diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
new file mode 100644
index 0000000..9814347
--- /dev/null
+++ b/fs/btrfs/dev-replace.c
@@ -0,0 +1,1042 @@
+// 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 "ctree.h"
+#include "extent_map.h"
+#include "disk-io.h"
+#include "transaction.h"
+#include "print-tree.h"
+#include "volumes.h"
+#include "async-thread.h"
+#include "check-integrity.h"
+#include "rcu-string.h"
+#include "dev-replace.h"
+#include "sysfs.h"
+
+static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
+				       int scrub_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);
+static int btrfs_dev_replace_kthread(void *data);
+
+int btrfs_init_dev_replace(struct btrfs_fs_info *fs_info)
+{
+	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;
+
+	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:
+		ret = 0;
+		dev_replace->replace_state =
+			BTRFS_DEV_REPLACE_ITEM_STATE_NEVER_STARTED;
+		dev_replace->cont_reading_from_srcdev_mode =
+		    BTRFS_DEV_REPLACE_ITEM_CONT_READING_FROM_SRCDEV_MODE_ALWAYS;
+		dev_replace->replace_state = 0;
+		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_nr(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:
+		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->srcdev = btrfs_find_device(fs_info, src_devid,
+							NULL, NULL);
+		dev_replace->tgtdev = btrfs_find_device(fs_info,
+							BTRFS_DEV_REPLACE_DEVID,
+							NULL, NULL);
+		/*
+		 * 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_device *device;
+	struct block_device *bdev;
+	struct list_head *devices;
+	struct rcu_string *name;
+	u64 devid = BTRFS_DEV_REPLACE_DEVID;
+	int ret = 0;
+
+	*device_out = NULL;
+	if (fs_info->fs_devices->seeding) {
+		btrfs_err(fs_info, "the filesystem is a seed filesystem!");
+		return -EINVAL;
+	}
+
+	bdev = blkdev_get_by_path(device_path, FMODE_WRITE | FMODE_EXCL,
+				  fs_info->bdev_holder);
+	if (IS_ERR(bdev)) {
+		btrfs_err(fs_info, "target device %s is invalid!", device_path);
+		return PTR_ERR(bdev);
+	}
+
+	filemap_write_and_wait(bdev->bd_inode->i_mapping);
+
+	devices = &fs_info->fs_devices->devices;
+	list_for_each_entry(device, devices, dev_list) {
+		if (device->bdev == bdev) {
+			btrfs_err(fs_info,
+				  "target device is in the filesystem!");
+			ret = -EEXIST;
+			goto error;
+		}
+	}
+
+
+	if (i_size_read(bdev->bd_inode) <
+	    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);
+	if (IS_ERR(device)) {
+		ret = PTR_ERR(device);
+		goto error;
+	}
+
+	name = rcu_string_strdup(device_path, GFP_KERNEL);
+	if (!name) {
+		btrfs_free_device(device);
+		ret = -ENOMEM;
+		goto error;
+	}
+	rcu_assign_pointer(device->name, name);
+
+	mutex_lock(&fs_info->fs_devices->device_list_mutex);
+	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;
+	set_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state);
+	set_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state);
+	device->mode = FMODE_EXCL;
+	device->dev_stats_valid = 1;
+	set_blocksize(device->bdev, BTRFS_BDEV_BLOCKSIZE);
+	device->fs_devices = fs_info->fs_devices;
+	list_add(&device->dev_list, &fs_info->fs_devices->devices);
+	fs_info->fs_devices->num_devices++;
+	fs_info->fs_devices->open_devices++;
+	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+
+	*device_out = device;
+	return 0;
+
+error:
+	blkdev_put(bdev, FMODE_EXCL);
+	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)
+{
+	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;
+
+	btrfs_dev_replace_read_lock(dev_replace);
+	if (!dev_replace->is_valid ||
+	    !dev_replace->item_needs_writeback) {
+		btrfs_dev_replace_read_unlock(dev_replace);
+		return 0;
+	}
+	btrfs_dev_replace_read_unlock(dev_replace);
+
+	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_nr(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);
+
+	btrfs_dev_replace_write_lock(dev_replace);
+	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;
+	btrfs_dev_replace_write_unlock(dev_replace);
+
+	btrfs_mark_buffer_dirty(eb);
+
+out:
+	btrfs_free_path(path);
+
+	return ret;
+}
+
+void btrfs_after_dev_replace_commit(struct btrfs_fs_info *fs_info)
+{
+	struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
+
+	dev_replace->committed_cursor_left =
+		dev_replace->cursor_left_last_write_of_item;
+}
+
+static char* btrfs_dev_name(struct btrfs_device *device)
+{
+	if (!device || test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
+		return "<missing disk>";
+	else
+		return rcu_str_deref(device->name);
+}
+
+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;
+
+	ret = btrfs_find_device_by_devspec(fs_info, srcdevid,
+					    srcdev_name, &src_device);
+	if (ret)
+		return ret;
+
+	ret = btrfs_init_dev_replace_tgtdev(fs_info, tgtdev_name,
+					    src_device, &tgt_device);
+	if (ret)
+		return ret;
+
+	/*
+	 * 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);
+	}
+
+	btrfs_dev_replace_write_lock(dev_replace);
+	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;
+		goto leave;
+	}
+
+	dev_replace->cont_reading_from_srcdev_mode = read_src;
+	WARN_ON(!src_device);
+	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,
+		      rcu_str_deref(tgt_device->name));
+
+	/*
+	 * 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);
+	btrfs_dev_replace_write_unlock(dev_replace);
+
+	ret = btrfs_sysfs_add_device_link(tgt_device->fs_devices, tgt_device);
+	if (ret)
+		btrfs_err(fs_info, "kobj add dev failed %d", ret);
+
+	btrfs_wait_ordered_roots(fs_info, U64_MAX, 0, (u64)-1);
+
+	/* force writing the updated state information to disk */
+	trans = btrfs_start_transaction(root, 0);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		btrfs_dev_replace_write_lock(dev_replace);
+		dev_replace->replace_state =
+			BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED;
+		dev_replace->srcdev = NULL;
+		dev_replace->tgtdev = NULL;
+		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;
+	} else {
+		WARN_ON(ret);
+	}
+
+	return ret;
+
+leave:
+	btrfs_dev_replace_write_unlock(dev_replace);
+	btrfs_destroy_dev_replace_tgtdev(tgt_device);
+	return ret;
+}
+
+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;
+	}
+
+	if ((args->start.srcdevid == 0 && args->start.srcdev_name[0] == '\0') ||
+	    args->start.tgtdev_name[0] == '\0')
+		return -EINVAL;
+
+	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 = 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->replace_wait, !percpu_counter_sum(
+		   &fs_info->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->replace_wait);
+}
+
+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_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);
+
+	btrfs_dev_replace_read_lock(dev_replace);
+	/* was the operation canceled, or is it finished? */
+	if (dev_replace->replace_state !=
+	    BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED) {
+		btrfs_dev_replace_read_unlock(dev_replace);
+		mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
+		return 0;
+	}
+
+	tgt_device = dev_replace->tgtdev;
+	src_device = dev_replace->srcdev;
+	btrfs_dev_replace_read_unlock(dev_replace);
+
+	/*
+	 * 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, -1);
+	if (ret) {
+		mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
+		return ret;
+	}
+	btrfs_wait_ordered_roots(fs_info, U64_MAX, 0, (u64)-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);
+
+	/* keep away write_all_supers() during the finishing procedure */
+	mutex_lock(&fs_info->fs_devices->device_list_mutex);
+	mutex_lock(&fs_info->chunk_mutex);
+	btrfs_dev_replace_write_lock(dev_replace);
+	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;
+
+	/* replace old device with new one in mapping tree */
+	if (!scrub_ret) {
+		btrfs_dev_replace_update_device_in_mapping_tree(fs_info,
+								src_device,
+								tgt_device);
+	} else {
+		btrfs_err_in_rcu(fs_info,
+				 "btrfs_scrub_dev(%s, %llu, %s) failed %d",
+				 btrfs_dev_name(src_device),
+				 src_device->devid,
+				 rcu_str_deref(tgt_device->name), scrub_ret);
+		btrfs_dev_replace_write_unlock(dev_replace);
+		mutex_unlock(&fs_info->chunk_mutex);
+		mutex_unlock(&fs_info->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,
+			  rcu_str_deref(tgt_device->name));
+	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);
+	ASSERT(list_empty(&src_device->resized_list));
+	tgt_device->commit_total_bytes = src_device->commit_total_bytes;
+	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_info->fs_devices->alloc_list);
+	fs_info->fs_devices->rw_devices++;
+
+	btrfs_dev_replace_write_unlock(dev_replace);
+
+	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_info->fs_devices->device_list_mutex);
+
+	/* replace the sysfs entry */
+	btrfs_sysfs_rm_device_link(fs_info->fs_devices, src_device);
+	btrfs_rm_dev_replace_free_srcdev(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);
+
+	return 0;
+}
+
+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 extent_map_tree *em_tree = &fs_info->mapping_tree.map_tree;
+	struct extent_map *em;
+	struct map_lookup *map;
+	u64 start = 0;
+	int i;
+
+	write_lock(&em_tree->lock);
+	do {
+		em = lookup_extent_mapping(em_tree, start, (u64)-1);
+		if (!em)
+			break;
+		map = em->map_lookup;
+		for (i = 0; i < map->num_stripes; i++)
+			if (srcdev == map->stripes[i].dev)
+				map->stripes[i].dev = tgtdev;
+		start = em->start + em->len;
+		free_extent_map(em);
+	} while (start);
+	write_unlock(&em_tree->lock);
+}
+
+/*
+ * 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;
+
+	btrfs_dev_replace_read_lock(dev_replace);
+	/* 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);
+	btrfs_dev_replace_read_unlock(dev_replace);
+}
+
+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);
+	btrfs_dev_replace_write_lock(dev_replace);
+	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;
+		btrfs_dev_replace_write_unlock(dev_replace);
+		goto leave;
+	case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
+	case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
+		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;
+		break;
+	}
+	dev_replace->replace_state = BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED;
+	dev_replace->time_stopped = ktime_get_real_seconds();
+	dev_replace->item_needs_writeback = 1;
+	btrfs_dev_replace_write_unlock(dev_replace);
+	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,
+		"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);
+
+leave:
+	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);
+	btrfs_dev_replace_write_lock(dev_replace);
+	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;
+	}
+
+	btrfs_dev_replace_write_unlock(dev_replace);
+	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;
+
+	btrfs_dev_replace_write_lock(dev_replace);
+	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:
+		btrfs_dev_replace_write_unlock(dev_replace);
+		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'");
+		btrfs_dev_replace_write_unlock(dev_replace);
+		return 0;
+	}
+	btrfs_dev_replace_write_unlock(dev_replace);
+
+	/*
+	 * 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 (test_and_set_bit(BTRFS_FS_EXCL_OP, &fs_info->flags)) {
+		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);
+
+	clear_bit(BTRFS_FS_EXCL_OP, &fs_info->flags);
+	return 0;
+}
+
+int 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 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_dev_replace_read_lock(struct btrfs_dev_replace *dev_replace)
+{
+	read_lock(&dev_replace->lock);
+	atomic_inc(&dev_replace->read_locks);
+}
+
+void btrfs_dev_replace_read_unlock(struct btrfs_dev_replace *dev_replace)
+{
+	ASSERT(atomic_read(&dev_replace->read_locks) > 0);
+	atomic_dec(&dev_replace->read_locks);
+	read_unlock(&dev_replace->lock);
+}
+
+void btrfs_dev_replace_write_lock(struct btrfs_dev_replace *dev_replace)
+{
+again:
+	wait_event(dev_replace->read_lock_wq,
+		   atomic_read(&dev_replace->blocking_readers) == 0);
+	write_lock(&dev_replace->lock);
+	if (atomic_read(&dev_replace->blocking_readers)) {
+		write_unlock(&dev_replace->lock);
+		goto again;
+	}
+}
+
+void btrfs_dev_replace_write_unlock(struct btrfs_dev_replace *dev_replace)
+{
+	ASSERT(atomic_read(&dev_replace->blocking_readers) == 0);
+	write_unlock(&dev_replace->lock);
+}
+
+/* inc blocking cnt and release read lock */
+void btrfs_dev_replace_set_lock_blocking(
+					struct btrfs_dev_replace *dev_replace)
+{
+	/* only set blocking for read lock */
+	ASSERT(atomic_read(&dev_replace->read_locks) > 0);
+	atomic_inc(&dev_replace->blocking_readers);
+	read_unlock(&dev_replace->lock);
+}
+
+/* acquire read lock and dec blocking cnt */
+void btrfs_dev_replace_clear_lock_blocking(
+					struct btrfs_dev_replace *dev_replace)
+{
+	/* only set blocking for read lock */
+	ASSERT(atomic_read(&dev_replace->read_locks) > 0);
+	ASSERT(atomic_read(&dev_replace->blocking_readers) > 0);
+	read_lock(&dev_replace->lock);
+	/* Barrier implied by atomic_dec_and_test */
+	if (atomic_dec_and_test(&dev_replace->blocking_readers))
+		cond_wake_up_nomb(&dev_replace->read_lock_wq);
+}
+
+void btrfs_bio_counter_inc_noblocked(struct btrfs_fs_info *fs_info)
+{
+	percpu_counter_inc(&fs_info->bio_counter);
+}
+
+void btrfs_bio_counter_sub(struct btrfs_fs_info *fs_info, s64 amount)
+{
+	percpu_counter_sub(&fs_info->bio_counter, amount);
+	cond_wake_up_nomb(&fs_info->replace_wait);
+}
+
+void btrfs_bio_counter_inc_blocked(struct btrfs_fs_info *fs_info)
+{
+	while (1) {
+		percpu_counter_inc(&fs_info->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->replace_wait,
+			   !test_bit(BTRFS_FS_STATE_DEV_REPLACING,
+				     &fs_info->fs_state));
+	}
+}