      trans->transid);
                        btrfs_mark_buffer_dirty(upper->eb);

                        btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF,
                                               node->eb->start, blocksize,
                                               upper->eb->start);
                        ref.real_root = root->root_key.objectid;
                        btrfs_init_tree_ref(&ref, node->level,
                                            btrfs_header_owner(upper->eb));
                        ret = btrfs_inc_extent_ref(trans, &ref);
                        BUG_ON(ret);

                        ret = btrfs_drop_subtree(trans, root, eb, upper->eb);
                        BUG_ON(ret);
                }
next:
                if (!upper->pending)
                        btrfs_backref_drop_node_buffer(upper);
                else
                        btrfs_backref_unlock_node_buffer(upper);
                if (ret)
                        break;
        }

        if (!ret && node->pending) {
                btrfs_backref_drop_node_buffer(node);
                list_move_tail(&node->list, &rc->backref_cache.changed);
                node->pending = 0;
        }

        path->lowest_level = 0;
        BUG_ON(ret == -ENOSPC);
        return ret;
}

static int link_to_upper(struct btrfs_trans_handle *trans,
                         struct reloc_control *rc,
                         struct btrfs_backref_node *node,
                         struct btrfs_path *path)
{
        struct btrfs_key key;

        btrfs_node_key_to_cpu(node->eb, &key, 0);
        return do_relocation(trans, rc, node, &key, path, 0);
}

static int finish_pending_nodes(struct btrfs_trans_handle *trans,
                                struct reloc_control *rc,
                                struct btrfs_path *path, int err)
{
        LIST_HEAD(list);
        struct btrfs_backref_cache *cache = &rc->backref_cache;
        struct btrfs_backref_node *node;
        int level;
        int ret;

        for (level = 0; level < BTRFS_MAX_LEVEL; level++) {
                while (!list_empty(&cache->pending[level])) {
                        node = list_entry(cache->pending[level].next,
                                          struct btrfs_backref_node, list);
                        list_move_tail(&node->list, &list);
                        BUG_ON(!node->pending);

                        if (!err) {
                                ret = link_to_upper(trans, rc, node, path);
                                if (ret < 0)
                                        err = ret;
                        }
                }
                list_splice_init(&list, &cache->pending[level]);
        }
        return err;
}

/*
 * mark a block and all blocks directly/indirectly reference the block
 * as processed.
 */
static void update_processed_blocks(struct reloc_control *rc,
                                    struct btrfs_backref_node *node)
{
        struct btrfs_backref_node *next = node;
        struct btrfs_backref_edge *edge;
        struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
        int index = 0;

        while (next) {
                cond_resched();
                while (1) {
                        if (next->processed)
                                break;

                        mark_block_processed(rc, next);

                        if (list_empty(&next->upper))
                                break;

                        edge = list_entry(next->upper.next,
                                        struct btrfs_backref_edge, list[LOWER]);
                        edges[index++] = edge;
                        next = edge->node[UPPER];
                }
                next = walk_down_backref(edges, &index);
        }
}

static int tree_block_processed(u64 bytenr, struct reloc_control *rc)
{
        u32 blocksize = rc->extent_root->fs_info->nodesize;

        if (test_range_bit(&rc->processed_blocks, bytenr,
                           bytenr + blocksize - 1, EXTENT_DIRTY, 1, NULL))
                return 1;
        return 0;
}

static int get_tree_block_key(struct btrfs_fs_info *fs_info,
                              struct tree_block *block)
{
        struct extent_buffer *eb;

        eb = read_tree_block(fs_info, block->bytenr, 0, block->key.offset,
                             block->level, NULL);
        if (IS_ERR(eb)) {
                return PTR_ERR(eb);
        } else if (!extent_buffer_uptodate(eb)) {
                free_extent_buffer(eb);
                return -EIO;
        }
        if (block->level == 0)
                btrfs_item_key_to_cpu(eb, &block->key, 0);
        else
                btrfs_node_key_to_cpu(eb, &block->key, 0);
        free_extent_buffer(eb);
        block->key_ready = 1;
        return 0;
}

/*
 * helper function to relocate a tree block
 */
static int relocate_tree_block(struct btrfs_trans_handle *trans,
                                struct reloc_control *rc,
                                struct btrfs_backref_node *node,
                                struct btrfs_key *key,
                                struct btrfs_path *path)
{
        struct btrfs_root *root;
        int ret = 0;

        if (!node)
                return 0;

        /*
         * If we fail here we want to drop our backref_node because we are going
         * to start over and regenerate the tree for it.
         */
        ret = reserve_metadata_space(trans, rc, node);
        if (ret)
                goto out;

        BUG_ON(node->processed);
        root = select_one_root(node);
        if (root == ERR_PTR(-ENOENT)) {
                update_processed_blocks(rc, node);
                goto out;
        }

        if (root) {
                if (test_bit(BTRFS_ROOT_SHAREABLE, &root->state)) {
                        BUG_ON(node->new_bytenr);
                        BUG_ON(!list_empty(&node->list));
                        btrfs_record_root_in_trans(trans, root);
                        root = root->reloc_root;
                        node->new_bytenr = root->node->start;
                        btrfs_put_root(node->root);
                        node->root = btrfs_grab_root(root);
                        ASSERT(node->root);
                        list_add_tail(&node->list, &rc->backref_cache.changed);
                } else {
                        path->lowest_level = node->level;
                        ret = btrfs_search_slot(trans, root, key, path, 0, 1);
                        btrfs_release_path(path);
                        if (ret > 0)
                                ret = 0;
                }
                if (!ret)
                        update_processed_blocks(rc, node);
        } else {
                ret = do_relocation(trans, rc, node, key, path, 1);
        }
out:
        if (ret || node->level == 0 || node->cowonly)
                btrfs_backref_cleanup_node(&rc->backref_cache, node);
        return ret;
}

/*
 * relocate a list of blocks
 */
static noinline_for_stack
int relocate_tree_blocks(struct btrfs_trans_handle *trans,
                         struct reloc_control *rc, struct rb_root *blocks)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct btrfs_backref_node *node;
        struct btrfs_path *path;
        struct tree_block *block;
        struct tree_block *next;
        int ret;
        int err = 0;

        path = btrfs_alloc_path();
        if (!path) {
                err = -ENOMEM;
                goto out_free_blocks;
        }

        /* Kick in readahead for tree blocks with missing keys */
        rbtree_postorder_for_each_entry_safe(block, next, blocks, rb_node) {
                if (!block->key_ready)
                        btrfs_readahead_tree_block(fs_info, block->bytenr, 0, 0,
                                                   block->level);
        }

        /* Get first keys */
        rbtree_postorder_for_each_entry_safe(block, next, blocks, rb_node) {
                if (!block->key_ready) {
                        err = get_tree_block_key(fs_info, block);
                        if (err)
                                goto out_free_path;
                }
        }

        /* Do tree relocation */
        rbtree_postorder_for_each_entry_safe(block, next, blocks, rb_node) {
                node = build_backref_tree(rc, &block->key,
                                          block->level, block->bytenr);
                if (IS_ERR(node)) {
                        err = PTR_ERR(node);
                        goto out;
                }

                ret = relocate_tree_block(trans, rc, node, &block->key,
                                          path);
                if (ret < 0) {
                        err = ret;
                        break;
                }
        }
out:
        err = finish_pending_nodes(trans, rc, path, err);

out_free_path:
        btrfs_free_path(path);
out_free_blocks:
        free_block_list(blocks);
        return err;
}

static noinline_for_stack int prealloc_file_extent_cluster(
                                struct btrfs_inode *inode,
                                struct file_extent_cluster *cluster)
{
        u64 alloc_hint = 0;
        u64 start;
        u64 end;
        u64 offset = inode->index_cnt;
        u64 num_bytes;
        int nr;
        int ret = 0;
        u64 prealloc_start = cluster->start - offset;
        u64 prealloc_end = cluster->end - offset;
        u64 cur_offset = prealloc_start;

        BUG_ON(cluster->start != cluster->boundary[0]);
        ret = btrfs_alloc_data_chunk_ondemand(inode,
                                              prealloc_end + 1 - prealloc_start);
        if (ret)
                return ret;

        inode_lock(&inode->vfs_inode);
        for (nr = 0; nr < cluster->nr; nr++) {
                start = cluster->boundary[nr] - offset;
                if (nr + 1 < cluster->nr)
                        end = cluster->boundary[nr + 1] - 1 - offset;
                else
                        end = cluster->end - offset;

                lock_extent(&inode->io_tree, start, end);
                num_bytes = end + 1 - start;
                ret = btrfs_prealloc_file_range(&inode->vfs_inode, 0, start,
                                                num_bytes, num_bytes,
                                                end + 1, &alloc_hint);
                cur_offset = end + 1;
                unlock_extent(&inode->io_tree, start, end);
                if (ret)
                        break;
        }
        inode_unlock(&inode->vfs_inode);

        if (cur_offset < prealloc_end)
                btrfs_free_reserved_data_space_noquota(inode->root->fs_info,
                                               prealloc_end + 1 - cur_offset);
        return ret;
}

static noinline_for_stack
int setup_extent_mapping(struct inode *inode, u64 start, u64 end,
                         u64 block_start)
{
        struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
        struct extent_map *em;
        int ret = 0;

        em = alloc_extent_map();
        if (!em)
                return -ENOMEM;

        em->start = start;
        em->len = end + 1 - start;
        em->block_len = em->len;
        em->block_start = block_start;
        set_bit(EXTENT_FLAG_PINNED, &em->flags);

        lock_extent(&BTRFS_I(inode)->io_tree, start, end);
        while (1) {
                write_lock(&em_tree->lock);
                ret = add_extent_mapping(em_tree, em, 0);
                write_unlock(&em_tree->lock);
                if (ret != -EEXIST) {
                        free_extent_map(em);
                        break;
                }
                btrfs_drop_extent_cache(BTRFS_I(inode), start, end, 0);
        }
        unlock_extent(&BTRFS_I(inode)->io_tree, start, end);
        return ret;
}

/*
 * Allow error injection to test balance cancellation
 */
int btrfs_should_cancel_balance(struct btrfs_fs_info *fs_info)
{
        return atomic_read(&fs_info->balance_cancel_req) ||
                fatal_signal_pending(current);
}
ALLOW_ERROR_INJECTION(btrfs_should_cancel_balance, TRUE);

static int relocate_file_extent_cluster(struct inode *inode,
                                        struct file_extent_cluster *cluster)
{
        struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
        u64 page_start;
        u64 page_end;
        u64 offset = BTRFS_I(inode)->index_cnt;
        unsigned long index;
        unsigned long last_index;
        struct page *page;
        struct file_ra_state *ra;
        gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
        int nr = 0;
        int ret = 0;

        if (!cluster->nr)
                return 0;

        ra = kzalloc(sizeof(*ra), GFP_NOFS);
        if (!ra)
                return -ENOMEM;

        ret = prealloc_file_extent_cluster(BTRFS_I(inode), cluster);
        if (ret)
                goto out;

        file_ra_state_init(ra, inode->i_mapping);

        ret = setup_extent_mapping(inode, cluster->start - offset,
                                   cluster->end - offset, cluster->start);
        if (ret)
                goto out;

        index = (cluster->start - offset) >> PAGE_SHIFT;
        last_index = (cluster->end - offset) >> PAGE_SHIFT;
        while (index <= last_index) {
                ret = btrfs_delalloc_reserve_metadata(BTRFS_I(inode),
                                PAGE_SIZE);
                if (ret)
                        goto out;

                page = find_lock_page(inode->i_mapping, index);
                if (!page) {
                        page_cache_sync_readahead(inode->i_mapping,
                                                  ra, NULL, index,
                                                  last_index + 1 - index);
                        page = find_or_create_page(inode->i_mapping, index,
                                                   mask);
                        if (!page) {
                                btrfs_delalloc_release_metadata(BTRFS_I(inode),
                                                        PAGE_SIZE, true);
                                btrfs_delalloc_release_extents(BTRFS_I(inode),
                                                        PAGE_SIZE);
                                ret = -ENOMEM;
                                goto out;
                        }
                }

                if (PageReadahead(page)) {
                        page_cache_async_readahead(inode->i_mapping,
                                                   ra, NULL, page, index,
                                                   last_index + 1 - index);
                }

                if (!PageUptodate(page)) {
                        btrfs_readpage(NULL, page);
                        lock_page(page);
                        if (!PageUptodate(page)) {
                                unlock_page(page);
                                put_page(page);
                                btrfs_delalloc_release_metadata(BTRFS_I(inode),
                                                        PAGE_SIZE, true);
                                btrfs_delalloc_release_extents(BTRFS_I(inode),
                                                               PAGE_SIZE);
                                ret = -EIO;
                                goto out;
                        }
                }

                page_start = page_offset(page);
                page_end = page_start + PAGE_SIZE - 1;

                lock_extent(&BTRFS_I(inode)->io_tree, page_start, page_end);

                set_page_extent_mapped(page);

                if (nr < cluster->nr &&
                    page_start + offset == cluster->boundary[nr]) {
                        set_extent_bits(&BTRFS_I(inode)->io_tree,
                                        page_start, page_end,
                                        EXTENT_BOUNDARY);
                        nr++;
                }

                ret = btrfs_set_extent_delalloc(BTRFS_I(inode), page_start,
                                                page_end, 0, NULL);
                if (ret) {
                        unlock_page(page);
                        put_page(page);
                        btrfs_delalloc_release_metadata(BTRFS_I(inode),
                                                         PAGE_SIZE, true);
                        btrfs_delalloc_release_extents(BTRFS_I(inode),
                                                       PAGE_SIZE);

                        clear_extent_bits(&BTRFS_I(inode)->io_tree,
                                          page_start, page_end,
                                          EXTENT_LOCKED | EXTENT_BOUNDARY);
                        goto out;

                }
                set_page_dirty(page);

                unlock_extent(&BTRFS_I(inode)->io_tree,
                              page_start, page_end);
                unlock_page(page);
                put_page(page);

                index++;
                btrfs_delalloc_release_extents(BTRFS_I(inode), PAGE_SIZE);
                balance_dirty_pages_ratelimited(inode->i_mapping);
                btrfs_throttle(fs_info);
                if (btrfs_should_cancel_balance(fs_info)) {
                        ret = -ECANCELED;
                        goto out;
                }
        }
        WARN_ON(nr != cluster->nr);
out:
        kfree(ra);
        return ret;
}

static noinline_for_stack
int relocate_data_extent(struct inode *inode, struct btrfs_key *extent_key,
                         struct file_extent_cluster *cluster)
{
        int ret;

        if (cluster->nr > 0 && extent_key->objectid != cluster->end + 1) {
                ret = relocate_file_extent_cluster(inode, cluster);
                if (ret)
                        return ret;
                cluster->nr = 0;
        }

        if (!cluster->nr)
                cluster->start = extent_key->objectid;
        else
                BUG_ON(cluster->nr >= MAX_EXTENTS);
        cluster->end = extent_key->objectid + extent_key->offset - 1;
        cluster->boundary[cluster->nr] = extent_key->objectid;
        cluster->nr++;

        if (cluster->nr >= MAX_EXTENTS) {
                ret = relocate_file_extent_cluster(inode, cluster);
                if (ret)
                        return ret;
                cluster->nr = 0;
        }
        return 0;
}

/*
 * helper to add a tree block to the list.
 * the major work is getting the generation and level of the block
 */
static int add_tree_block(struct reloc_control *rc,
                          struct btrfs_key *extent_key,
                          struct btrfs_path *path,
                          struct rb_root *blocks)
{
        struct extent_buffer *eb;
        struct btrfs_extent_item *ei;
        struct btrfs_tree_block_info *bi;
        struct tree_block *block;
        struct rb_node *rb_node;
        u32 item_size;
        int level = -1;
        u64 generation;

        eb =  path->nodes[0];
        item_size = btrfs_item_size_nr(eb, path->slots[0]);

        if (extent_key->type == BTRFS_METADATA_ITEM_KEY ||
            item_size >= sizeof(*ei) + sizeof(*bi)) {
                ei = btrfs_item_ptr(eb, path->slots[0],
                                struct btrfs_extent_item);
                if (extent_key->type == BTRFS_EXTENT_ITEM_KEY) {
                        bi = (struct btrfs_tree_block_info *)(ei + 1);
                        level = btrfs_tree_block_level(eb, bi);
                } else {
                        level = (int)extent_key->offset;
                }
                generation = btrfs_extent_generation(eb, ei);
        } else if (unlikely(item_size == sizeof(struct btrfs_extent_item_v0))) {
                btrfs_print_v0_err(eb->fs_info);
                btrfs_handle_fs_error(eb->fs_info, -EINVAL, NULL);
                return -EINVAL;
        } else {
                BUG();
        }

        btrfs_release_path(path);

        BUG_ON(level == -1);

        block = kmalloc(sizeof(*block), GFP_NOFS);
        if (!block)
                return -ENOMEM;

        block->bytenr = extent_key->objectid;
        block->key.objectid = rc->extent_root->fs_info->nodesize;
        block->key.offset = generation;
        block->level = level;
        block->key_ready = 0;

        rb_node = rb_simple_insert(blocks, block->bytenr, &block->rb_node);
        if (rb_node)
                btrfs_backref_panic(rc->extent_root->fs_info, block->bytenr,
                                    -EEXIST);

        return 0;
}

/*
 * helper to add tree blocks for backref of type BTRFS_SHARED_DATA_REF_KEY
 */
static int __add_tree_block(struct reloc_control *rc,
                            u64 bytenr, u32 blocksize,
                            struct rb_root *blocks)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct btrfs_path *path;
        struct btrfs_key key;
        int ret;
        bool skinny = btrfs_fs_incompat(fs_info, SKINNY_METADATA);

        if (tree_block_processed(bytenr, rc))
                return 0;

        if (rb_simple_search(blocks, bytenr))
                return 0;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;
again:
        key.objectid = bytenr;
        if (skinny) {
                key.type = BTRFS_METADATA_ITEM_KEY;
                key.offset = (u64)-1;
        } else {
                key.type = BTRFS_EXTENT_ITEM_KEY;
                key.offset = blocksize;
        }

        path->search_commit_root = 1;
        path->skip_locking = 1;
        ret = btrfs_search_slot(NULL, rc->extent_root, &key, path, 0, 0);
        if (ret < 0)
                goto out;

        if (ret > 0 && skinny) {
                if (path->slots[0]) {
                        path->slots[0]--;
                        btrfs_item_key_to_cpu(path->nodes[0], &key,
                                              path->slots[0]);
                        if (key.objectid == bytenr &&
                            (key.type == BTRFS_METADATA_ITEM_KEY ||
                             (key.type == BTRFS_EXTENT_ITEM_KEY &&
                              key.offset == blocksize)))
                                ret = 0;
                }

                if (ret) {
                        skinny = false;
                        btrfs_release_path(path);
                        goto again;
                }
        }
        if (ret) {
                ASSERT(ret == 1);
                btrfs_print_leaf(path->nodes[0]);
                btrfs_err(fs_info,
             "tree block extent item (%llu) is not found in extent tree",
                     bytenr);
                WARN_ON(1);
                ret = -EINVAL;
                goto out;
        }

        ret = add_tree_block(rc, &key, path, blocks);
out:
        btrfs_free_path(path);
        return ret;
}

static int delete_block_group_cache(struct btrfs_fs_info *fs_info,
                                    struct btrfs_block_group *block_group,
                                    struct inode *inode,
                                    u64 ino)
{
        struct btrfs_root *root = fs_info->tree_root;
        struct btrfs_trans_handle *trans;
        int ret = 0;

        if (inode)
                goto truncate;

        inode = btrfs_iget(fs_info->sb, ino, root);
        if (IS_ERR(inode))
                return -ENOENT;

truncate:
        ret = btrfs_check_trunc_cache_free_space(fs_info,
                                                 &fs_info->global_block_rsv);
        if (ret)
                goto out;

        trans = btrfs_join_transaction(root);
        if (IS_ERR(trans)) {
                ret = PTR_ERR(trans);
                goto out;
        }

        ret = btrfs_truncate_free_space_cache(trans, block_group, inode);

        btrfs_end_transaction(trans);
        btrfs_btree_balance_dirty(fs_info);
out:
        iput(inode);
        return ret;
}

/*
 * Locate the free space cache EXTENT_DATA in root tree leaf and delete the
 * cache inode, to avoid free space cache data extent blocking data relocation.
 */
static int delete_v1_space_cache(struct extent_buffer *leaf,
                                 struct btrfs_block_group *block_group,
                                 u64 data_bytenr)
{
        u64 space_cache_ino;
        struct btrfs_file_extent_item *ei;
        struct btrfs_key key;
        bool found = false;
        int i;
        int ret;

        if (btrfs_header_owner(leaf) != BTRFS_ROOT_TREE_OBJECTID)
                return 0;

        for (i = 0; i < btrfs_header_nritems(leaf); i++) {
                u8 type;

                btrfs_item_key_to_cpu(leaf, &key, i);
                if (key.type != BTRFS_EXTENT_DATA_KEY)
                        continue;
                ei = btrfs_item_ptr(leaf, i, struct btrfs_file_extent_item);
                type = btrfs_file_extent_type(leaf, ei);

                if ((type == BTRFS_FILE_EXTENT_REG ||
                     type == BTRFS_FILE_EXTENT_PREALLOC) &&
                    btrfs_file_extent_disk_bytenr(leaf, ei) == data_bytenr) {
                        found = true;
                        space_cache_ino = key.objectid;
                        break;
                }
        }
        if (!found)
                return -ENOENT;
        ret = delete_block_group_cache(leaf->fs_info, block_group, NULL,
                                        space_cache_ino);
        return ret;
}

/*
 * helper to find all tree blocks that reference a given data extent
 */
static noinline_for_stack
int add_data_references(struct reloc_control *rc,
                        struct btrfs_key *extent_key,
                        struct btrfs_path *path,
                        struct rb_root *blocks)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct ulist *leaves = NULL;
        struct ulist_iterator leaf_uiter;
        struct ulist_node *ref_node = NULL;
        const u32 blocksize = fs_info->nodesize;
        int ret = 0;

        btrfs_release_path(path);
        ret = btrfs_find_all_leafs(NULL, fs_info, extent_key->objectid,
                                   0, &leaves, NULL, true);
        if (ret < 0)
                return ret;

        ULIST_ITER_INIT(&leaf_uiter);
        while ((ref_node = ulist_next(leaves, &leaf_uiter))) {
                struct extent_buffer *eb;

                eb = read_tree_block(fs_info, ref_node->val, 0, 0, 0, NULL);
                if (IS_ERR(eb)) {
                        ret = PTR_ERR(eb);
                        break;
                }
                ret = delete_v1_space_cache(eb, rc->block_group,
                                            extent_key->objectid);
                free_extent_buffer(eb);
                if (ret < 0)
                        break;
                ret = __add_tree_block(rc, ref_node->val, blocksize, blocks);
                if (ret < 0)
                        break;
        }
        if (ret < 0)
                free_block_list(blocks);
        ulist_free(leaves);
        return ret;
}

/*
 * helper to find next unprocessed extent
 */
static noinline_for_stack
int find_next_extent(struct reloc_control *rc, struct btrfs_path *path,
                     struct btrfs_key *extent_key)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct btrfs_key key;
        struct extent_buffer *leaf;
        u64 start, end, last;
        int ret;

        last = rc->block_group->start + rc->block_group->length;
        while (1) {
                cond_resched();
                if (rc->search_start >= last) {
                        ret = 1;
                        break;
                }

                key.objectid = rc->search_start;
                key.type = BTRFS_EXTENT_ITEM_KEY;
                key.offset = 0;

                path->search_commit_root = 1;
                path->skip_locking = 1;
                ret = btrfs_search_slot(NULL, rc->extent_root, &key, path,
                                        0, 0);
                if (ret < 0)
                        break;
next:
                leaf = path->nodes[0];
                if (path->slots[0] >= btrfs_header_nritems(leaf)) {
                        ret = btrfs_next_leaf(rc->extent_root, path);
                        if (ret != 0)
                                break;
                        leaf = path->nodes[0];
                }

                btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
                if (key.objectid >= last) {
                        ret = 1;
                        break;
                }

                if (key.type != BTRFS_EXTENT_ITEM_KEY &&
                    key.type != BTRFS_METADATA_ITEM_KEY) {
                        path->slots[0]++;
                        goto next;
                }

                if (key.type == BTRFS_EXTENT_ITEM_KEY &&
                    key.objectid + key.offset <= rc->search_start) {
                        path->slots[0]++;
                        goto next;
                }

                if (key.type == BTRFS_METADATA_ITEM_KEY &&
                    key.objectid + fs_info->nodesize <=
                    rc->search_start) {
                        path->slots[0]++;
                        goto next;
                }

                ret = find_first_extent_bit(&rc->processed_blocks,
                                            key.objectid, &start, &end,
                                            EXTENT_DIRTY, NULL);

                if (ret == 0 && start <= key.objectid) {
                        btrfs_release_path(path);
                        rc->search_start = end + 1;
                } else {
                        if (key.type == BTRFS_EXTENT_ITEM_KEY)
                                rc->search_start = key.objectid + key.offset;
                        else
                                rc->search_start = key.objectid +
                                        fs_info->nodesize;
                        memcpy(extent_key, &key, sizeof(key));
                        return 0;
                }
        }
        btrfs_release_path(path);
        return ret;
}

static void set_reloc_control(struct reloc_control *rc)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;

        mutex_lock(&fs_info->reloc_mutex);
        fs_info->reloc_ctl = rc;
        mutex_unlock(&fs_info->reloc_mutex);
}

static void unset_reloc_control(struct reloc_control *rc)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;

        mutex_lock(&fs_info->reloc_mutex);
        fs_info->reloc_ctl = NULL;
        mutex_unlock(&fs_info->reloc_mutex);
}

static int check_extent_flags(u64 flags)
{
        if ((flags & BTRFS_EXTENT_FLAG_DATA) &&
            (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK))
                return 1;
        if (!(flags & BTRFS_EXTENT_FLAG_DATA) &&
            !(flags & BTRFS_EXTENT_FLAG_TREE_BLOCK))
                return 1;
        if ((flags & BTRFS_EXTENT_FLAG_DATA) &&
            (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF))
                return 1;
        return 0;
}

static noinline_for_stack
int prepare_to_relocate(struct reloc_control *rc)
{
        struct btrfs_trans_handle *trans;
        int ret;

        rc->block_rsv = btrfs_alloc_block_rsv(rc->extent_root->fs_info,
                                              BTRFS_BLOCK_RSV_TEMP);
        if (!rc->block_rsv)
                return -ENOMEM;

        memset(&rc->cluster, 0, sizeof(rc->cluster));
        rc->search_start = rc->block_group->start;
        rc->extents_found = 0;
        rc->nodes_relocated = 0;
        rc->merging_rsv_size = 0;
        rc->reserved_bytes = 0;
        rc->block_rsv->size = rc->extent_root->fs_info->nodesize *
                              RELOCATION_RESERVED_NODES;
        ret = btrfs_block_rsv_refill(rc->extent_root,
                                     rc->block_rsv, rc->block_rsv->size,
                                     BTRFS_RESERVE_FLUSH_ALL);
        if (ret)
                return ret;

        rc->create_reloc_tree = 1;
        set_reloc_control(rc);

        trans = btrfs_join_transaction(rc->extent_root);
        if (IS_ERR(trans)) {
                unset_reloc_control(rc);
                /*
                 * extent tree is not a ref_cow tree and has no reloc_root to
                 * cleanup.  And callers are responsible to free the above
                 * block rsv.
                 */
                return PTR_ERR(trans);
        }
        btrfs_commit_transaction(trans);
        return 0;
}

static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
{
        struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
        struct rb_root blocks = RB_ROOT;
        struct btrfs_key key;
        struct btrfs_trans_handle *trans = NULL;
        struct btrfs_path *path;
        struct btrfs_extent_item *ei;
        u64 flags;
        u32 item_size;
        int ret;
        int err = 0;
        int progress = 0;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;
        path->reada = READA_FORWARD;

        ret = prepare_to_relocate(rc);
        if (ret) {
                err = ret;
                goto out_free;
        }

        while (1) {
                rc->reserved_bytes = 0;
                ret = btrfs_block_rsv_refill(rc->extent_root,
                                        rc->block_rsv, rc->block_rsv->size,
                                        BTRFS_RESERVE_FLUSH_ALL);
                if (ret) {
                        err = ret;
                        break;
                }
                progress++;
                trans = btrfs_start_transaction(rc->extent_root, 0);
                if (IS_ERR(trans)) {
                        err = PTR_ERR(trans);
                        trans = NULL;
                        break;
                }
restart:
                if (update_backref_cache(trans, &rc->backref_cache)) {
                        btrfs_end_transaction(trans);
                        trans = NULL;
                        continue;
                }

                ret = find_next_extent(rc, path, &key);
                if (ret < 0)
                        err = ret;
                if (ret != 0)
                        break;

                rc->extents_found++;

                ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
                                    struct btrfs_extent_item);
                item_size = btrfs_item_size_nr(path->nodes[0], path->slots[0]);
                if (item_size >= sizeof(*ei)) {
                        flags = btrfs_extent_flags(path->nodes[0], ei);
                        ret = check_extent_flags(flags);
                        BUG_ON(ret);
                } else if (unlikely(item_size == sizeof(struct btrfs_extent_item_v0))) {
                        err = -EINVAL;
                        btrfs_print_v0_err(trans->fs_info);
                        btrfs_abort_transaction(trans, err);
                        break;
                } else {
                        BUG();
                }

                if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                        ret = add_tree_block(rc, &key, path, &blocks);
                } else if (rc->stage == UPDATE_DATA_PTRS &&
                           (flags & BTRFS_EXTENT_FLAG_DATA)) {
                        ret = add_data_references(rc, &key, path, &blocks);
                } else {
                        btrfs_release_path(path);
                        ret = 0;
                }
                if (ret < 0) {
                        err = ret;
                        break;
                }

                if (!RB_EMPTY_ROOT(&blocks)) {
                        ret = relocate_tree_blocks(trans, rc, &blocks);
                        if (ret < 0) {
                                if (ret != -EAGAIN) {
                                        err = ret;
                                        break;
                                }
                                rc->extents_found--;
                                rc->search_start = key.objectid;
                        }
                }

                btrfs_end_transaction_throttle(trans);
                btrfs_btree_balance_dirty(fs_info);
                trans = NULL;

                if (rc->stage == MOVE_DATA_EXTENTS &&
                    (flags & BTRFS_EXTENT_FLAG_DATA)) {
                        rc->found_file_extent = 1;
                        ret = relocate_data_extent(rc->data_inode,
                                                   &key, &rc->cluster);
                        if (ret < 0) {
                                err = ret;
                                break;
                        }
                }
                if (btrfs_should_cancel_balance(fs_info)) {
                        err = -ECANCELED;
                        break;
                }
        }
        if (trans && progress && err == -ENOSPC) {
                ret = btrfs_force_chunk_alloc(trans, rc->block_group->flags);
                if (ret == 1) {
                        err = 0;
                        progress = 0;
                        goto restart;
                }
        }

        btrfs_release_path(path);
        clear_extent_bits(&rc->processed_blocks, 0, (u64)-1, EXTENT_DIRTY);

        if (trans) {
                btrfs_end_transaction_throttle(trans);
                btrfs_btree_balance_dirty(fs_info);
        }

        if (!err) {
                ret = relocate_file_extent_cluster(rc->data_inode,
                                                   &rc->cluster);
                if (ret < 0)
                        err = ret;
        }

        rc->create_reloc_tree = 0;
        set_reloc_control(rc);

        btrfs_backref_release_cache(&rc->backref_cache);
        btrfs_block_rsv_release(fs_info, rc->block_rsv, (u64)-1, NULL);

        /*
         * Even in the case when the relocation is cancelled, we should all go
         * through prepare_to_merge() and merge_reloc_roots().
         *
         * For error (including cancelled balance), prepare_to_merge() will
         * mark all reloc trees orphan, then queue them for cleanup in
         * merge_reloc_roots()
         */
        err = prepare_to_merge(rc, err);

        merge_reloc_roots(rc);

        rc->merge_reloc_tree = 0;
        unset_reloc_control(rc);
        btrfs_block_rsv_release(fs_info, rc->block_rsv, (u64)-1, NULL);

        /* get rid of pinned extents */
        trans = btrfs_join_transaction(rc->extent_root);
        if (IS_ERR(trans)) {
                err = PTR_ERR(trans);
                goto out_free;
        }
        btrfs_commit_transaction(trans);
out_free:
        ret = clean_dirty_subvols(rc);
        if (ret < 0 && !err)
                err = ret;
        btrfs_free_block_rsv(fs_info, rc->block_rsv);
        btrfs_free_path(path);
        return err;
}

static int __insert_orphan_inode(struct btrfs_trans_handle *trans,
                                 struct btrfs_root *root, u64 objectid)
{
        struct btrfs_path *path;
        struct btrfs_inode_item *item;
        struct extent_buffer *leaf;
        int ret;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;

        ret = btrfs_insert_empty_inode(trans, root, path, objectid);
        if (ret)
                goto out;

        leaf = path->nodes[0];
        item = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_inode_item);
        memzero_extent_buffer(leaf, (unsigned long)item, sizeof(*item));
        btrfs_set_inode_generation(leaf, item, 1);
        btrfs_set_inode_size(leaf, item, 0);
        btrfs_set_inode_mode(leaf, item, S_IFREG | 0600);
        btrfs_set_inode_flags(leaf, item, BTRFS_INODE_NOCOMPRESS |
                                          BTRFS_INODE_PREALLOC);
        btrfs_mark_buffer_dirty(leaf);
out:
        btrfs_free_path(path);
        return ret;
}

/*
 * helper to create inode for data relocation.
 * the inode is in data relocation tree and its link count is 0
 */
static noinline_for_stack
struct inode *create_reloc_inode(struct btrfs_fs_info *fs_info,
                                 struct btrfs_block_group *group)
{
        struct inode *inode = NULL;
        struct btrfs_trans_handle *trans;
        struct btrfs_root *root;
        u64 objectid;
        int err = 0;

        root = btrfs_grab_root(fs_info->data_reloc_root);
        trans = btrfs_start_transaction(root, 6);
        if (IS_ERR(trans)) {
                btrfs_put_root(root);
                return ERR_CAST(trans);
        }

        err = btrfs_find_free_objectid(root, &objectid);
        if (err)
                goto out;

        err = __insert_orphan_inode(trans, root, objectid);
        BUG_ON(err);

        inode = btrfs_iget(fs_info->sb, objectid, root);
        BUG_ON(IS_ERR(inode));
        BTRFS_I(inode)->index_cnt = group->start;

        err = btrfs_orphan_add(trans, BTRFS_I(inode));
out:
        btrfs_put_root(root);
        btrfs_end_transaction(trans);
        btrfs_btree_balance_dirty(fs_info);
        if (err) {
                if (inode)
                        iput(inode);
                inode = ERR_PTR(err);
        }
        return inode;
}

static struct reloc_control *alloc_reloc_control(struct btrfs_fs_info *fs_info)
{
        struct reloc_control *rc;

        rc = kzalloc(sizeof(*rc), GFP_NOFS);
        if (!rc)
                return NULL;

        INIT_LIST_HEAD(&rc->reloc_roots);
        INIT_LIST_HEAD(&rc->dirty_subvol_roots);
        btrfs_backref_init_cache(fs_info, &rc->backref_cache, 1);
        mapping_tree_init(&rc->reloc_root_tree);
        extent_io_tree_init(fs_info, &rc->processed_blocks,
                            IO_TREE_RELOC_BLOCKS, NULL);
        return rc;
}

static void free_reloc_control(struct reloc_control *rc)
{
        struct mapping_node *node, *tmp;

        free_reloc_roots(&rc->reloc_roots);
        rbtree_postorder_for_each_entry_safe(node, tmp,
                        &rc->reloc_root_tree.rb_root, rb_node)
                kfree(node);

        kfree(rc);
}

/*
 * Print the block group being relocated
 */
static void describe_relocation(struct btrfs_fs_info *fs_info,
                                struct btrfs_block_group *block_group)
{
        char buf[128] = {'\0'};

        btrfs_describe_block_groups(block_group->flags, buf, sizeof(buf));

        btrfs_info(fs_info,
                   "relocating block group %llu flags %s",
                   block_group->start, buf);
}

static const char *stage_to_string(int stage)
{
        if (stage == MOVE_DATA_EXTENTS)
                return "move data extents";
        if (stage == UPDATE_DATA_PTRS)
                return "update data pointers";
        return "unknown";
}

/*
 * function to relocate all extents in a block group.
 */
int btrfs_relocate_block_group(struct btrfs_fs_info *fs_info, u64 group_start)
{
        struct btrfs_block_group *bg;
        struct btrfs_root *extent_root = fs_info->extent_root;
        struct reloc_control *rc;
        struct inode *inode;
        struct btrfs_path *path;
        int ret;
        int rw = 0;
        int err = 0;

        bg = btrfs_lookup_block_group(fs_info, group_start);
        if (!bg)
                return -ENOENT;

        if (btrfs_pinned_by_swapfile(fs_info, bg)) {
                btrfs_put_block_group(bg);
                return -ETXTBSY;
        }

        rc = alloc_reloc_control(fs_info);
        if (!rc) {
                btrfs_put_block_group(bg);
                return -ENOMEM;
        }

        rc->extent_root = extent_root;
        rc->block_group = bg;

        ret = btrfs_inc_block_group_ro(rc->block_group, true);
        if (ret) {
                err = ret;
                goto out;
        }
        rw = 1;

        path = btrfs_alloc_path();
        if (!path) {
                err = -ENOMEM;
                goto out;
        }

        inode = lookup_free_space_inode(rc->block_group, path);
        btrfs_free_path(path);

        if (!IS_ERR(inode))
                ret = delete_block_group_cache(fs_info, rc->block_group, inode, 0);
        else
                ret = PTR_ERR(inode);

        if (ret && ret != -ENOENT) {
                err = ret;
                goto out;
        }

        rc->data_inode = create_reloc_inode(fs_info, rc->block_group);
        if (IS_ERR(rc->data_inode)) {
                err = PTR_ERR(rc->data_inode);
                rc->data_inode = NULL;
                goto out;
        }

        describe_relocation(fs_info, rc->block_group);

        btrfs_wait_block_group_reservations(rc->block_group);
        btrfs_wait_nocow_writers(rc->block_group);
        btrfs_wait_ordered_roots(fs_info, U64_MAX,
                                 rc->block_group->start,
                                 rc->block_group->length);

        while (1) {
                int finishes_stage;

                mutex_lock(&fs_info->cleaner_mutex);
                ret = relocate_block_group(rc);
                mutex_unlock(&fs_info->cleaner_mutex);
                if (ret < 0)
                        err = ret;

                finishes_stage = rc->stage;
                /*
                 * We may have gotten ENOSPC after we already dirtied some
                 * extents.  If writeout happens while we're relocating a
                 * different block group we could end up hitting the
                 * BUG_ON(rc->stage == UPDATE_DATA_PTRS) in
                 * btrfs_reloc_cow_block.  Make sure we write everything out
                 * properly so we don't trip over this problem, and then break
                 * out of the loop if we hit an error.
                 */
                if (rc->stage == MOVE_DATA_EXTENTS && rc->found_file_extent) {
                        ret = btrfs_wait_ordered_range(rc->data_inode, 0,
                                                       (u64)-1);
                        if (ret)
                                err = ret;
                        invalidate_mapping_pages(rc->data_inode->i_mapping,
                                                 0, -1);
                        rc->stage = UPDATE_DATA_PTRS;
                }

                if (err < 0)
                        goto out;

                if (rc->extents_found == 0)
                        break;

                btrfs_info(fs_info, "found %llu extents, stage: %s",
                           rc->extents_found, stage_to_string(finishes_stage));
        }

        WARN_ON(rc->block_group->pinned > 0);
        WARN_ON(rc->block_group->reserved > 0);
        WARN_ON(rc->block_group->used > 0);
out:
        if (err && rw)
                btrfs_dec_block_group_ro(rc->block_group);
        iput(rc->data_inode);
        btrfs_put_block_group(rc->block_group);
        free_reloc_control(rc);
        return err;
}

static noinline_for_stack int mark_garbage_root(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_trans_handle *trans;
        int ret, err;

        trans = btrfs_start_transaction(fs_info->tree_root, 0);
        if (IS_ERR(trans))
                return PTR_ERR(trans);

        memset(&root->root_item.drop_progress, 0,
                sizeof(root->root_item.drop_progress));
        btrfs_set_root_drop_level(&root->root_item, 0);
        btrfs_set_root_refs(&root->root_item, 0);
        ret = btrfs_update_root(trans, fs_info->tree_root,
                                &root->root_key, &root->root_item);

        err = btrfs_end_transaction(trans);
        if (err)
                return err;
        return ret;
}

/*
 * recover relocation interrupted by system crash.
 *
 * this function resumes merging reloc trees with corresponding fs trees.
 * this is important for keeping the sharing of tree blocks
 */
int btrfs_recover_relocation(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        LIST_HEAD(reloc_roots);
        struct btrfs_key key;
        struct btrfs_root *fs_root;
        struct btrfs_root *reloc_root;
        struct btrfs_path *path;
        struct extent_buffer *leaf;
        struct reloc_control *rc = NULL;
        struct btrfs_trans_handle *trans;
        int ret;
        int err = 0;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;
        path->reada = READA_BACK;

        key.objectid = BTRFS_TREE_RELOC_OBJECTID;
        key.type = BTRFS_ROOT_ITEM_KEY;
        key.offset = (u64)-1;

        while (1) {
                ret = btrfs_search_slot(NULL, fs_info->tree_root, &key,
                                        path, 0, 0);
                if (ret < 0) {
                        err = ret;
                        goto out;
                }
                if (ret > 0) {
                        if (path->slots[0] == 0)
                                break;
                        path->slots[0]--;
                }
                leaf = path->nodes[0];
                btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
                btrfs_release_path(path);

                if (key.objectid != BTRFS_TREE_RELOC_OBJECTID ||
                    key.type != BTRFS_ROOT_ITEM_KEY)
                        break;

                reloc_root = btrfs_read_tree_root(root, &key);
                if (IS_ERR(reloc_root)) {
                        err = PTR_ERR(reloc_root);
                        goto out;
                }

                set_bit(BTRFS_ROOT_SHAREABLE, &reloc_root->state);
                list_add(&reloc_root->root_list, &reloc_roots);

                if (btrfs_root_refs(&reloc_root->root_item) > 0) {
                        fs_root = btrfs_get_fs_root(fs_info,
                                        reloc_root->root_key.offset, false);
                        if (IS_ERR(fs_root)) {
                                ret = PTR_ERR(fs_root);
                                if (ret != -ENOENT) {
                                        err = ret;
                                        goto out;
                                }
                                ret = mark_garbage_root(reloc_root);
                                if (ret < 0) {
                                        err = ret;
                                        goto out;
                                }
                        } else {
                                btrfs_put_root(fs_root);
                        }
                }

                if (key.offset == 0)
                        break;

                key.offset--;
        }
        btrfs_release_path(path);

        if (list_empty(&reloc_roots))
                goto out;

        rc = alloc_reloc_control(fs_info);
        if (!rc) {
                err = -ENOMEM;
                goto out;
        }

        rc->extent_root = fs_info->extent_root;

        set_reloc_control(rc);

        trans = btrfs_join_transaction(rc->extent_root);
        if (IS_ERR(trans)) {
                err = PTR_ERR(trans);
                goto out_unset;
        }

        rc->merge_reloc_tree = 1;

        while (!list_empty(&reloc_roots)) {
                reloc_root = list_entry(reloc_roots.next,
                                        struct btrfs_root, root_list);
                list_del(&reloc_root->root_list);

                if (btrfs_root_refs(&reloc_root->root_item) == 0) {
                        list_add_tail(&reloc_root->root_list,
                                      &rc->reloc_roots);
                        continue;
                }

                fs_root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
                                            false);
                if (IS_ERR(fs_root)) {
                        err = PTR_ERR(fs_root);
                        list_add_tail(&reloc_root->root_list, &reloc_roots);
                        btrfs_end_transaction(trans);
                        goto out_unset;
                }

                err = __add_reloc_root(reloc_root);
                BUG_ON(err < 0); /* -ENOMEM or logic error */
                fs_root->reloc_root = btrfs_grab_root(reloc_root);
                btrfs_put_root(fs_root);
        }

        err = btrfs_commit_transaction(trans);
        if (err)
                goto out_unset;

        merge_reloc_roots(rc);

        unset_reloc_control(rc);

        trans = btrfs_join_transaction(rc->extent_root);
        if (IS_ERR(trans)) {
                err = PTR_ERR(trans);
                goto out_clean;
        }
        err = btrfs_commit_transaction(trans);
out_clean:
        ret = clean_dirty_subvols(rc);
        if (ret < 0 && !err)
                err = ret;
out_unset:
        unset_reloc_control(rc);
        free_reloc_control(rc);
out:
        free_reloc_roots(&reloc_roots);

        btrfs_free_path(path);

        if (err == 0) {
                /* cleanup orphan inode in data relocation tree */
                fs_root = btrfs_grab_root(fs_info->data_reloc_root);
                ASSERT(fs_root);
                err = btrfs_orphan_cleanup(fs_root);
                btrfs_put_root(fs_root);
        }
        return err;
}

/*
 * helper to add ordered checksum for data relocation.
 *
 * cloning checksum properly handles the nodatasum extents.
 * it also saves CPU time to re-calculate the checksum.
 */
int btrfs_reloc_clone_csums(struct btrfs_inode *inode, u64 file_pos, u64 len)
{
        struct btrfs_fs_info *fs_info = inode->root->fs_info;
        struct btrfs_ordered_sum *sums;
        struct btrfs_ordered_extent *ordered;
        int ret;
        u64 disk_bytenr;
        u64 new_bytenr;
        LIST_HEAD(list);

        ordered = btrfs_lookup_ordered_extent(inode, file_pos);
        BUG_ON(ordered->file_offset != file_pos || ordered->num_bytes != len);

        disk_bytenr = file_pos + inode->index_cnt;
        ret = btrfs_lookup_csums_range(fs_info->csum_root, disk_bytenr,
                                       disk_bytenr + len - 1, &list, 0);
        if (ret)
                goto out;

        while (!list_empty(&list)) {
                sums = list_entry(list.next, struct btrfs_ordered_sum, list);
                list_del_init(&sums->list);

                /*
                 * We need to offset the new_bytenr based on where the csum is.
                 * We need to do this because we will read in entire prealloc
                 * extents but we may have written to say the middle of the
                 * prealloc extent, so we need to make sure the csum goes with
                 * the right disk offset.
                 *
                 * We can do this because the data reloc inode refers strictly
                 * to the on disk bytes, so we don't have to worry about
                 * disk_len vs real len like with real inodes since it's all
                 * disk length.
                 */
                new_bytenr = ordered->disk_bytenr + sums->bytenr - disk_bytenr;
                sums->bytenr = new_bytenr;

                btrfs_add_ordered_sum(ordered, sums);
        }
out:
        btrfs_put_ordered_extent(ordered);
        return ret;
}

int btrfs_reloc_cow_block(struct btrfs_trans_handle *trans,
                          struct btrfs_root *root, struct extent_buffer *buf,
                          struct extent_buffer *cow)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct reloc_control *rc;
        struct btrfs_backref_node *node;
        int first_cow = 0;
        int level;
        int ret = 0;

        rc = fs_info->reloc_ctl;
        if (!rc)
                return 0;

        BUG_ON(rc->stage == UPDATE_DATA_PTRS &&
               root->root_key.objectid == BTRFS_DATA_RELOC_TREE_OBJECTID);

        level = btrfs_header_level(buf);
        if (btrfs_header_generation(buf) <=
            btrfs_root_last_snapshot(&root->root_item))
                first_cow = 1;

        if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID &&
            rc->create_reloc_tree) {
                WARN_ON(!first_cow && level == 0);

                node = rc->backref_cache.path[level];
                BUG_ON(node->bytenr != buf->start &&
                       node->new_bytenr != buf->start);

                btrfs_backref_drop_node_buffer(node);
                atomic_inc(&cow->refs);
                node->eb = cow;
                node->new_bytenr = cow->start;

                if (!node->pending) {
                        list_move_tail(&node->list,
                                       &rc->backref_cache.pending[level]);
                        node->pending = 1;
                }

                if (first_cow)
                        mark_block_processed(rc, node);

                if (first_cow && level > 0)
                        rc->nodes_relocated += buf->len;
        }

        if (level == 0 && first_cow && rc->stage == UPDATE_DATA_PTRS)
                ret = replace_file_extents(trans, rc, root, cow);
        return ret;
}

/*
 * called before creating snapshot. it calculates metadata reservation
 * required for relocating tree blocks in the snapshot
 */
void btrfs_reloc_pre_snapshot(struct btrfs_pending_snapshot *pending,
                              u64 *bytes_to_reserve)
{
        struct btrfs_root *root = pending->root;
        struct reloc_control *rc = root->fs_info->reloc_ctl;

        if (!rc || !have_reloc_root(root))
                return;

        if (!rc->merge_reloc_tree)
                return;

        root = root->reloc_root;
        BUG_ON(btrfs_root_refs(&root->root_item) == 0);
        /*
         * relocation is in the stage of merging trees. the space
         * used by merging a reloc tree is twice the size of
         * relocated tree nodes in the worst case. half for cowing
         * the reloc tree, half for cowing the fs tree. the space
         * used by cowing the reloc tree will be freed after the
         * tree is dropped. if we create snapshot, cowing the fs
         * tree may use more space than it frees. so we need
         * reserve extra space.
         */
        *bytes_to_reserve += rc->nodes_relocated;
}

/*
 * called after snapshot is created. migrate block reservation
 * and create reloc root for the newly created snapshot
 *
 * This is similar to btrfs_init_reloc_root(), we come out of here with two
 * references held on the reloc_root, one for root->reloc_root and one for
 * rc->reloc_roots.
 */
int btrfs_reloc_post_snapshot(struct btrfs_trans_handle *trans,
                               struct btrfs_pending_snapshot *pending)
{
        struct btrfs_root *root = pending->root;
        struct btrfs_root *reloc_root;
        struct btrfs_root *new_root;
        struct reloc_control *rc = root->fs_info->reloc_ctl;
        int ret;

        if (!rc || !have_reloc_root(root))
                return 0;

        rc = root->fs_info->reloc_ctl;
        rc->merging_rsv_size += rc->nodes_relocated;

        if (rc->merge_reloc_tree) {
                ret = btrfs_block_rsv_migrate(&pending->block_rsv,
                                              rc->block_rsv,
                                              rc->nodes_relocated, true);
                if (ret)
                        return ret;
        }

        new_root = pending->snap;
        reloc_root = create_reloc_root(trans, root->reloc_root,
                                       new_root->root_key.objectid);
        if (IS_ERR(reloc_root))
                return PTR_ERR(reloc_root);

        ret = __add_reloc_root(reloc_root);
        BUG_ON(ret < 0);
        new_root->reloc_root = btrfs_grab_root(reloc_root);

        if (rc->create_reloc_tree)
                ret = clone_backref_node(trans, rc, root, reloc_root);
        return ret;
}