WIP FPC-III support

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

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2011 STRATO.  All rights reserved.
 */

#include <linux/sched.h>
#include <linux/pagemap.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/slab.h>
#include <linux/workqueue.h>
#include "ctree.h"
#include "volumes.h"
#include "disk-io.h"
#include "transaction.h"
#include "dev-replace.h"
#include "block-group.h"

#undef DEBUG

/*
 * This is the implementation for the generic read ahead framework.
 *
 * To trigger a readahead, btrfs_reada_add must be called. It will start
 * a read ahead for the given range [start, end) on tree root. The returned
 * handle can either be used to wait on the readahead to finish
 * (btrfs_reada_wait), or to send it to the background (btrfs_reada_detach).
 *
 * The read ahead works as follows:
 * On btrfs_reada_add, the root of the tree is inserted into a radix_tree.
 * reada_start_machine will then search for extents to prefetch and trigger
 * some reads. When a read finishes for a node, all contained node/leaf
 * pointers that lie in the given range will also be enqueued. The reads will
 * be triggered in sequential order, thus giving a big win over a naive
 * enumeration. It will also make use of multi-device layouts. Each disk
 * will have its on read pointer and all disks will by utilized in parallel.
 * Also will no two disks read both sides of a mirror simultaneously, as this
 * would waste seeking capacity. Instead both disks will read different parts
 * of the filesystem.
 * Any number of readaheads can be started in parallel. The read order will be
 * determined globally, i.e. 2 parallel readaheads will normally finish faster
 * than the 2 started one after another.
 */

#define MAX_IN_FLIGHT 6

struct reada_extctl {
        struct list_head        list;
        struct reada_control    *rc;
        u64                     generation;
};

struct reada_extent {
        u64                     logical;
        u64                     owner_root;
        struct btrfs_key        top;
        struct list_head        extctl;
        int                     refcnt;
        spinlock_t              lock;
        struct reada_zone       *zones[BTRFS_MAX_MIRRORS];
        int                     nzones;
        int                     scheduled;
        int                     level;
};

struct reada_zone {
        u64                     start;
        u64                     end;
        u64                     elems;
        struct list_head        list;
        spinlock_t              lock;
        int                     locked;
        struct btrfs_device     *device;
        struct btrfs_device     *devs[BTRFS_MAX_MIRRORS]; /* full list, incl
                                                           * self */
        int                     ndevs;
        struct kref             refcnt;
};

struct reada_machine_work {
        struct btrfs_work       work;
        struct btrfs_fs_info    *fs_info;
};

static void reada_extent_put(struct btrfs_fs_info *, struct reada_extent *);
static void reada_control_release(struct kref *kref);
static void reada_zone_release(struct kref *kref);
static void reada_start_machine(struct btrfs_fs_info *fs_info);
static void __reada_start_machine(struct btrfs_fs_info *fs_info);

static int reada_add_block(struct reada_control *rc, u64 logical,
                           struct btrfs_key *top, u64 owner_root,
                           u64 generation, int level);

/* recurses */
/* in case of err, eb might be NULL */
static void __readahead_hook(struct btrfs_fs_info *fs_info,
                             struct reada_extent *re, struct extent_buffer *eb,
                             int err)
{
        int nritems;
        int i;
        u64 bytenr;
        u64 generation;
        struct list_head list;

        spin_lock(&re->lock);
        /*
         * just take the full list from the extent. afterwards we
         * don't need the lock anymore
         */
        list_replace_init(&re->extctl, &list);
        re->scheduled = 0;
        spin_unlock(&re->lock);

        /*
         * this is the error case, the extent buffer has not been
         * read correctly. We won't access anything from it and
         * just cleanup our data structures. Effectively this will
         * cut the branch below this node from read ahead.
         */
        if (err)
                goto cleanup;

        /*
         * FIXME: currently we just set nritems to 0 if this is a leaf,
         * effectively ignoring the content. In a next step we could
         * trigger more readahead depending from the content, e.g.
         * fetch the checksums for the extents in the leaf.
         */
        if (!btrfs_header_level(eb))
                goto cleanup;

        nritems = btrfs_header_nritems(eb);
        generation = btrfs_header_generation(eb);
        for (i = 0; i < nritems; i++) {
                struct reada_extctl *rec;
                u64 n_gen;
                struct btrfs_key key;
                struct btrfs_key next_key;

                btrfs_node_key_to_cpu(eb, &key, i);
                if (i + 1 < nritems)
                        btrfs_node_key_to_cpu(eb, &next_key, i + 1);
                else
                        next_key = re->top;
                bytenr = btrfs_node_blockptr(eb, i);
                n_gen = btrfs_node_ptr_generation(eb, i);

                list_for_each_entry(rec, &list, list) {
                        struct reada_control *rc = rec->rc;

                        /*
                         * if the generation doesn't match, just ignore this
                         * extctl. This will probably cut off a branch from
                         * prefetch. Alternatively one could start a new (sub-)
                         * prefetch for this branch, starting again from root.
                         * FIXME: move the generation check out of this loop
                         */
#ifdef DEBUG
                        if (rec->generation != generation) {
                                btrfs_debug(fs_info,
                                            "generation mismatch for (%llu,%d,%llu) %llu != %llu",
                                            key.objectid, key.type, key.offset,
                                            rec->generation, generation);
                        }
#endif
                        if (rec->generation == generation &&
                            btrfs_comp_cpu_keys(&key, &rc->key_end) < 0 &&
                            btrfs_comp_cpu_keys(&next_key, &rc->key_start) > 0)
                                reada_add_block(rc, bytenr, &next_key,
                                                btrfs_header_owner(eb), n_gen,
                                                btrfs_header_level(eb) - 1);
                }
        }

cleanup:
        /*
         * free extctl records
         */
        while (!list_empty(&list)) {
                struct reada_control *rc;
                struct reada_extctl *rec;

                rec = list_first_entry(&list, struct reada_extctl, list);
                list_del(&rec->list);
                rc = rec->rc;
                kfree(rec);

                kref_get(&rc->refcnt);
                if (atomic_dec_and_test(&rc->elems)) {
                        kref_put(&rc->refcnt, reada_control_release);
                        wake_up(&rc->wait);
                }
                kref_put(&rc->refcnt, reada_control_release);

                reada_extent_put(fs_info, re);  /* one ref for each entry */
        }

        return;
}

int btree_readahead_hook(struct extent_buffer *eb, int err)
{
        struct btrfs_fs_info *fs_info = eb->fs_info;
        int ret = 0;
        struct reada_extent *re;

        /* find extent */
        spin_lock(&fs_info->reada_lock);
        re = radix_tree_lookup(&fs_info->reada_tree,
                               eb->start >> PAGE_SHIFT);
        if (re)
                re->refcnt++;
        spin_unlock(&fs_info->reada_lock);
        if (!re) {
                ret = -1;
                goto start_machine;
        }

        __readahead_hook(fs_info, re, eb, err);
        reada_extent_put(fs_info, re);  /* our ref */

start_machine:
        reada_start_machine(fs_info);
        return ret;
}

static struct reada_zone *reada_find_zone(struct btrfs_device *dev, u64 logical,
                                          struct btrfs_bio *bbio)
{
        struct btrfs_fs_info *fs_info = dev->fs_info;
        int ret;
        struct reada_zone *zone;
        struct btrfs_block_group *cache = NULL;
        u64 start;
        u64 end;
        int i;

        zone = NULL;
        spin_lock(&fs_info->reada_lock);
        ret = radix_tree_gang_lookup(&dev->reada_zones, (void **)&zone,
                                     logical >> PAGE_SHIFT, 1);
        if (ret == 1 && logical >= zone->start && logical <= zone->end) {
                kref_get(&zone->refcnt);
                spin_unlock(&fs_info->reada_lock);
                return zone;
        }

        spin_unlock(&fs_info->reada_lock);

        cache = btrfs_lookup_block_group(fs_info, logical);
        if (!cache)
                return NULL;

        start = cache->start;
        end = start + cache->length - 1;
        btrfs_put_block_group(cache);

        zone = kzalloc(sizeof(*zone), GFP_KERNEL);
        if (!zone)
                return NULL;

        ret = radix_tree_preload(GFP_KERNEL);
        if (ret) {
                kfree(zone);
                return NULL;
        }

        zone->start = start;
        zone->end = end;
        INIT_LIST_HEAD(&zone->list);
        spin_lock_init(&zone->lock);
        zone->locked = 0;
        kref_init(&zone->refcnt);
        zone->elems = 0;
        zone->device = dev; /* our device always sits at index 0 */
        for (i = 0; i < bbio->num_stripes; ++i) {
                /* bounds have already been checked */
                zone->devs[i] = bbio->stripes[i].dev;
        }
        zone->ndevs = bbio->num_stripes;

        spin_lock(&fs_info->reada_lock);
        ret = radix_tree_insert(&dev->reada_zones,
                                (unsigned long)(zone->end >> PAGE_SHIFT),
                                zone);

        if (ret == -EEXIST) {
                kfree(zone);
                ret = radix_tree_gang_lookup(&dev->reada_zones, (void **)&zone,
                                             logical >> PAGE_SHIFT, 1);
                if (ret == 1 && logical >= zone->start && logical <= zone->end)
                        kref_get(&zone->refcnt);
                else
                        zone = NULL;
        }
        spin_unlock(&fs_info->reada_lock);
        radix_tree_preload_end();

        return zone;
}

static struct reada_extent *reada_find_extent(struct btrfs_fs_info *fs_info,
                                              u64 logical,
                                              struct btrfs_key *top,
                                              u64 owner_root, int level)
{
        int ret;
        struct reada_extent *re = NULL;
        struct reada_extent *re_exist = NULL;
        struct btrfs_bio *bbio = NULL;
        struct btrfs_device *dev;
        struct btrfs_device *prev_dev;
        u64 length;
        int real_stripes;
        int nzones = 0;
        unsigned long index = logical >> PAGE_SHIFT;
        int dev_replace_is_ongoing;
        int have_zone = 0;

        spin_lock(&fs_info->reada_lock);
        re = radix_tree_lookup(&fs_info->reada_tree, index);
        if (re)
                re->refcnt++;
        spin_unlock(&fs_info->reada_lock);

        if (re)
                return re;

        re = kzalloc(sizeof(*re), GFP_KERNEL);
        if (!re)
                return NULL;

        re->logical = logical;
        re->top = *top;
        INIT_LIST_HEAD(&re->extctl);
        spin_lock_init(&re->lock);
        re->refcnt = 1;
        re->owner_root = owner_root;
        re->level = level;

        /*
         * map block
         */
        length = fs_info->nodesize;
        ret = btrfs_map_block(fs_info, BTRFS_MAP_GET_READ_MIRRORS, logical,
                        &length, &bbio, 0);
        if (ret || !bbio || length < fs_info->nodesize)
                goto error;

        if (bbio->num_stripes > BTRFS_MAX_MIRRORS) {
                btrfs_err(fs_info,
                           "readahead: more than %d copies not supported",
                           BTRFS_MAX_MIRRORS);
                goto error;
        }

        real_stripes = bbio->num_stripes - bbio->num_tgtdevs;
        for (nzones = 0; nzones < real_stripes; ++nzones) {
                struct reada_zone *zone;

                dev = bbio->stripes[nzones].dev;

                /* cannot read ahead on missing device. */
                if (!dev->bdev)
                        continue;

                zone = reada_find_zone(dev, logical, bbio);
                if (!zone)
                        continue;

                re->zones[re->nzones++] = zone;
                spin_lock(&zone->lock);
                if (!zone->elems)
                        kref_get(&zone->refcnt);
                ++zone->elems;
                spin_unlock(&zone->lock);
                spin_lock(&fs_info->reada_lock);
                kref_put(&zone->refcnt, reada_zone_release);
                spin_unlock(&fs_info->reada_lock);
        }
        if (re->nzones == 0) {
                /* not a single zone found, error and out */
                goto error;
        }

        /* Insert extent in reada tree + all per-device trees, all or nothing */
        down_read(&fs_info->dev_replace.rwsem);
        ret = radix_tree_preload(GFP_KERNEL);
        if (ret) {
                up_read(&fs_info->dev_replace.rwsem);
                goto error;
        }

        spin_lock(&fs_info->reada_lock);
        ret = radix_tree_insert(&fs_info->reada_tree, index, re);
        if (ret == -EEXIST) {
                re_exist = radix_tree_lookup(&fs_info->reada_tree, index);
                re_exist->refcnt++;
                spin_unlock(&fs_info->reada_lock);
                radix_tree_preload_end();
                up_read(&fs_info->dev_replace.rwsem);
                goto error;
        }
        if (ret) {
                spin_unlock(&fs_info->reada_lock);
                radix_tree_preload_end();
                up_read(&fs_info->dev_replace.rwsem);
                goto error;
        }
        radix_tree_preload_end();
        prev_dev = NULL;
        dev_replace_is_ongoing = btrfs_dev_replace_is_ongoing(
                        &fs_info->dev_replace);
        for (nzones = 0; nzones < re->nzones; ++nzones) {
                dev = re->zones[nzones]->device;

                if (dev == prev_dev) {
                        /*
                         * in case of DUP, just add the first zone. As both
                         * are on the same device, there's nothing to gain
                         * from adding both.
                         * Also, it wouldn't work, as the tree is per device
                         * and adding would fail with EEXIST
                         */
                        continue;
                }
                if (!dev->bdev)
                        continue;

                if (test_bit(BTRFS_DEV_STATE_NO_READA, &dev->dev_state))
                        continue;

                if (dev_replace_is_ongoing &&
                    dev == fs_info->dev_replace.tgtdev) {
                        /*
                         * as this device is selected for reading only as
                         * a last resort, skip it for read ahead.
                         */
                        continue;
                }
                prev_dev = dev;
                ret = radix_tree_insert(&dev->reada_extents, index, re);
                if (ret) {
                        while (--nzones >= 0) {
                                dev = re->zones[nzones]->device;
                                BUG_ON(dev == NULL);
                                /* ignore whether the entry was inserted */
                                radix_tree_delete(&dev->reada_extents, index);
                        }
                        radix_tree_delete(&fs_info->reada_tree, index);
                        spin_unlock(&fs_info->reada_lock);
                        up_read(&fs_info->dev_replace.rwsem);
                        goto error;
                }
                have_zone = 1;
        }
        if (!have_zone)
                radix_tree_delete(&fs_info->reada_tree, index);
        spin_unlock(&fs_info->reada_lock);
        up_read(&fs_info->dev_replace.rwsem);

        if (!have_zone)
                goto error;

        btrfs_put_bbio(bbio);
        return re;

error:
        for (nzones = 0; nzones < re->nzones; ++nzones) {
                struct reada_zone *zone;

                zone = re->zones[nzones];
                kref_get(&zone->refcnt);
                spin_lock(&zone->lock);
                --zone->elems;
                if (zone->elems == 0) {
                        /*
                         * no fs_info->reada_lock needed, as this can't be
                         * the last ref
                         */
                        kref_put(&zone->refcnt, reada_zone_release);
                }
                spin_unlock(&zone->lock);

                spin_lock(&fs_info->reada_lock);
                kref_put(&zone->refcnt, reada_zone_release);
                spin_unlock(&fs_info->reada_lock);
        }
        btrfs_put_bbio(bbio);
        kfree(re);
        return re_exist;
}

static void reada_extent_put(struct btrfs_fs_info *fs_info,
                             struct reada_extent *re)
{
        int i;
        unsigned long index = re->logical >> PAGE_SHIFT;

        spin_lock(&fs_info->reada_lock);
        if (--re->refcnt) {
                spin_unlock(&fs_info->reada_lock);
                return;
        }

        radix_tree_delete(&fs_info->reada_tree, index);
        for (i = 0; i < re->nzones; ++i) {
                struct reada_zone *zone = re->zones[i];

                radix_tree_delete(&zone->device->reada_extents, index);
        }

        spin_unlock(&fs_info->reada_lock);

        for (i = 0; i < re->nzones; ++i) {
                struct reada_zone *zone = re->zones[i];

                kref_get(&zone->refcnt);
                spin_lock(&zone->lock);
                --zone->elems;
                if (zone->elems == 0) {
                        /* no fs_info->reada_lock needed, as this can't be
                         * the last ref */
                        kref_put(&zone->refcnt, reada_zone_release);
                }
                spin_unlock(&zone->lock);

                spin_lock(&fs_info->reada_lock);
                kref_put(&zone->refcnt, reada_zone_release);
                spin_unlock(&fs_info->reada_lock);
        }

        kfree(re);
}

static void reada_zone_release(struct kref *kref)
{
        struct reada_zone *zone = container_of(kref, struct reada_zone, refcnt);

        lockdep_assert_held(&zone->device->fs_info->reada_lock);

        radix_tree_delete(&zone->device->reada_zones,
                          zone->end >> PAGE_SHIFT);

        kfree(zone);
}

static void reada_control_release(struct kref *kref)
{
        struct reada_control *rc = container_of(kref, struct reada_control,
                                                refcnt);

        kfree(rc);
}

static int reada_add_block(struct reada_control *rc, u64 logical,
                           struct btrfs_key *top, u64 owner_root,
                           u64 generation, int level)
{
        struct btrfs_fs_info *fs_info = rc->fs_info;
        struct reada_extent *re;
        struct reada_extctl *rec;

        /* takes one ref */
        re = reada_find_extent(fs_info, logical, top, owner_root, level);
        if (!re)
                return -1;

        rec = kzalloc(sizeof(*rec), GFP_KERNEL);
        if (!rec) {
                reada_extent_put(fs_info, re);
                return -ENOMEM;
        }

        rec->rc = rc;
        rec->generation = generation;
        atomic_inc(&rc->elems);

        spin_lock(&re->lock);
        list_add_tail(&rec->list, &re->extctl);
        spin_unlock(&re->lock);

        /* leave the ref on the extent */

        return 0;
}

/*
 * called with fs_info->reada_lock held
 */
static void reada_peer_zones_set_lock(struct reada_zone *zone, int lock)
{
        int i;
        unsigned long index = zone->end >> PAGE_SHIFT;

        for (i = 0; i < zone->ndevs; ++i) {
                struct reada_zone *peer;
                peer = radix_tree_lookup(&zone->devs[i]->reada_zones, index);
                if (peer && peer->device != zone->device)
                        peer->locked = lock;
        }
}

/*
 * called with fs_info->reada_lock held
 */
static int reada_pick_zone(struct btrfs_device *dev)
{
        struct reada_zone *top_zone = NULL;
        struct reada_zone *top_locked_zone = NULL;
        u64 top_elems = 0;
        u64 top_locked_elems = 0;
        unsigned long index = 0;
        int ret;

        if (dev->reada_curr_zone) {
                reada_peer_zones_set_lock(dev->reada_curr_zone, 0);
                kref_put(&dev->reada_curr_zone->refcnt, reada_zone_release);
                dev->reada_curr_zone = NULL;
        }
        /* pick the zone with the most elements */
        while (1) {
                struct reada_zone *zone;

                ret = radix_tree_gang_lookup(&dev->reada_zones,
                                             (void **)&zone, index, 1);
                if (ret == 0)
                        break;
                index = (zone->end >> PAGE_SHIFT) + 1;
                if (zone->locked) {
                        if (zone->elems > top_locked_elems) {
                                top_locked_elems = zone->elems;
                                top_locked_zone = zone;
                        }
                } else {
                        if (zone->elems > top_elems) {
                                top_elems = zone->elems;
                                top_zone = zone;
                        }
                }
        }
        if (top_zone)
                dev->reada_curr_zone = top_zone;
        else if (top_locked_zone)
                dev->reada_curr_zone = top_locked_zone;
        else
                return 0;

        dev->reada_next = dev->reada_curr_zone->start;
        kref_get(&dev->reada_curr_zone->refcnt);
        reada_peer_zones_set_lock(dev->reada_curr_zone, 1);

        return 1;
}

static int reada_tree_block_flagged(struct btrfs_fs_info *fs_info, u64 bytenr,
                                    u64 owner_root, int level, int mirror_num,
                                    struct extent_buffer **eb)
{
        struct extent_buffer *buf = NULL;
        int ret;

        buf = btrfs_find_create_tree_block(fs_info, bytenr, owner_root, level);
        if (IS_ERR(buf))
                return 0;

        set_bit(EXTENT_BUFFER_READAHEAD, &buf->bflags);

        ret = read_extent_buffer_pages(buf, WAIT_PAGE_LOCK, mirror_num);
        if (ret) {
                free_extent_buffer_stale(buf);
                return ret;
        }

        if (test_bit(EXTENT_BUFFER_CORRUPT, &buf->bflags)) {
                free_extent_buffer_stale(buf);
                return -EIO;
        } else if (extent_buffer_uptodate(buf)) {
                *eb = buf;
        } else {
                free_extent_buffer(buf);
        }
        return 0;
}

static int reada_start_machine_dev(struct btrfs_device *dev)
{
        struct btrfs_fs_info *fs_info = dev->fs_info;
        struct reada_extent *re = NULL;
        int mirror_num = 0;
        struct extent_buffer *eb = NULL;
        u64 logical;
        int ret;
        int i;

        spin_lock(&fs_info->reada_lock);
        if (dev->reada_curr_zone == NULL) {
                ret = reada_pick_zone(dev);
                if (!ret) {
                        spin_unlock(&fs_info->reada_lock);
                        return 0;
                }
        }
        /*
         * FIXME currently we issue the reads one extent at a time. If we have
         * a contiguous block of extents, we could also coagulate them or use
         * plugging to speed things up
         */
        ret = radix_tree_gang_lookup(&dev->reada_extents, (void **)&re,
                                     dev->reada_next >> PAGE_SHIFT, 1);
        if (ret == 0 || re->logical > dev->reada_curr_zone->end) {
                ret = reada_pick_zone(dev);
                if (!ret) {
                        spin_unlock(&fs_info->reada_lock);
                        return 0;
                }
                re = NULL;
                ret = radix_tree_gang_lookup(&dev->reada_extents, (void **)&re,
                                        dev->reada_next >> PAGE_SHIFT, 1);
        }
        if (ret == 0) {
                spin_unlock(&fs_info->reada_lock);
                return 0;
        }
        dev->reada_next = re->logical + fs_info->nodesize;
        re->refcnt++;

        spin_unlock(&fs_info->reada_lock);

        spin_lock(&re->lock);
        if (re->scheduled || list_empty(&re->extctl)) {
                spin_unlock(&re->lock);
                reada_extent_put(fs_info, re);
                return 0;
        }
        re->scheduled = 1;
        spin_unlock(&re->lock);

        /*
         * find mirror num
         */
        for (i = 0; i < re->nzones; ++i) {
                if (re->zones[i]->device == dev) {
                        mirror_num = i + 1;
                        break;
                }
        }
        logical = re->logical;

        atomic_inc(&dev->reada_in_flight);
        ret = reada_tree_block_flagged(fs_info, logical, re->owner_root,
                                       re->level, mirror_num, &eb);
        if (ret)
                __readahead_hook(fs_info, re, NULL, ret);
        else if (eb)
                __readahead_hook(fs_info, re, eb, ret);

        if (eb)
                free_extent_buffer(eb);

        atomic_dec(&dev->reada_in_flight);
        reada_extent_put(fs_info, re);

        return 1;

}

static void reada_start_machine_worker(struct btrfs_work *work)
{
        struct reada_machine_work *rmw;
        int old_ioprio;

        rmw = container_of(work, struct reada_machine_work, work);

        old_ioprio = IOPRIO_PRIO_VALUE(task_nice_ioclass(current),
                                       task_nice_ioprio(current));
        set_task_ioprio(current, BTRFS_IOPRIO_READA);
        __reada_start_machine(rmw->fs_info);
        set_task_ioprio(current, old_ioprio);

        atomic_dec(&rmw->fs_info->reada_works_cnt);

        kfree(rmw);
}

/* Try to start up to 10k READA requests for a group of devices */
static int reada_start_for_fsdevs(struct btrfs_fs_devices *fs_devices)
{
        u64 enqueued;
        u64 total = 0;
        struct btrfs_device *device;

        do {
                enqueued = 0;
                list_for_each_entry(device, &fs_devices->devices, dev_list) {
                        if (atomic_read(&device->reada_in_flight) <
                            MAX_IN_FLIGHT)
                                enqueued += reada_start_machine_dev(device);
                }
                total += enqueued;
        } while (enqueued && total < 10000);

        return total;
}

static void __reada_start_machine(struct btrfs_fs_info *fs_info)
{
        struct btrfs_fs_devices *fs_devices = fs_info->fs_devices, *seed_devs;
        int i;
        u64 enqueued = 0;

        mutex_lock(&fs_devices->device_list_mutex);

        enqueued += reada_start_for_fsdevs(fs_devices);
        list_for_each_entry(seed_devs, &fs_devices->seed_list, seed_list)
                enqueued += reada_start_for_fsdevs(seed_devs);

        mutex_unlock(&fs_devices->device_list_mutex);
        if (enqueued == 0)
                return;

        /*
         * If everything is already in the cache, this is effectively single
         * threaded. To a) not hold the caller for too long and b) to utilize
         * more cores, we broke the loop above after 10000 iterations and now
         * enqueue to workers to finish it. This will distribute the load to
         * the cores.
         */
        for (i = 0; i < 2; ++i) {
                reada_start_machine(fs_info);
                if (atomic_read(&fs_info->reada_works_cnt) >
                    BTRFS_MAX_MIRRORS * 2)
                        break;
        }
}

static void reada_start_machine(struct btrfs_fs_info *fs_info)
{
        struct reada_machine_work *rmw;

        rmw = kzalloc(sizeof(*rmw), GFP_KERNEL);
        if (!rmw) {
                /* FIXME we cannot handle this properly right now */
                BUG();
        }
        btrfs_init_work(&rmw->work, reada_start_machine_worker, NULL, NULL);
        rmw->fs_info = fs_info;

        btrfs_queue_work(fs_info->readahead_workers, &rmw->work);
        atomic_inc(&fs_info->reada_works_cnt);
}

#ifdef DEBUG
static void dump_devs(struct btrfs_fs_info *fs_info, int all)
{
        struct btrfs_device *device;
        struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
        unsigned long index;
        int ret;
        int i;
        int j;
        int cnt;

        spin_lock(&fs_info->reada_lock);
        list_for_each_entry(device, &fs_devices->devices, dev_list) {
                btrfs_debug(fs_info, "dev %lld has %d in flight", device->devid,
                        atomic_read(&device->reada_in_flight));
                index = 0;
                while (1) {
                        struct reada_zone *zone;
                        ret = radix_tree_gang_lookup(&device->reada_zones,
                                                     (void **)&zone, index, 1);
                        if (ret == 0)
                                break;
                        pr_debug("  zone %llu-%llu elems %llu locked %d devs",
                                    zone->start, zone->end, zone->elems,
                                    zone->locked);
                        for (j = 0; j < zone->ndevs; ++j) {
                                pr_cont(" %lld",
                                        zone->devs[j]->devid);
                        }
                        if (device->reada_curr_zone == zone)
                                pr_cont(" curr off %llu",
                                        device->reada_next - zone->start);
                        pr_cont("\n");
                        index = (zone->end >> PAGE_SHIFT) + 1;
                }
                cnt = 0;
                index = 0;
                while (all) {
                        struct reada_extent *re = NULL;

                        ret = radix_tree_gang_lookup(&device->reada_extents,
                                                     (void **)&re, index, 1);
                        if (ret == 0)
                                break;
                        pr_debug("  re: logical %llu size %u empty %d scheduled %d",
                                re->logical, fs_info->nodesize,
                                list_empty(&re->extctl), re->scheduled);

                        for (i = 0; i < re->nzones; ++i) {
                                pr_cont(" zone %llu-%llu devs",
                                        re->zones[i]->start,
                                        re->zones[i]->end);
                                for (j = 0; j < re->zones[i]->ndevs; ++j) {
                                        pr_cont(" %lld",
                                                re->zones[i]->devs[j]->devid);
                                }
                        }
                        pr_cont("\n");
                        index = (re->logical >> PAGE_SHIFT) + 1;
                        if (++cnt > 15)
                                break;
                }
        }

        index = 0;
        cnt = 0;
        while (all) {
                struct reada_extent *re = NULL;

                ret = radix_tree_gang_lookup(&fs_info->reada_tree, (void **)&re,
                                             index, 1);
                if (ret == 0)
                        break;
                if (!re->scheduled) {
                        index = (re->logical >> PAGE_SHIFT) + 1;
                        continue;
                }
                pr_debug("re: logical %llu size %u list empty %d scheduled %d",
                        re->logical, fs_info->nodesize,
                        list_empty(&re->extctl), re->scheduled);
                for (i = 0; i < re->nzones; ++i) {
                        pr_cont(" zone %llu-%llu devs",
                                re->zones[i]->start,
                                re->zones[i]->end);
                        for (j = 0; j < re->zones[i]->ndevs; ++j) {
                                pr_cont(" %lld",
                                       re->zones[i]->devs[j]->devid);
                        }
                }
                pr_cont("\n");
                index = (re->logical >> PAGE_SHIFT) + 1;
        }
        spin_unlock(&fs_info->reada_lock);
}
#endif

/*
 * interface
 */
struct reada_control *btrfs_reada_add(struct btrfs_root *root,
                        struct btrfs_key *key_start, struct btrfs_key *key_end)
{
        struct reada_control *rc;
        u64 start;
        u64 generation;
        int ret;
        int level;
        struct extent_buffer *node;
        static struct btrfs_key max_key = {
                .objectid = (u64)-1,
                .type = (u8)-1,
                .offset = (u64)-1
        };

        rc = kzalloc(sizeof(*rc), GFP_KERNEL);
        if (!rc)
                return ERR_PTR(-ENOMEM);

        rc->fs_info = root->fs_info;
        rc->key_start = *key_start;
        rc->key_end = *key_end;
        atomic_set(&rc->elems, 0);
        init_waitqueue_head(&rc->wait);
        kref_init(&rc->refcnt);
        kref_get(&rc->refcnt); /* one ref for having elements */

        node = btrfs_root_node(root);
        start = node->start;
        generation = btrfs_header_generation(node);
        level = btrfs_header_level(node);
        free_extent_buffer(node);

        ret = reada_add_block(rc, start, &max_key, root->root_key.objectid,
                              generation, level);
        if (ret) {
                kfree(rc);
                return ERR_PTR(ret);
        }

        reada_start_machine(root->fs_info);

        return rc;
}

#ifdef DEBUG
int btrfs_reada_wait(void *handle)
{
        struct reada_control *rc = handle;
        struct btrfs_fs_info *fs_info = rc->fs_info;

        while (atomic_read(&rc->elems)) {
                if (!atomic_read(&fs_info->reada_works_cnt))
                        reada_start_machine(fs_info);
                wait_event_timeout(rc->wait, atomic_read(&rc->elems) == 0,
                                   5 * HZ);
                dump_devs(fs_info, atomic_read(&rc->elems) < 10 ? 1 : 0);
        }

        dump_devs(fs_info, atomic_read(&rc->elems) < 10 ? 1 : 0);

        kref_put(&rc->refcnt, reada_control_release);

        return 0;
}
#else
int btrfs_reada_wait(void *handle)
{
        struct reada_control *rc = handle;
        struct btrfs_fs_info *fs_info = rc->fs_info;

        while (atomic_read(&rc->elems)) {
                if (!atomic_read(&fs_info->reada_works_cnt))
                        reada_start_machine(fs_info);
                wait_event_timeout(rc->wait, atomic_read(&rc->elems) == 0,
                                   (HZ + 9) / 10);
        }

        kref_put(&rc->refcnt, reada_control_release);

        return 0;
}
#endif

void btrfs_reada_detach(void *handle)
{
        struct reada_control *rc = handle;

        kref_put(&rc->refcnt, reada_control_release);
}

/*
 * Before removing a device (device replace or device remove ioctls), call this
 * function to wait for all existing readahead requests on the device and to
 * make sure no one queues more readahead requests for the device.
 *
 * Must be called without holding neither the device list mutex nor the device
 * replace semaphore, otherwise it will deadlock.
 */
void btrfs_reada_remove_dev(struct btrfs_device *dev)
{
        struct btrfs_fs_info *fs_info = dev->fs_info;

        /* Serialize with readahead extent creation at reada_find_extent(). */
        spin_lock(&fs_info->reada_lock);
        set_bit(BTRFS_DEV_STATE_NO_READA, &dev->dev_state);
        spin_unlock(&fs_info->reada_lock);

        /*
         * There might be readahead requests added to the radix trees which
         * were not yet added to the readahead work queue. We need to start
         * them and wait for their completion, otherwise we can end up with
         * use-after-free problems when dropping the last reference on the
         * readahead extents and their zones, as they need to access the
         * device structure.
         */
        reada_start_machine(fs_info);
        btrfs_flush_workqueue(fs_info->readahead_workers);
}

/*
 * If when removing a device (device replace or device remove ioctls) an error
 * happens after calling btrfs_reada_remove_dev(), call this to undo what that
 * function did. This is safe to call even if btrfs_reada_remove_dev() was not
 * called before.
 */
void btrfs_reada_undo_remove_dev(struct btrfs_device *dev)
{
        spin_lock(&dev->fs_info->reada_lock);
        clear_bit(BTRFS_DEV_STATE_NO_READA, &dev->dev_state);
        spin_unlock(&dev->fs_info->reada_lock);
}