Linux 6.13-rc4

[linux.git] / fs / btrfs / dev-replace.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) STRATO AG 2012.  All rights reserved.
 */

#include <linux/sched.h>
#include <linux/bio.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/kthread.h>
#include <linux/math64.h>
#include "misc.h"
#include "ctree.h"
#include "disk-io.h"
#include "transaction.h"
#include "volumes.h"
#include "async-thread.h"
#include "dev-replace.h"
#include "sysfs.h"
#include "zoned.h"
#include "block-group.h"
#include "fs.h"
#include "accessors.h"
#include "scrub.h"

/*
 * Device replace overview
 *
 * [Objective]
 * To copy all extents (both new and on-disk) from source device to target
 * device, while still keeping the filesystem read-write.
 *
 * [Method]
 * There are two main methods involved:
 *
 * - Write duplication
 *
 *   All new writes will be written to both target and source devices, so even
 *   if replace gets canceled, sources device still contains up-to-date data.
 *
 *   Location:          handle_ops_on_dev_replace() from btrfs_map_block()
 *   Start:             btrfs_dev_replace_start()
 *   End:               btrfs_dev_replace_finishing()
 *   Content:           Latest data/metadata
 *
 * - Copy existing extents
 *
 *   This happens by reusing scrub facility, as scrub also iterates through
 *   existing extents from commit root.
 *
 *   Location:          scrub_write_block_to_dev_replace() from
 *                      scrub_block_complete()
 *   Content:           Data/meta from commit root.
 *
 * Due to the content difference, we need to avoid nocow write when dev-replace
 * is happening.  This is done by marking the block group read-only and waiting
 * for NOCOW writes.
 *
 * After replace is done, the finishing part is done by swapping the target and
 * source devices.
 *
 *   Location:          btrfs_dev_replace_update_device_in_mapping_tree() from
 *                      btrfs_dev_replace_finishing()
 */

static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
                                       int scrub_ret);
static int btrfs_dev_replace_kthread(void *data);

int btrfs_init_dev_replace(struct btrfs_fs_info *fs_info)
{
        struct btrfs_dev_lookup_args args = { .devid = BTRFS_DEV_REPLACE_DEVID };
        struct btrfs_key key;
        struct btrfs_root *dev_root = fs_info->dev_root;
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
        struct extent_buffer *eb;
        int slot;
        int ret = 0;
        struct btrfs_path *path = NULL;
        int item_size;
        struct btrfs_dev_replace_item *ptr;
        u64 src_devid;

        if (!dev_root)
                return 0;

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

        key.objectid = 0;
        key.type = BTRFS_DEV_REPLACE_KEY;
        key.offset = 0;
        ret = btrfs_search_slot(NULL, dev_root, &key, path, 0, 0);
        if (ret) {
no_valid_dev_replace_entry_found:
                /*
                 * We don't have a replace item or it's corrupted.  If there is
                 * a replace target, fail the mount.
                 */
                if (btrfs_find_device(fs_info->fs_devices, &args)) {
                        btrfs_err(fs_info,
                        "found replace target device without a valid replace item");
                        ret = -EUCLEAN;
                        goto out;
                }
                ret = 0;
                dev_replace->replace_state =
                        BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED;
                dev_replace->cont_reading_from_srcdev_mode =
                    BTRFS_DEV_REPLACE_ITEM_CONT_READING_FROM_SRCDEV_MODE_ALWAYS;
                dev_replace->time_started = 0;
                dev_replace->time_stopped = 0;
                atomic64_set(&dev_replace->num_write_errors, 0);
                atomic64_set(&dev_replace->num_uncorrectable_read_errors, 0);
                dev_replace->cursor_left = 0;
                dev_replace->committed_cursor_left = 0;
                dev_replace->cursor_left_last_write_of_item = 0;
                dev_replace->cursor_right = 0;
                dev_replace->srcdev = NULL;
                dev_replace->tgtdev = NULL;
                dev_replace->is_valid = 0;
                dev_replace->item_needs_writeback = 0;
                goto out;
        }
        slot = path->slots[0];
        eb = path->nodes[0];
        item_size = btrfs_item_size(eb, slot);
        ptr = btrfs_item_ptr(eb, slot, struct btrfs_dev_replace_item);

        if (item_size != sizeof(struct btrfs_dev_replace_item)) {
                btrfs_warn(fs_info,
                        "dev_replace entry found has unexpected size, ignore entry");
                goto no_valid_dev_replace_entry_found;
        }

        src_devid = btrfs_dev_replace_src_devid(eb, ptr);
        dev_replace->cont_reading_from_srcdev_mode =
                btrfs_dev_replace_cont_reading_from_srcdev_mode(eb, ptr);
        dev_replace->replace_state = btrfs_dev_replace_replace_state(eb, ptr);
        dev_replace->time_started = btrfs_dev_replace_time_started(eb, ptr);
        dev_replace->time_stopped =
                btrfs_dev_replace_time_stopped(eb, ptr);
        atomic64_set(&dev_replace->num_write_errors,
                     btrfs_dev_replace_num_write_errors(eb, ptr));
        atomic64_set(&dev_replace->num_uncorrectable_read_errors,
                     btrfs_dev_replace_num_uncorrectable_read_errors(eb, ptr));
        dev_replace->cursor_left = btrfs_dev_replace_cursor_left(eb, ptr);
        dev_replace->committed_cursor_left = dev_replace->cursor_left;
        dev_replace->cursor_left_last_write_of_item = dev_replace->cursor_left;
        dev_replace->cursor_right = btrfs_dev_replace_cursor_right(eb, ptr);
        dev_replace->is_valid = 1;

        dev_replace->item_needs_writeback = 0;
        switch (dev_replace->replace_state) {
        case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
                /*
                 * We don't have an active replace item but if there is a
                 * replace target, fail the mount.
                 */
                if (btrfs_find_device(fs_info->fs_devices, &args)) {
                        btrfs_err(fs_info,
"replace without active item, run 'device scan --forget' on the target device");
                        ret = -EUCLEAN;
                } else {
                        dev_replace->srcdev = NULL;
                        dev_replace->tgtdev = NULL;
                }
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
                dev_replace->tgtdev = btrfs_find_device(fs_info->fs_devices, &args);
                args.devid = src_devid;
                dev_replace->srcdev = btrfs_find_device(fs_info->fs_devices, &args);

                /*
                 * allow 'btrfs dev replace_cancel' if src/tgt device is
                 * missing
                 */
                if (!dev_replace->srcdev &&
                    !btrfs_test_opt(fs_info, DEGRADED)) {
                        ret = -EIO;
                        btrfs_warn(fs_info,
                           "cannot mount because device replace operation is ongoing and");
                        btrfs_warn(fs_info,
                           "srcdev (devid %llu) is missing, need to run 'btrfs dev scan'?",
                           src_devid);
                }
                if (!dev_replace->tgtdev &&
                    !btrfs_test_opt(fs_info, DEGRADED)) {
                        ret = -EIO;
                        btrfs_warn(fs_info,
                           "cannot mount because device replace operation is ongoing and");
                        btrfs_warn(fs_info,
                           "tgtdev (devid %llu) is missing, need to run 'btrfs dev scan'?",
                                BTRFS_DEV_REPLACE_DEVID);
                }
                if (dev_replace->tgtdev) {
                        if (dev_replace->srcdev) {
                                dev_replace->tgtdev->total_bytes =
                                        dev_replace->srcdev->total_bytes;
                                dev_replace->tgtdev->disk_total_bytes =
                                        dev_replace->srcdev->disk_total_bytes;
                                dev_replace->tgtdev->commit_total_bytes =
                                        dev_replace->srcdev->commit_total_bytes;
                                dev_replace->tgtdev->bytes_used =
                                        dev_replace->srcdev->bytes_used;
                                dev_replace->tgtdev->commit_bytes_used =
                                        dev_replace->srcdev->commit_bytes_used;
                        }
                        set_bit(BTRFS_DEV_STATE_REPLACE_TGT,
                                &dev_replace->tgtdev->dev_state);

                        WARN_ON(fs_info->fs_devices->rw_devices == 0);
                        dev_replace->tgtdev->io_width = fs_info->sectorsize;
                        dev_replace->tgtdev->io_align = fs_info->sectorsize;
                        dev_replace->tgtdev->sector_size = fs_info->sectorsize;
                        dev_replace->tgtdev->fs_info = fs_info;
                        set_bit(BTRFS_DEV_STATE_IN_FS_METADATA,
                                &dev_replace->tgtdev->dev_state);
                }
                break;
        }

out:
        btrfs_free_path(path);
        return ret;
}

/*
 * Initialize a new device for device replace target from a given source dev
 * and path.
 *
 * Return 0 and new device in @device_out, otherwise return < 0
 */
static int btrfs_init_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,
                                  const char *device_path,
                                  struct btrfs_device *srcdev,
                                  struct btrfs_device **device_out)
{
        struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
        struct btrfs_device *device;
        struct file *bdev_file;
        struct block_device *bdev;
        u64 devid = BTRFS_DEV_REPLACE_DEVID;
        int ret = 0;

        *device_out = NULL;
        if (srcdev->fs_devices->seeding) {
                btrfs_err(fs_info, "the filesystem is a seed filesystem!");
                return -EINVAL;
        }

        bdev_file = bdev_file_open_by_path(device_path, BLK_OPEN_WRITE,
                                        fs_info->bdev_holder, NULL);
        if (IS_ERR(bdev_file)) {
                btrfs_err(fs_info, "target device %s is invalid!", device_path);
                return PTR_ERR(bdev_file);
        }
        bdev = file_bdev(bdev_file);

        if (!btrfs_check_device_zone_type(fs_info, bdev)) {
                btrfs_err(fs_info,
                "dev-replace: zoned type of target device mismatch with filesystem");
                ret = -EINVAL;
                goto error;
        }

        sync_blockdev(bdev);

        list_for_each_entry(device, &fs_devices->devices, dev_list) {
                if (device->bdev == bdev) {
                        btrfs_err(fs_info,
                                  "target device is in the filesystem!");
                        ret = -EEXIST;
                        goto error;
                }
        }


        if (bdev_nr_bytes(bdev) < btrfs_device_get_total_bytes(srcdev)) {
                btrfs_err(fs_info,
                          "target device is smaller than source device!");
                ret = -EINVAL;
                goto error;
        }


        device = btrfs_alloc_device(NULL, &devid, NULL, device_path);
        if (IS_ERR(device)) {
                ret = PTR_ERR(device);
                goto error;
        }

        ret = lookup_bdev(device_path, &device->devt);
        if (ret)
                goto error;

        set_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
        device->generation = 0;
        device->io_width = fs_info->sectorsize;
        device->io_align = fs_info->sectorsize;
        device->sector_size = fs_info->sectorsize;
        device->total_bytes = btrfs_device_get_total_bytes(srcdev);
        device->disk_total_bytes = btrfs_device_get_disk_total_bytes(srcdev);
        device->bytes_used = btrfs_device_get_bytes_used(srcdev);
        device->commit_total_bytes = srcdev->commit_total_bytes;
        device->commit_bytes_used = device->bytes_used;
        device->fs_info = fs_info;
        device->bdev = bdev;
        device->bdev_file = bdev_file;
        set_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state);
        set_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state);
        device->dev_stats_valid = 1;
        set_blocksize(bdev_file, BTRFS_BDEV_BLOCKSIZE);
        device->fs_devices = fs_devices;

        ret = btrfs_get_dev_zone_info(device, false);
        if (ret)
                goto error;

        mutex_lock(&fs_devices->device_list_mutex);
        list_add(&device->dev_list, &fs_devices->devices);
        fs_devices->num_devices++;
        fs_devices->open_devices++;
        mutex_unlock(&fs_devices->device_list_mutex);

        *device_out = device;
        return 0;

error:
        fput(bdev_file);
        return ret;
}

/*
 * called from commit_transaction. Writes changed device replace state to
 * disk.
 */
int btrfs_run_dev_replace(struct btrfs_trans_handle *trans)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;
        int ret;
        struct btrfs_root *dev_root = fs_info->dev_root;
        struct btrfs_path *path;
        struct btrfs_key key;
        struct extent_buffer *eb;
        struct btrfs_dev_replace_item *ptr;
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;

        down_read(&dev_replace->rwsem);
        if (!dev_replace->is_valid ||
            !dev_replace->item_needs_writeback) {
                up_read(&dev_replace->rwsem);
                return 0;
        }
        up_read(&dev_replace->rwsem);

        key.objectid = 0;
        key.type = BTRFS_DEV_REPLACE_KEY;
        key.offset = 0;

        path = btrfs_alloc_path();
        if (!path) {
                ret = -ENOMEM;
                goto out;
        }
        ret = btrfs_search_slot(trans, dev_root, &key, path, -1, 1);
        if (ret < 0) {
                btrfs_warn(fs_info,
                           "error %d while searching for dev_replace item!",
                           ret);
                goto out;
        }

        if (ret == 0 &&
            btrfs_item_size(path->nodes[0], path->slots[0]) < sizeof(*ptr)) {
                /*
                 * need to delete old one and insert a new one.
                 * Since no attempt is made to recover any old state, if the
                 * dev_replace state is 'running', the data on the target
                 * drive is lost.
                 * It would be possible to recover the state: just make sure
                 * that the beginning of the item is never changed and always
                 * contains all the essential information. Then read this
                 * minimal set of information and use it as a base for the
                 * new state.
                 */
                ret = btrfs_del_item(trans, dev_root, path);
                if (ret != 0) {
                        btrfs_warn(fs_info,
                                   "delete too small dev_replace item failed %d!",
                                   ret);
                        goto out;
                }
                ret = 1;
        }

        if (ret == 1) {
                /* need to insert a new item */
                btrfs_release_path(path);
                ret = btrfs_insert_empty_item(trans, dev_root, path,
                                              &key, sizeof(*ptr));
                if (ret < 0) {
                        btrfs_warn(fs_info,
                                   "insert dev_replace item failed %d!", ret);
                        goto out;
                }
        }

        eb = path->nodes[0];
        ptr = btrfs_item_ptr(eb, path->slots[0],
                             struct btrfs_dev_replace_item);

        down_write(&dev_replace->rwsem);
        if (dev_replace->srcdev)
                btrfs_set_dev_replace_src_devid(eb, ptr,
                        dev_replace->srcdev->devid);
        else
                btrfs_set_dev_replace_src_devid(eb, ptr, (u64)-1);
        btrfs_set_dev_replace_cont_reading_from_srcdev_mode(eb, ptr,
                dev_replace->cont_reading_from_srcdev_mode);
        btrfs_set_dev_replace_replace_state(eb, ptr,
                dev_replace->replace_state);
        btrfs_set_dev_replace_time_started(eb, ptr, dev_replace->time_started);
        btrfs_set_dev_replace_time_stopped(eb, ptr, dev_replace->time_stopped);
        btrfs_set_dev_replace_num_write_errors(eb, ptr,
                atomic64_read(&dev_replace->num_write_errors));
        btrfs_set_dev_replace_num_uncorrectable_read_errors(eb, ptr,
                atomic64_read(&dev_replace->num_uncorrectable_read_errors));
        dev_replace->cursor_left_last_write_of_item =
                dev_replace->cursor_left;
        btrfs_set_dev_replace_cursor_left(eb, ptr,
                dev_replace->cursor_left_last_write_of_item);
        btrfs_set_dev_replace_cursor_right(eb, ptr,
                dev_replace->cursor_right);
        dev_replace->item_needs_writeback = 0;
        up_write(&dev_replace->rwsem);

        btrfs_mark_buffer_dirty(trans, eb);

out:
        btrfs_free_path(path);

        return ret;
}

static int mark_block_group_to_copy(struct btrfs_fs_info *fs_info,
                                    struct btrfs_device *src_dev)
{
        struct btrfs_path *path;
        struct btrfs_key key;
        struct btrfs_key found_key;
        struct btrfs_root *root = fs_info->dev_root;
        struct btrfs_dev_extent *dev_extent = NULL;
        struct btrfs_block_group *cache;
        struct btrfs_trans_handle *trans;
        int iter_ret = 0;
        int ret = 0;
        u64 chunk_offset;

        /* Do not use "to_copy" on non zoned filesystem for now */
        if (!btrfs_is_zoned(fs_info))
                return 0;

        mutex_lock(&fs_info->chunk_mutex);

        /* Ensure we don't have pending new block group */
        spin_lock(&fs_info->trans_lock);
        while (fs_info->running_transaction &&
               !list_empty(&fs_info->running_transaction->dev_update_list)) {
                spin_unlock(&fs_info->trans_lock);
                mutex_unlock(&fs_info->chunk_mutex);
                trans = btrfs_attach_transaction(root);
                if (IS_ERR(trans)) {
                        ret = PTR_ERR(trans);
                        mutex_lock(&fs_info->chunk_mutex);
                        if (ret == -ENOENT) {
                                spin_lock(&fs_info->trans_lock);
                                continue;
                        } else {
                                goto unlock;
                        }
                }

                ret = btrfs_commit_transaction(trans);
                mutex_lock(&fs_info->chunk_mutex);
                if (ret)
                        goto unlock;

                spin_lock(&fs_info->trans_lock);
        }
        spin_unlock(&fs_info->trans_lock);

        path = btrfs_alloc_path();
        if (!path) {
                ret = -ENOMEM;
                goto unlock;
        }

        path->reada = READA_FORWARD;
        path->search_commit_root = 1;
        path->skip_locking = 1;

        key.objectid = src_dev->devid;
        key.type = BTRFS_DEV_EXTENT_KEY;
        key.offset = 0;

        btrfs_for_each_slot(root, &key, &found_key, path, iter_ret) {
                struct extent_buffer *leaf = path->nodes[0];

                if (found_key.objectid != src_dev->devid)
                        break;

                if (found_key.type != BTRFS_DEV_EXTENT_KEY)
                        break;

                if (found_key.offset < key.offset)
                        break;

                dev_extent = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_dev_extent);

                chunk_offset = btrfs_dev_extent_chunk_offset(leaf, dev_extent);

                cache = btrfs_lookup_block_group(fs_info, chunk_offset);
                if (!cache)
                        continue;

                set_bit(BLOCK_GROUP_FLAG_TO_COPY, &cache->runtime_flags);
                btrfs_put_block_group(cache);
        }
        if (iter_ret < 0)
                ret = iter_ret;

        btrfs_free_path(path);
unlock:
        mutex_unlock(&fs_info->chunk_mutex);

        return ret;
}

bool btrfs_finish_block_group_to_copy(struct btrfs_device *srcdev,
                                      struct btrfs_block_group *cache,
                                      u64 physical)
{
        struct btrfs_fs_info *fs_info = cache->fs_info;
        struct btrfs_chunk_map *map;
        u64 chunk_offset = cache->start;
        int num_extents, cur_extent;
        int i;

        /* Do not use "to_copy" on non zoned filesystem for now */
        if (!btrfs_is_zoned(fs_info))
                return true;

        spin_lock(&cache->lock);
        if (test_bit(BLOCK_GROUP_FLAG_REMOVED, &cache->runtime_flags)) {
                spin_unlock(&cache->lock);
                return true;
        }
        spin_unlock(&cache->lock);

        map = btrfs_get_chunk_map(fs_info, chunk_offset, 1);
        ASSERT(!IS_ERR(map));

        num_extents = 0;
        cur_extent = 0;
        for (i = 0; i < map->num_stripes; i++) {
                /* We have more device extent to copy */
                if (srcdev != map->stripes[i].dev)
                        continue;

                num_extents++;
                if (physical == map->stripes[i].physical)
                        cur_extent = i;
        }

        btrfs_free_chunk_map(map);

        if (num_extents > 1 && cur_extent < num_extents - 1) {
                /*
                 * Has more stripes on this device. Keep this block group
                 * readonly until we finish all the stripes.
                 */
                return false;
        }

        /* Last stripe on this device */
        clear_bit(BLOCK_GROUP_FLAG_TO_COPY, &cache->runtime_flags);

        return true;
}

static int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
                const char *tgtdev_name, u64 srcdevid, const char *srcdev_name,
                int read_src)
{
        struct btrfs_root *root = fs_info->dev_root;
        struct btrfs_trans_handle *trans;
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
        int ret;
        struct btrfs_device *tgt_device = NULL;
        struct btrfs_device *src_device = NULL;

        src_device = btrfs_find_device_by_devspec(fs_info, srcdevid,
                                                  srcdev_name);
        if (IS_ERR(src_device))
                return PTR_ERR(src_device);

        if (btrfs_pinned_by_swapfile(fs_info, src_device)) {
                btrfs_warn_in_rcu(fs_info,
          "cannot replace device %s (devid %llu) due to active swapfile",
                        btrfs_dev_name(src_device), src_device->devid);
                return -ETXTBSY;
        }

        /*
         * Here we commit the transaction to make sure commit_total_bytes
         * of all the devices are updated.
         */
        trans = btrfs_attach_transaction(root);
        if (!IS_ERR(trans)) {
                ret = btrfs_commit_transaction(trans);
                if (ret)
                        return ret;
        } else if (PTR_ERR(trans) != -ENOENT) {
                return PTR_ERR(trans);
        }

        ret = btrfs_init_dev_replace_tgtdev(fs_info, tgtdev_name,
                                            src_device, &tgt_device);
        if (ret)
                return ret;

        ret = mark_block_group_to_copy(fs_info, src_device);
        if (ret)
                return ret;

        down_write(&dev_replace->rwsem);
        dev_replace->replace_task = current;
        switch (dev_replace->replace_state) {
        case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
                ASSERT(0);
                ret = BTRFS_IOCTL_DEV_REPLACE_RESULT_ALREADY_STARTED;
                up_write(&dev_replace->rwsem);
                goto leave;
        }

        dev_replace->cont_reading_from_srcdev_mode = read_src;
        dev_replace->srcdev = src_device;
        dev_replace->tgtdev = tgt_device;

        btrfs_info_in_rcu(fs_info,
                      "dev_replace from %s (devid %llu) to %s started",
                      btrfs_dev_name(src_device),
                      src_device->devid,
                      btrfs_dev_name(tgt_device));

        /*
         * from now on, the writes to the srcdev are all duplicated to
         * go to the tgtdev as well (refer to btrfs_map_block()).
         */
        dev_replace->replace_state = BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED;
        dev_replace->time_started = ktime_get_real_seconds();
        dev_replace->cursor_left = 0;
        dev_replace->committed_cursor_left = 0;
        dev_replace->cursor_left_last_write_of_item = 0;
        dev_replace->cursor_right = 0;
        dev_replace->is_valid = 1;
        dev_replace->item_needs_writeback = 1;
        atomic64_set(&dev_replace->num_write_errors, 0);
        atomic64_set(&dev_replace->num_uncorrectable_read_errors, 0);
        up_write(&dev_replace->rwsem);

        ret = btrfs_sysfs_add_device(tgt_device);
        if (ret)
                btrfs_err(fs_info, "kobj add dev failed %d", ret);

        btrfs_wait_ordered_roots(fs_info, U64_MAX, NULL);

        /*
         * Commit dev_replace state and reserve 1 item for it.
         * This is crucial to ensure we won't miss copying extents for new block
         * groups that are allocated after we started the device replace, and
         * must be done after setting up the device replace state.
         */
        trans = btrfs_start_transaction(root, 1);
        if (IS_ERR(trans)) {
                ret = PTR_ERR(trans);
                down_write(&dev_replace->rwsem);
                dev_replace->replace_state =
                        BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED;
                dev_replace->srcdev = NULL;
                dev_replace->tgtdev = NULL;
                up_write(&dev_replace->rwsem);
                goto leave;
        }

        ret = btrfs_commit_transaction(trans);
        WARN_ON(ret);

        /* the disk copy procedure reuses the scrub code */
        ret = btrfs_scrub_dev(fs_info, src_device->devid, 0,
                              btrfs_device_get_total_bytes(src_device),
                              &dev_replace->scrub_progress, 0, 1);

        ret = btrfs_dev_replace_finishing(fs_info, ret);
        if (ret == -EINPROGRESS)
                ret = BTRFS_IOCTL_DEV_REPLACE_RESULT_SCRUB_INPROGRESS;

        return ret;

leave:
        btrfs_destroy_dev_replace_tgtdev(tgt_device);
        return ret;
}

static int btrfs_check_replace_dev_names(struct btrfs_ioctl_dev_replace_args *args)
{
        if (args->start.srcdevid == 0) {
                if (memchr(args->start.srcdev_name, 0,
                           sizeof(args->start.srcdev_name)) == NULL)
                        return -ENAMETOOLONG;
        } else {
                args->start.srcdev_name[0] = 0;
        }

        if (memchr(args->start.tgtdev_name, 0,
                   sizeof(args->start.tgtdev_name)) == NULL)
            return -ENAMETOOLONG;

        return 0;
}

int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
                            struct btrfs_ioctl_dev_replace_args *args)
{
        int ret;

        switch (args->start.cont_reading_from_srcdev_mode) {
        case BTRFS_IOCTL_DEV_REPLACE_CONT_READING_FROM_SRCDEV_MODE_ALWAYS:
        case BTRFS_IOCTL_DEV_REPLACE_CONT_READING_FROM_SRCDEV_MODE_AVOID:
                break;
        default:
                return -EINVAL;
        }
        ret = btrfs_check_replace_dev_names(args);
        if (ret < 0)
                return ret;

        ret = btrfs_dev_replace_start(fs_info, args->start.tgtdev_name,
                                        args->start.srcdevid,
                                        args->start.srcdev_name,
                                        args->start.cont_reading_from_srcdev_mode);
        args->result = ret;
        /* don't warn if EINPROGRESS, someone else might be running scrub */
        if (ret == BTRFS_IOCTL_DEV_REPLACE_RESULT_SCRUB_INPROGRESS ||
            ret == BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR)
                return 0;

        return ret;
}

/*
 * blocked until all in-flight bios operations are finished.
 */
static void btrfs_rm_dev_replace_blocked(struct btrfs_fs_info *fs_info)
{
        set_bit(BTRFS_FS_STATE_DEV_REPLACING, &fs_info->fs_state);
        wait_event(fs_info->dev_replace.replace_wait, !percpu_counter_sum(
                   &fs_info->dev_replace.bio_counter));
}

/*
 * we have removed target device, it is safe to allow new bios request.
 */
static void btrfs_rm_dev_replace_unblocked(struct btrfs_fs_info *fs_info)
{
        clear_bit(BTRFS_FS_STATE_DEV_REPLACING, &fs_info->fs_state);
        wake_up(&fs_info->dev_replace.replace_wait);
}

/*
 * When finishing the device replace, before swapping the source device with the
 * target device we must update the chunk allocation state in the target device,
 * as it is empty because replace works by directly copying the chunks and not
 * through the normal chunk allocation path.
 */
static int btrfs_set_target_alloc_state(struct btrfs_device *srcdev,
                                        struct btrfs_device *tgtdev)
{
        struct extent_state *cached_state = NULL;
        u64 start = 0;
        u64 found_start;
        u64 found_end;
        int ret = 0;

        lockdep_assert_held(&srcdev->fs_info->chunk_mutex);

        while (find_first_extent_bit(&srcdev->alloc_state, start,
                                     &found_start, &found_end,
                                     CHUNK_ALLOCATED, &cached_state)) {
                ret = set_extent_bit(&tgtdev->alloc_state, found_start,
                                     found_end, CHUNK_ALLOCATED, NULL);
                if (ret)
                        break;
                start = found_end + 1;
        }

        free_extent_state(cached_state);
        return ret;
}

static void btrfs_dev_replace_update_device_in_mapping_tree(
                                                struct btrfs_fs_info *fs_info,
                                                struct btrfs_device *srcdev,
                                                struct btrfs_device *tgtdev)
{
        struct rb_node *node;

        /*
         * The chunk mutex must be held so that no new chunks can be created
         * while we are updating existing chunks. This guarantees we don't miss
         * any new chunk that gets created for a range that falls before the
         * range of the last chunk we processed.
         */
        lockdep_assert_held(&fs_info->chunk_mutex);

        write_lock(&fs_info->mapping_tree_lock);
        node = rb_first_cached(&fs_info->mapping_tree);
        while (node) {
                struct rb_node *next = rb_next(node);
                struct btrfs_chunk_map *map;
                u64 next_start;

                map = rb_entry(node, struct btrfs_chunk_map, rb_node);
                next_start = map->start + map->chunk_len;

                for (int i = 0; i < map->num_stripes; i++)
                        if (srcdev == map->stripes[i].dev)
                                map->stripes[i].dev = tgtdev;

                if (cond_resched_rwlock_write(&fs_info->mapping_tree_lock)) {
                        map = btrfs_find_chunk_map_nolock(fs_info, next_start, U64_MAX);
                        if (!map)
                                break;
                        node = &map->rb_node;
                        /*
                         * Drop the lookup reference since we are holding the
                         * lock in write mode and no one can remove the chunk
                         * map from the tree and drop its tree reference.
                         */
                        btrfs_free_chunk_map(map);
                } else {
                        node = next;
                }
        }
        write_unlock(&fs_info->mapping_tree_lock);
}

static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
                                       int scrub_ret)
{
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
        struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
        struct btrfs_device *tgt_device;
        struct btrfs_device *src_device;
        struct btrfs_root *root = fs_info->tree_root;
        u8 uuid_tmp[BTRFS_UUID_SIZE];
        struct btrfs_trans_handle *trans;
        int ret = 0;

        /* don't allow cancel or unmount to disturb the finishing procedure */
        mutex_lock(&dev_replace->lock_finishing_cancel_unmount);

        down_read(&dev_replace->rwsem);
        /* was the operation canceled, or is it finished? */
        if (dev_replace->replace_state !=
            BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED) {
                up_read(&dev_replace->rwsem);
                mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
                return 0;
        }

        tgt_device = dev_replace->tgtdev;
        src_device = dev_replace->srcdev;
        up_read(&dev_replace->rwsem);

        /*
         * flush all outstanding I/O and inode extent mappings before the
         * copy operation is declared as being finished
         */
        ret = btrfs_start_delalloc_roots(fs_info, LONG_MAX, false);
        if (ret) {
                mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
                return ret;
        }
        btrfs_wait_ordered_roots(fs_info, U64_MAX, NULL);

        /*
         * We have to use this loop approach because at this point src_device
         * has to be available for transaction commit to complete, yet new
         * chunks shouldn't be allocated on the device.
         */
        while (1) {
                trans = btrfs_start_transaction(root, 0);
                if (IS_ERR(trans)) {
                        mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
                        return PTR_ERR(trans);
                }
                ret = btrfs_commit_transaction(trans);
                WARN_ON(ret);

                /* Prevent write_all_supers() during the finishing procedure */
                mutex_lock(&fs_devices->device_list_mutex);
                /* Prevent new chunks being allocated on the source device */
                mutex_lock(&fs_info->chunk_mutex);

                if (!list_empty(&src_device->post_commit_list)) {
                        mutex_unlock(&fs_devices->device_list_mutex);
                        mutex_unlock(&fs_info->chunk_mutex);
                } else {
                        break;
                }
        }

        down_write(&dev_replace->rwsem);
        dev_replace->replace_state =
                scrub_ret ? BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED
                          : BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED;
        dev_replace->tgtdev = NULL;
        dev_replace->srcdev = NULL;
        dev_replace->time_stopped = ktime_get_real_seconds();
        dev_replace->item_needs_writeback = 1;

        /*
         * Update allocation state in the new device and replace the old device
         * with the new one in the mapping tree.
         */
        if (!scrub_ret) {
                scrub_ret = btrfs_set_target_alloc_state(src_device, tgt_device);
                if (scrub_ret)
                        goto error;
                btrfs_dev_replace_update_device_in_mapping_tree(fs_info,
                                                                src_device,
                                                                tgt_device);
        } else {
                if (scrub_ret != -ECANCELED)
                        btrfs_err_in_rcu(fs_info,
                                 "btrfs_scrub_dev(%s, %llu, %s) failed %d",
                                 btrfs_dev_name(src_device),
                                 src_device->devid,
                                 btrfs_dev_name(tgt_device), scrub_ret);
error:
                up_write(&dev_replace->rwsem);
                mutex_unlock(&fs_info->chunk_mutex);
                mutex_unlock(&fs_devices->device_list_mutex);
                btrfs_rm_dev_replace_blocked(fs_info);
                if (tgt_device)
                        btrfs_destroy_dev_replace_tgtdev(tgt_device);
                btrfs_rm_dev_replace_unblocked(fs_info);
                mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);

                return scrub_ret;
        }

        btrfs_info_in_rcu(fs_info,
                          "dev_replace from %s (devid %llu) to %s finished",
                          btrfs_dev_name(src_device),
                          src_device->devid,
                          btrfs_dev_name(tgt_device));
        clear_bit(BTRFS_DEV_STATE_REPLACE_TGT, &tgt_device->dev_state);
        tgt_device->devid = src_device->devid;
        src_device->devid = BTRFS_DEV_REPLACE_DEVID;
        memcpy(uuid_tmp, tgt_device->uuid, sizeof(uuid_tmp));
        memcpy(tgt_device->uuid, src_device->uuid, sizeof(tgt_device->uuid));
        memcpy(src_device->uuid, uuid_tmp, sizeof(src_device->uuid));
        btrfs_device_set_total_bytes(tgt_device, src_device->total_bytes);
        btrfs_device_set_disk_total_bytes(tgt_device,
                                          src_device->disk_total_bytes);
        btrfs_device_set_bytes_used(tgt_device, src_device->bytes_used);
        tgt_device->commit_bytes_used = src_device->bytes_used;

        btrfs_assign_next_active_device(src_device, tgt_device);

        list_add(&tgt_device->dev_alloc_list, &fs_devices->alloc_list);
        fs_devices->rw_devices++;

        dev_replace->replace_task = NULL;
        up_write(&dev_replace->rwsem);
        btrfs_rm_dev_replace_blocked(fs_info);

        btrfs_rm_dev_replace_remove_srcdev(src_device);

        btrfs_rm_dev_replace_unblocked(fs_info);

        /*
         * Increment dev_stats_ccnt so that btrfs_run_dev_stats() will
         * update on-disk dev stats value during commit transaction
         */
        atomic_inc(&tgt_device->dev_stats_ccnt);

        /*
         * this is again a consistent state where no dev_replace procedure
         * is running, the target device is part of the filesystem, the
         * source device is not part of the filesystem anymore and its 1st
         * superblock is scratched out so that it is no longer marked to
         * belong to this filesystem.
         */
        mutex_unlock(&fs_info->chunk_mutex);
        mutex_unlock(&fs_devices->device_list_mutex);

        /* replace the sysfs entry */
        btrfs_sysfs_remove_device(src_device);
        btrfs_sysfs_update_devid(tgt_device);
        if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &src_device->dev_state))
                btrfs_scratch_superblocks(fs_info, src_device);

        /* write back the superblocks */
        trans = btrfs_start_transaction(root, 0);
        if (!IS_ERR(trans))
                btrfs_commit_transaction(trans);

        mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);

        btrfs_rm_dev_replace_free_srcdev(src_device);

        return 0;
}

/*
 * Read progress of device replace status according to the state and last
 * stored position. The value format is the same as for
 * btrfs_dev_replace::progress_1000
 */
static u64 btrfs_dev_replace_progress(struct btrfs_fs_info *fs_info)
{
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
        u64 ret = 0;

        switch (dev_replace->replace_state) {
        case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
                ret = 0;
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
                ret = 1000;
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
                ret = div64_u64(dev_replace->cursor_left,
                                div_u64(btrfs_device_get_total_bytes(
                                                dev_replace->srcdev), 1000));
                break;
        }

        return ret;
}

void btrfs_dev_replace_status(struct btrfs_fs_info *fs_info,
                              struct btrfs_ioctl_dev_replace_args *args)
{
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;

        down_read(&dev_replace->rwsem);
        /* even if !dev_replace_is_valid, the values are good enough for
         * the replace_status ioctl */
        args->result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR;
        args->status.replace_state = dev_replace->replace_state;
        args->status.time_started = dev_replace->time_started;
        args->status.time_stopped = dev_replace->time_stopped;
        args->status.num_write_errors =
                atomic64_read(&dev_replace->num_write_errors);
        args->status.num_uncorrectable_read_errors =
                atomic64_read(&dev_replace->num_uncorrectable_read_errors);
        args->status.progress_1000 = btrfs_dev_replace_progress(fs_info);
        up_read(&dev_replace->rwsem);
}

int btrfs_dev_replace_cancel(struct btrfs_fs_info *fs_info)
{
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
        struct btrfs_device *tgt_device = NULL;
        struct btrfs_device *src_device = NULL;
        struct btrfs_trans_handle *trans;
        struct btrfs_root *root = fs_info->tree_root;
        int result;
        int ret;

        if (sb_rdonly(fs_info->sb))
                return -EROFS;

        mutex_lock(&dev_replace->lock_finishing_cancel_unmount);
        down_write(&dev_replace->rwsem);
        switch (dev_replace->replace_state) {
        case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
                result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NOT_STARTED;
                up_write(&dev_replace->rwsem);
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
                tgt_device = dev_replace->tgtdev;
                src_device = dev_replace->srcdev;
                up_write(&dev_replace->rwsem);
                ret = btrfs_scrub_cancel(fs_info);
                if (ret < 0) {
                        result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NOT_STARTED;
                } else {
                        result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR;
                        /*
                         * btrfs_dev_replace_finishing() will handle the
                         * cleanup part
                         */
                        btrfs_info_in_rcu(fs_info,
                                "dev_replace from %s (devid %llu) to %s canceled",
                                btrfs_dev_name(src_device), src_device->devid,
                                btrfs_dev_name(tgt_device));
                }
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
                /*
                 * Scrub doing the replace isn't running so we need to do the
                 * cleanup step of btrfs_dev_replace_finishing() here
                 */
                result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR;
                tgt_device = dev_replace->tgtdev;
                src_device = dev_replace->srcdev;
                dev_replace->tgtdev = NULL;
                dev_replace->srcdev = NULL;
                dev_replace->replace_state =
                                BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED;
                dev_replace->time_stopped = ktime_get_real_seconds();
                dev_replace->item_needs_writeback = 1;

                up_write(&dev_replace->rwsem);

                /* Scrub for replace must not be running in suspended state */
                btrfs_scrub_cancel(fs_info);

                trans = btrfs_start_transaction(root, 0);
                if (IS_ERR(trans)) {
                        mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
                        return PTR_ERR(trans);
                }
                ret = btrfs_commit_transaction(trans);
                WARN_ON(ret);

                btrfs_info_in_rcu(fs_info,
                "suspended dev_replace from %s (devid %llu) to %s canceled",
                        btrfs_dev_name(src_device), src_device->devid,
                        btrfs_dev_name(tgt_device));

                if (tgt_device)
                        btrfs_destroy_dev_replace_tgtdev(tgt_device);
                break;
        default:
                up_write(&dev_replace->rwsem);
                result = -EINVAL;
        }

        mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
        return result;
}

void btrfs_dev_replace_suspend_for_unmount(struct btrfs_fs_info *fs_info)
{
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;

        mutex_lock(&dev_replace->lock_finishing_cancel_unmount);
        down_write(&dev_replace->rwsem);

        switch (dev_replace->replace_state) {
        case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
                dev_replace->replace_state =
                        BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED;
                dev_replace->time_stopped = ktime_get_real_seconds();
                dev_replace->item_needs_writeback = 1;
                btrfs_info(fs_info, "suspending dev_replace for unmount");
                break;
        }

        up_write(&dev_replace->rwsem);
        mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
}

/* resume dev_replace procedure that was interrupted by unmount */
int btrfs_resume_dev_replace_async(struct btrfs_fs_info *fs_info)
{
        struct task_struct *task;
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;

        down_write(&dev_replace->rwsem);

        switch (dev_replace->replace_state) {
        case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
                up_write(&dev_replace->rwsem);
                return 0;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
                dev_replace->replace_state =
                        BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED;
                break;
        }
        if (!dev_replace->tgtdev || !dev_replace->tgtdev->bdev) {
                btrfs_info(fs_info,
                           "cannot continue dev_replace, tgtdev is missing");
                btrfs_info(fs_info,
                           "you may cancel the operation after 'mount -o degraded'");
                dev_replace->replace_state =
                                        BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED;
                up_write(&dev_replace->rwsem);
                return 0;
        }
        up_write(&dev_replace->rwsem);

        /*
         * This could collide with a paused balance, but the exclusive op logic
         * should never allow both to start and pause. We don't want to allow
         * dev-replace to start anyway.
         */
        if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_DEV_REPLACE)) {
                down_write(&dev_replace->rwsem);
                dev_replace->replace_state =
                                        BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED;
                up_write(&dev_replace->rwsem);
                btrfs_info(fs_info,
                "cannot resume dev-replace, other exclusive operation running");
                return 0;
        }

        task = kthread_run(btrfs_dev_replace_kthread, fs_info, "btrfs-devrepl");
        return PTR_ERR_OR_ZERO(task);
}

static int btrfs_dev_replace_kthread(void *data)
{
        struct btrfs_fs_info *fs_info = data;
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
        u64 progress;
        int ret;

        progress = btrfs_dev_replace_progress(fs_info);
        progress = div_u64(progress, 10);
        btrfs_info_in_rcu(fs_info,
                "continuing dev_replace from %s (devid %llu) to target %s @%u%%",
                btrfs_dev_name(dev_replace->srcdev),
                dev_replace->srcdev->devid,
                btrfs_dev_name(dev_replace->tgtdev),
                (unsigned int)progress);

        ret = btrfs_scrub_dev(fs_info, dev_replace->srcdev->devid,
                              dev_replace->committed_cursor_left,
                              btrfs_device_get_total_bytes(dev_replace->srcdev),
                              &dev_replace->scrub_progress, 0, 1);
        ret = btrfs_dev_replace_finishing(fs_info, ret);
        WARN_ON(ret && ret != -ECANCELED);

        btrfs_exclop_finish(fs_info);
        return 0;
}

int __pure btrfs_dev_replace_is_ongoing(struct btrfs_dev_replace *dev_replace)
{
        if (!dev_replace->is_valid)
                return 0;

        switch (dev_replace->replace_state) {
        case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
                return 0;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
                /*
                 * return true even if tgtdev is missing (this is
                 * something that can happen if the dev_replace
                 * procedure is suspended by an umount and then
                 * the tgtdev is missing (or "btrfs dev scan") was
                 * not called and the filesystem is remounted
                 * in degraded state. This does not stop the
                 * dev_replace procedure. It needs to be canceled
                 * manually if the cancellation is wanted.
                 */
                break;
        }
        return 1;
}

void btrfs_bio_counter_sub(struct btrfs_fs_info *fs_info, s64 amount)
{
        percpu_counter_sub(&fs_info->dev_replace.bio_counter, amount);
        cond_wake_up_nomb(&fs_info->dev_replace.replace_wait);
}

void btrfs_bio_counter_inc_blocked(struct btrfs_fs_info *fs_info)
{
        while (1) {
                percpu_counter_inc(&fs_info->dev_replace.bio_counter);
                if (likely(!test_bit(BTRFS_FS_STATE_DEV_REPLACING,
                                     &fs_info->fs_state)))
                        break;

                btrfs_bio_counter_dec(fs_info);
                wait_event(fs_info->dev_replace.replace_wait,
                           !test_bit(BTRFS_FS_STATE_DEV_REPLACING,
                                     &fs_info->fs_state));
        }
}