Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 4532 insertions, 0 deletions

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
new file mode 100644
index 000000000..d2bdcc2cc
--- /dev/null
+++ b/fs/btrfs/relocation.c
@@ -0,0 +1,4532 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2009 Oracle.  All rights reserved.
+ */
+
+#include <linux/sched.h>
+#include <linux/pagemap.h>
+#include <linux/writeback.h>
+#include <linux/blkdev.h>
+#include <linux/rbtree.h>
+#include <linux/slab.h>
+#include <linux/error-injection.h>
+#include "ctree.h"
+#include "disk-io.h"
+#include "transaction.h"
+#include "volumes.h"
+#include "locking.h"
+#include "btrfs_inode.h"
+#include "async-thread.h"
+#include "free-space-cache.h"
+#include "qgroup.h"
+#include "print-tree.h"
+#include "delalloc-space.h"
+#include "block-group.h"
+#include "backref.h"
+#include "misc.h"
+#include "subpage.h"
+#include "zoned.h"
+#include "inode-item.h"
+
+/*
+ * Relocation overview
+ *
+ * [What does relocation do]
+ *
+ * The objective of relocation is to relocate all extents of the target block
+ * group to other block groups.
+ * This is utilized by resize (shrink only), profile converting, compacting
+ * space, or balance routine to spread chunks over devices.
+ *
+ * 		Before		|		After
+ * ------------------------------------------------------------------
+ *  BG A: 10 data extents	| BG A: deleted
+ *  BG B:  2 data extents	| BG B: 10 data extents (2 old + 8 relocated)
+ *  BG C:  1 extents		| BG C:  3 data extents (1 old + 2 relocated)
+ *
+ * [How does relocation work]
+ *
+ * 1.   Mark the target block group read-only
+ *      New extents won't be allocated from the target block group.
+ *
+ * 2.1  Record each extent in the target block group
+ *      To build a proper map of extents to be relocated.
+ *
+ * 2.2  Build data reloc tree and reloc trees
+ *      Data reloc tree will contain an inode, recording all newly relocated
+ *      data extents.
+ *      There will be only one data reloc tree for one data block group.
+ *
+ *      Reloc tree will be a special snapshot of its source tree, containing
+ *      relocated tree blocks.
+ *      Each tree referring to a tree block in target block group will get its
+ *      reloc tree built.
+ *
+ * 2.3  Swap source tree with its corresponding reloc tree
+ *      Each involved tree only refers to new extents after swap.
+ *
+ * 3.   Cleanup reloc trees and data reloc tree.
+ *      As old extents in the target block group are still referenced by reloc
+ *      trees, we need to clean them up before really freeing the target block
+ *      group.
+ *
+ * The main complexity is in steps 2.2 and 2.3.
+ *
+ * The entry point of relocation is relocate_block_group() function.
+ */
+
+#define RELOCATION_RESERVED_NODES	256
+/*
+ * map address of tree root to tree
+ */
+struct mapping_node {
+	struct {
+		struct rb_node rb_node;
+		u64 bytenr;
+	}; /* Use rb_simle_node for search/insert */
+	void *data;
+};
+
+struct mapping_tree {
+	struct rb_root rb_root;
+	spinlock_t lock;
+};
+
+/*
+ * present a tree block to process
+ */
+struct tree_block {
+	struct {
+		struct rb_node rb_node;
+		u64 bytenr;
+	}; /* Use rb_simple_node for search/insert */
+	u64 owner;
+	struct btrfs_key key;
+	unsigned int level:8;
+	unsigned int key_ready:1;
+};
+
+#define MAX_EXTENTS 128
+
+struct file_extent_cluster {
+	u64 start;
+	u64 end;
+	u64 boundary[MAX_EXTENTS];
+	unsigned int nr;
+};
+
+struct reloc_control {
+	/* block group to relocate */
+	struct btrfs_block_group *block_group;
+	/* extent tree */
+	struct btrfs_root *extent_root;
+	/* inode for moving data */
+	struct inode *data_inode;
+
+	struct btrfs_block_rsv *block_rsv;
+
+	struct btrfs_backref_cache backref_cache;
+
+	struct file_extent_cluster cluster;
+	/* tree blocks have been processed */
+	struct extent_io_tree processed_blocks;
+	/* map start of tree root to corresponding reloc tree */
+	struct mapping_tree reloc_root_tree;
+	/* list of reloc trees */
+	struct list_head reloc_roots;
+	/* list of subvolume trees that get relocated */
+	struct list_head dirty_subvol_roots;
+	/* size of metadata reservation for merging reloc trees */
+	u64 merging_rsv_size;
+	/* size of relocated tree nodes */
+	u64 nodes_relocated;
+	/* reserved size for block group relocation*/
+	u64 reserved_bytes;
+
+	u64 search_start;
+	u64 extents_found;
+
+	unsigned int stage:8;
+	unsigned int create_reloc_tree:1;
+	unsigned int merge_reloc_tree:1;
+	unsigned int found_file_extent:1;
+};
+
+/* stages of data relocation */
+#define MOVE_DATA_EXTENTS	0
+#define UPDATE_DATA_PTRS	1
+
+static void mark_block_processed(struct reloc_control *rc,
+				 struct btrfs_backref_node *node)
+{
+	u32 blocksize;
+
+	if (node->level == 0 ||
+	    in_range(node->bytenr, rc->block_group->start,
+		     rc->block_group->length)) {
+		blocksize = rc->extent_root->fs_info->nodesize;
+		set_extent_bits(&rc->processed_blocks, node->bytenr,
+				node->bytenr + blocksize - 1, EXTENT_DIRTY);
+	}
+	node->processed = 1;
+}
+
+
+static void mapping_tree_init(struct mapping_tree *tree)
+{
+	tree->rb_root = RB_ROOT;
+	spin_lock_init(&tree->lock);
+}
+
+/*
+ * walk up backref nodes until reach node presents tree root
+ */
+static struct btrfs_backref_node *walk_up_backref(
+		struct btrfs_backref_node *node,
+		struct btrfs_backref_edge *edges[], int *index)
+{
+	struct btrfs_backref_edge *edge;
+	int idx = *index;
+
+	while (!list_empty(&node->upper)) {
+		edge = list_entry(node->upper.next,
+				  struct btrfs_backref_edge, list[LOWER]);
+		edges[idx++] = edge;
+		node = edge->node[UPPER];
+	}
+	BUG_ON(node->detached);
+	*index = idx;
+	return node;
+}
+
+/*
+ * walk down backref nodes to find start of next reference path
+ */
+static struct btrfs_backref_node *walk_down_backref(
+		struct btrfs_backref_edge *edges[], int *index)
+{
+	struct btrfs_backref_edge *edge;
+	struct btrfs_backref_node *lower;
+	int idx = *index;
+
+	while (idx > 0) {
+		edge = edges[idx - 1];
+		lower = edge->node[LOWER];
+		if (list_is_last(&edge->list[LOWER], &lower->upper)) {
+			idx--;
+			continue;
+		}
+		edge = list_entry(edge->list[LOWER].next,
+				  struct btrfs_backref_edge, list[LOWER]);
+		edges[idx - 1] = edge;
+		*index = idx;
+		return edge->node[UPPER];
+	}
+	*index = 0;
+	return NULL;
+}
+
+static void update_backref_node(struct btrfs_backref_cache *cache,
+				struct btrfs_backref_node *node, u64 bytenr)
+{
+	struct rb_node *rb_node;
+	rb_erase(&node->rb_node, &cache->rb_root);
+	node->bytenr = bytenr;
+	rb_node = rb_simple_insert(&cache->rb_root, node->bytenr, &node->rb_node);
+	if (rb_node)
+		btrfs_backref_panic(cache->fs_info, bytenr, -EEXIST);
+}
+
+/*
+ * update backref cache after a transaction commit
+ */
+static int update_backref_cache(struct btrfs_trans_handle *trans,
+				struct btrfs_backref_cache *cache)
+{
+	struct btrfs_backref_node *node;
+	int level = 0;
+
+	if (cache->last_trans == 0) {
+		cache->last_trans = trans->transid;
+		return 0;
+	}
+
+	if (cache->last_trans == trans->transid)
+		return 0;
+
+	/*
+	 * detached nodes are used to avoid unnecessary backref
+	 * lookup. transaction commit changes the extent tree.
+	 * so the detached nodes are no longer useful.
+	 */
+	while (!list_empty(&cache->detached)) {
+		node = list_entry(cache->detached.next,
+				  struct btrfs_backref_node, list);
+		btrfs_backref_cleanup_node(cache, node);
+	}
+
+	while (!list_empty(&cache->changed)) {
+		node = list_entry(cache->changed.next,
+				  struct btrfs_backref_node, list);
+		list_del_init(&node->list);
+		BUG_ON(node->pending);
+		update_backref_node(cache, node, node->new_bytenr);
+	}
+
+	/*
+	 * some nodes can be left in the pending list if there were
+	 * errors during processing the pending nodes.
+	 */
+	for (level = 0; level < BTRFS_MAX_LEVEL; level++) {
+		list_for_each_entry(node, &cache->pending[level], list) {
+			BUG_ON(!node->pending);
+			if (node->bytenr == node->new_bytenr)
+				continue;
+			update_backref_node(cache, node, node->new_bytenr);
+		}
+	}
+
+	cache->last_trans = 0;
+	return 1;
+}
+
+static bool reloc_root_is_dead(struct btrfs_root *root)
+{
+	/*
+	 * Pair with set_bit/clear_bit in clean_dirty_subvols and
+	 * btrfs_update_reloc_root. We need to see the updated bit before
+	 * trying to access reloc_root
+	 */
+	smp_rmb();
+	if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
+		return true;
+	return false;
+}
+
+/*
+ * Check if this subvolume tree has valid reloc tree.
+ *
+ * Reloc tree after swap is considered dead, thus not considered as valid.
+ * This is enough for most callers, as they don't distinguish dead reloc root
+ * from no reloc root.  But btrfs_should_ignore_reloc_root() below is a
+ * special case.
+ */
+static bool have_reloc_root(struct btrfs_root *root)
+{
+	if (reloc_root_is_dead(root))
+		return false;
+	if (!root->reloc_root)
+		return false;
+	return true;
+}
+
+int btrfs_should_ignore_reloc_root(struct btrfs_root *root)
+{
+	struct btrfs_root *reloc_root;
+
+	if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
+		return 0;
+
+	/* This root has been merged with its reloc tree, we can ignore it */
+	if (reloc_root_is_dead(root))
+		return 1;
+
+	reloc_root = root->reloc_root;
+	if (!reloc_root)
+		return 0;
+
+	if (btrfs_header_generation(reloc_root->commit_root) ==
+	    root->fs_info->running_transaction->transid)
+		return 0;
+	/*
+	 * if there is reloc tree and it was created in previous
+	 * transaction backref lookup can find the reloc tree,
+	 * so backref node for the fs tree root is useless for
+	 * relocation.
+	 */
+	return 1;
+}
+
+/*
+ * find reloc tree by address of tree root
+ */
+struct btrfs_root *find_reloc_root(struct btrfs_fs_info *fs_info, u64 bytenr)
+{
+	struct reloc_control *rc = fs_info->reloc_ctl;
+	struct rb_node *rb_node;
+	struct mapping_node *node;
+	struct btrfs_root *root = NULL;
+
+	ASSERT(rc);
+	spin_lock(&rc->reloc_root_tree.lock);
+	rb_node = rb_simple_search(&rc->reloc_root_tree.rb_root, bytenr);
+	if (rb_node) {
+		node = rb_entry(rb_node, struct mapping_node, rb_node);
+		root = node->data;
+	}
+	spin_unlock(&rc->reloc_root_tree.lock);
+	return btrfs_grab_root(root);
+}
+
+/*
+ * For useless nodes, do two major clean ups:
+ *
+ * - Cleanup the children edges and nodes
+ *   If child node is also orphan (no parent) during cleanup, then the child
+ *   node will also be cleaned up.
+ *
+ * - Freeing up leaves (level 0), keeps nodes detached
+ *   For nodes, the node is still cached as "detached"
+ *
+ * Return false if @node is not in the @useless_nodes list.
+ * Return true if @node is in the @useless_nodes list.
+ */
+static bool handle_useless_nodes(struct reloc_control *rc,
+				 struct btrfs_backref_node *node)
+{
+	struct btrfs_backref_cache *cache = &rc->backref_cache;
+	struct list_head *useless_node = &cache->useless_node;
+	bool ret = false;
+
+	while (!list_empty(useless_node)) {
+		struct btrfs_backref_node *cur;
+
+		cur = list_first_entry(useless_node, struct btrfs_backref_node,
+				 list);
+		list_del_init(&cur->list);
+
+		/* Only tree root nodes can be added to @useless_nodes */
+		ASSERT(list_empty(&cur->upper));
+
+		if (cur == node)
+			ret = true;
+
+		/* The node is the lowest node */
+		if (cur->lowest) {
+			list_del_init(&cur->lower);
+			cur->lowest = 0;
+		}
+
+		/* Cleanup the lower edges */
+		while (!list_empty(&cur->lower)) {
+			struct btrfs_backref_edge *edge;
+			struct btrfs_backref_node *lower;
+
+			edge = list_entry(cur->lower.next,
+					struct btrfs_backref_edge, list[UPPER]);
+			list_del(&edge->list[UPPER]);
+			list_del(&edge->list[LOWER]);
+			lower = edge->node[LOWER];
+			btrfs_backref_free_edge(cache, edge);
+
+			/* Child node is also orphan, queue for cleanup */
+			if (list_empty(&lower->upper))
+				list_add(&lower->list, useless_node);
+		}
+		/* Mark this block processed for relocation */
+		mark_block_processed(rc, cur);
+
+		/*
+		 * Backref nodes for tree leaves are deleted from the cache.
+		 * Backref nodes for upper level tree blocks are left in the
+		 * cache to avoid unnecessary backref lookup.
+		 */
+		if (cur->level > 0) {
+			list_add(&cur->list, &cache->detached);
+			cur->detached = 1;
+		} else {
+			rb_erase(&cur->rb_node, &cache->rb_root);
+			btrfs_backref_free_node(cache, cur);
+		}
+	}
+	return ret;
+}
+
+/*
+ * Build backref tree for a given tree block. Root of the backref tree
+ * corresponds the tree block, leaves of the backref tree correspond roots of
+ * b-trees that reference the tree block.
+ *
+ * The basic idea of this function is check backrefs of a given block to find
+ * upper level blocks that reference the block, and then check backrefs of
+ * these upper level blocks recursively. The recursion stops when tree root is
+ * reached or backrefs for the block is cached.
+ *
+ * NOTE: if we find that backrefs for a block are cached, we know backrefs for
+ * all upper level blocks that directly/indirectly reference the block are also
+ * cached.
+ */
+static noinline_for_stack struct btrfs_backref_node *build_backref_tree(
+			struct reloc_control *rc, struct btrfs_key *node_key,
+			int level, u64 bytenr)
+{
+	struct btrfs_backref_iter *iter;
+	struct btrfs_backref_cache *cache = &rc->backref_cache;
+	/* For searching parent of TREE_BLOCK_REF */
+	struct btrfs_path *path;
+	struct btrfs_backref_node *cur;
+	struct btrfs_backref_node *node = NULL;
+	struct btrfs_backref_edge *edge;
+	int ret;
+	int err = 0;
+
+	iter = btrfs_backref_iter_alloc(rc->extent_root->fs_info, GFP_NOFS);
+	if (!iter)
+		return ERR_PTR(-ENOMEM);
+	path = btrfs_alloc_path();
+	if (!path) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	node = btrfs_backref_alloc_node(cache, bytenr, level);
+	if (!node) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	node->lowest = 1;
+	cur = node;
+
+	/* Breadth-first search to build backref cache */
+	do {
+		ret = btrfs_backref_add_tree_node(cache, path, iter, node_key,
+						  cur);
+		if (ret < 0) {
+			err = ret;
+			goto out;
+		}
+		edge = list_first_entry_or_null(&cache->pending_edge,
+				struct btrfs_backref_edge, list[UPPER]);
+		/*
+		 * The pending list isn't empty, take the first block to
+		 * process
+		 */
+		if (edge) {
+			list_del_init(&edge->list[UPPER]);
+			cur = edge->node[UPPER];
+		}
+	} while (edge);
+
+	/* Finish the upper linkage of newly added edges/nodes */
+	ret = btrfs_backref_finish_upper_links(cache, node);
+	if (ret < 0) {
+		err = ret;
+		goto out;
+	}
+
+	if (handle_useless_nodes(rc, node))
+		node = NULL;
+out:
+	btrfs_backref_iter_free(iter);
+	btrfs_free_path(path);
+	if (err) {
+		btrfs_backref_error_cleanup(cache, node);
+		return ERR_PTR(err);
+	}
+	ASSERT(!node || !node->detached);
+	ASSERT(list_empty(&cache->useless_node) &&
+	       list_empty(&cache->pending_edge));
+	return node;
+}
+
+/*
+ * helper to add backref node for the newly created snapshot.
+ * the backref node is created by cloning backref node that
+ * corresponds to root of source tree
+ */
+static int clone_backref_node(struct btrfs_trans_handle *trans,
+			      struct reloc_control *rc,
+			      struct btrfs_root *src,
+			      struct btrfs_root *dest)
+{
+	struct btrfs_root *reloc_root = src->reloc_root;
+	struct btrfs_backref_cache *cache = &rc->backref_cache;
+	struct btrfs_backref_node *node = NULL;
+	struct btrfs_backref_node *new_node;
+	struct btrfs_backref_edge *edge;
+	struct btrfs_backref_edge *new_edge;
+	struct rb_node *rb_node;
+
+	if (cache->last_trans > 0)
+		update_backref_cache(trans, cache);
+
+	rb_node = rb_simple_search(&cache->rb_root, src->commit_root->start);
+	if (rb_node) {
+		node = rb_entry(rb_node, struct btrfs_backref_node, rb_node);
+		if (node->detached)
+			node = NULL;
+		else
+			BUG_ON(node->new_bytenr != reloc_root->node->start);
+	}
+
+	if (!node) {
+		rb_node = rb_simple_search(&cache->rb_root,
+					   reloc_root->commit_root->start);
+		if (rb_node) {
+			node = rb_entry(rb_node, struct btrfs_backref_node,
+					rb_node);
+			BUG_ON(node->detached);
+		}
+	}
+
+	if (!node)
+		return 0;
+
+	new_node = btrfs_backref_alloc_node(cache, dest->node->start,
+					    node->level);
+	if (!new_node)
+		return -ENOMEM;
+
+	new_node->lowest = node->lowest;
+	new_node->checked = 1;
+	new_node->root = btrfs_grab_root(dest);
+	ASSERT(new_node->root);
+
+	if (!node->lowest) {
+		list_for_each_entry(edge, &node->lower, list[UPPER]) {
+			new_edge = btrfs_backref_alloc_edge(cache);
+			if (!new_edge)
+				goto fail;
+
+			btrfs_backref_link_edge(new_edge, edge->node[LOWER],
+						new_node, LINK_UPPER);
+		}
+	} else {
+		list_add_tail(&new_node->lower, &cache->leaves);
+	}
+
+	rb_node = rb_simple_insert(&cache->rb_root, new_node->bytenr,
+				   &new_node->rb_node);
+	if (rb_node)
+		btrfs_backref_panic(trans->fs_info, new_node->bytenr, -EEXIST);
+
+	if (!new_node->lowest) {
+		list_for_each_entry(new_edge, &new_node->lower, list[UPPER]) {
+			list_add_tail(&new_edge->list[LOWER],
+				      &new_edge->node[LOWER]->upper);
+		}
+	}
+	return 0;
+fail:
+	while (!list_empty(&new_node->lower)) {
+		new_edge = list_entry(new_node->lower.next,
+				      struct btrfs_backref_edge, list[UPPER]);
+		list_del(&new_edge->list[UPPER]);
+		btrfs_backref_free_edge(cache, new_edge);
+	}
+	btrfs_backref_free_node(cache, new_node);
+	return -ENOMEM;
+}
+
+/*
+ * helper to add 'address of tree root -> reloc tree' mapping
+ */
+static int __must_check __add_reloc_root(struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct rb_node *rb_node;
+	struct mapping_node *node;
+	struct reloc_control *rc = fs_info->reloc_ctl;
+
+	node = kmalloc(sizeof(*node), GFP_NOFS);
+	if (!node)
+		return -ENOMEM;
+
+	node->bytenr = root->commit_root->start;
+	node->data = root;
+
+	spin_lock(&rc->reloc_root_tree.lock);
+	rb_node = rb_simple_insert(&rc->reloc_root_tree.rb_root,
+				   node->bytenr, &node->rb_node);
+	spin_unlock(&rc->reloc_root_tree.lock);
+	if (rb_node) {
+		btrfs_err(fs_info,
+			    "Duplicate root found for start=%llu while inserting into relocation tree",
+			    node->bytenr);
+		return -EEXIST;
+	}
+
+	list_add_tail(&root->root_list, &rc->reloc_roots);
+	return 0;
+}
+
+/*
+ * helper to delete the 'address of tree root -> reloc tree'
+ * mapping
+ */
+static void __del_reloc_root(struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct rb_node *rb_node;
+	struct mapping_node *node = NULL;
+	struct reloc_control *rc = fs_info->reloc_ctl;
+	bool put_ref = false;
+
+	if (rc && root->node) {
+		spin_lock(&rc->reloc_root_tree.lock);
+		rb_node = rb_simple_search(&rc->reloc_root_tree.rb_root,
+					   root->commit_root->start);
+		if (rb_node) {
+			node = rb_entry(rb_node, struct mapping_node, rb_node);
+			rb_erase(&node->rb_node, &rc->reloc_root_tree.rb_root);
+			RB_CLEAR_NODE(&node->rb_node);
+		}
+		spin_unlock(&rc->reloc_root_tree.lock);
+		ASSERT(!node || (struct btrfs_root *)node->data == root);
+	}
+
+	/*
+	 * We only put the reloc root here if it's on the list.  There's a lot
+	 * of places where the pattern is to splice the rc->reloc_roots, process
+	 * the reloc roots, and then add the reloc root back onto
+	 * rc->reloc_roots.  If we call __del_reloc_root while it's off of the
+	 * list we don't want the reference being dropped, because the guy
+	 * messing with the list is in charge of the reference.
+	 */
+	spin_lock(&fs_info->trans_lock);
+	if (!list_empty(&root->root_list)) {
+		put_ref = true;
+		list_del_init(&root->root_list);
+	}
+	spin_unlock(&fs_info->trans_lock);
+	if (put_ref)
+		btrfs_put_root(root);
+	kfree(node);
+}
+
+/*
+ * helper to update the 'address of tree root -> reloc tree'
+ * mapping
+ */
+static int __update_reloc_root(struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct rb_node *rb_node;
+	struct mapping_node *node = NULL;
+	struct reloc_control *rc = fs_info->reloc_ctl;
+
+	spin_lock(&rc->reloc_root_tree.lock);
+	rb_node = rb_simple_search(&rc->reloc_root_tree.rb_root,
+				   root->commit_root->start);
+	if (rb_node) {
+		node = rb_entry(rb_node, struct mapping_node, rb_node);
+		rb_erase(&node->rb_node, &rc->reloc_root_tree.rb_root);
+	}
+	spin_unlock(&rc->reloc_root_tree.lock);
+
+	if (!node)
+		return 0;
+	BUG_ON((struct btrfs_root *)node->data != root);
+
+	spin_lock(&rc->reloc_root_tree.lock);
+	node->bytenr = root->node->start;
+	rb_node = rb_simple_insert(&rc->reloc_root_tree.rb_root,
+				   node->bytenr, &node->rb_node);
+	spin_unlock(&rc->reloc_root_tree.lock);
+	if (rb_node)
+		btrfs_backref_panic(fs_info, node->bytenr, -EEXIST);
+	return 0;
+}
+
+static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
+					struct btrfs_root *root, u64 objectid)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_root *reloc_root;
+	struct extent_buffer *eb;
+	struct btrfs_root_item *root_item;
+	struct btrfs_key root_key;
+	int ret = 0;
+	bool must_abort = false;
+
+	root_item = kmalloc(sizeof(*root_item), GFP_NOFS);
+	if (!root_item)
+		return ERR_PTR(-ENOMEM);
+
+	root_key.objectid = BTRFS_TREE_RELOC_OBJECTID;
+	root_key.type = BTRFS_ROOT_ITEM_KEY;
+	root_key.offset = objectid;
+
+	if (root->root_key.objectid == objectid) {
+		u64 commit_root_gen;
+
+		/* called by btrfs_init_reloc_root */
+		ret = btrfs_copy_root(trans, root, root->commit_root, &eb,
+				      BTRFS_TREE_RELOC_OBJECTID);
+		if (ret)
+			goto fail;
+
+		/*
+		 * Set the last_snapshot field to the generation of the commit
+		 * root - like this ctree.c:btrfs_block_can_be_shared() behaves
+		 * correctly (returns true) when the relocation root is created
+		 * either inside the critical section of a transaction commit
+		 * (through transaction.c:qgroup_account_snapshot()) and when
+		 * it's created before the transaction commit is started.
+		 */
+		commit_root_gen = btrfs_header_generation(root->commit_root);
+		btrfs_set_root_last_snapshot(&root->root_item, commit_root_gen);
+	} else {
+		/*
+		 * called by btrfs_reloc_post_snapshot_hook.
+		 * the source tree is a reloc tree, all tree blocks
+		 * modified after it was created have RELOC flag
+		 * set in their headers. so it's OK to not update
+		 * the 'last_snapshot'.
+		 */
+		ret = btrfs_copy_root(trans, root, root->node, &eb,
+				      BTRFS_TREE_RELOC_OBJECTID);
+		if (ret)
+			goto fail;
+	}
+
+	/*
+	 * We have changed references at this point, we must abort the
+	 * transaction if anything fails.
+	 */
+	must_abort = true;
+
+	memcpy(root_item, &root->root_item, sizeof(*root_item));
+	btrfs_set_root_bytenr(root_item, eb->start);
+	btrfs_set_root_level(root_item, btrfs_header_level(eb));
+	btrfs_set_root_generation(root_item, trans->transid);
+
+	if (root->root_key.objectid == objectid) {
+		btrfs_set_root_refs(root_item, 0);
+		memset(&root_item->drop_progress, 0,
+		       sizeof(struct btrfs_disk_key));
+		btrfs_set_root_drop_level(root_item, 0);
+	}
+
+	btrfs_tree_unlock(eb);
+	free_extent_buffer(eb);
+
+	ret = btrfs_insert_root(trans, fs_info->tree_root,
+				&root_key, root_item);
+	if (ret)
+		goto fail;
+
+	kfree(root_item);
+
+	reloc_root = btrfs_read_tree_root(fs_info->tree_root, &root_key);
+	if (IS_ERR(reloc_root)) {
+		ret = PTR_ERR(reloc_root);
+		goto abort;
+	}
+	set_bit(BTRFS_ROOT_SHAREABLE, &reloc_root->state);
+	reloc_root->last_trans = trans->transid;
+	return reloc_root;
+fail:
+	kfree(root_item);
+abort:
+	if (must_abort)
+		btrfs_abort_transaction(trans, ret);
+	return ERR_PTR(ret);
+}
+
+/*
+ * create reloc tree for a given fs tree. reloc tree is just a
+ * snapshot of the fs tree with special root objectid.
+ *
+ * The reloc_root comes out of here with two references, one for
+ * root->reloc_root, and another for being on the rc->reloc_roots list.
+ */
+int btrfs_init_reloc_root(struct btrfs_trans_handle *trans,
+			  struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_root *reloc_root;
+	struct reloc_control *rc = fs_info->reloc_ctl;
+	struct btrfs_block_rsv *rsv;
+	int clear_rsv = 0;
+	int ret;
+
+	if (!rc)
+		return 0;
+
+	/*
+	 * The subvolume has reloc tree but the swap is finished, no need to
+	 * create/update the dead reloc tree
+	 */
+	if (reloc_root_is_dead(root))
+		return 0;
+
+	/*
+	 * This is subtle but important.  We do not do
+	 * record_root_in_transaction for reloc roots, instead we record their
+	 * corresponding fs root, and then here we update the last trans for the
+	 * reloc root.  This means that we have to do this for the entire life
+	 * of the reloc root, regardless of which stage of the relocation we are
+	 * in.
+	 */
+	if (root->reloc_root) {
+		reloc_root = root->reloc_root;
+		reloc_root->last_trans = trans->transid;
+		return 0;
+	}
+
+	/*
+	 * We are merging reloc roots, we do not need new reloc trees.  Also
+	 * reloc trees never need their own reloc tree.
+	 */
+	if (!rc->create_reloc_tree ||
+	    root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID)
+		return 0;
+
+	if (!trans->reloc_reserved) {
+		rsv = trans->block_rsv;
+		trans->block_rsv = rc->block_rsv;
+		clear_rsv = 1;
+	}
+	reloc_root = create_reloc_root(trans, root, root->root_key.objectid);
+	if (clear_rsv)
+		trans->block_rsv = rsv;
+	if (IS_ERR(reloc_root))
+		return PTR_ERR(reloc_root);
+
+	ret = __add_reloc_root(reloc_root);
+	ASSERT(ret != -EEXIST);
+	if (ret) {
+		/* Pairs with create_reloc_root */
+		btrfs_put_root(reloc_root);
+		return ret;
+	}
+	root->reloc_root = btrfs_grab_root(reloc_root);
+	return 0;
+}
+
+/*
+ * update root item of reloc tree
+ */
+int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
+			    struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_root *reloc_root;
+	struct btrfs_root_item *root_item;
+	int ret;
+
+	if (!have_reloc_root(root))
+		return 0;
+
+	reloc_root = root->reloc_root;
+	root_item = &reloc_root->root_item;
+
+	/*
+	 * We are probably ok here, but __del_reloc_root() will drop its ref of
+	 * the root.  We have the ref for root->reloc_root, but just in case
+	 * hold it while we update the reloc root.
+	 */
+	btrfs_grab_root(reloc_root);
+
+	/* root->reloc_root will stay until current relocation finished */
+	if (fs_info->reloc_ctl->merge_reloc_tree &&
+	    btrfs_root_refs(root_item) == 0) {
+		set_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
+		/*
+		 * Mark the tree as dead before we change reloc_root so
+		 * have_reloc_root will not touch it from now on.
+		 */
+		smp_wmb();
+		__del_reloc_root(reloc_root);
+	}
+
+	if (reloc_root->commit_root != reloc_root->node) {
+		__update_reloc_root(reloc_root);
+		btrfs_set_root_node(root_item, reloc_root->node);
+		free_extent_buffer(reloc_root->commit_root);
+		reloc_root->commit_root = btrfs_root_node(reloc_root);
+	}
+
+	ret = btrfs_update_root(trans, fs_info->tree_root,
+				&reloc_root->root_key, root_item);
+	btrfs_put_root(reloc_root);
+	return ret;
+}
+
+/*
+ * helper to find first cached inode with inode number >= objectid
+ * in a subvolume
+ */
+static struct inode *find_next_inode(struct btrfs_root *root, u64 objectid)
+{
+	struct rb_node *node;
+	struct rb_node *prev;
+	struct btrfs_inode *entry;
+	struct inode *inode;
+
+	spin_lock(&root->inode_lock);
+again:
+	node = root->inode_tree.rb_node;
+	prev = NULL;
+	while (node) {
+		prev = node;
+		entry = rb_entry(node, struct btrfs_inode, rb_node);
+
+		if (objectid < btrfs_ino(entry))
+			node = node->rb_left;
+		else if (objectid > btrfs_ino(entry))
+			node = node->rb_right;
+		else
+			break;
+	}
+	if (!node) {
+		while (prev) {
+			entry = rb_entry(prev, struct btrfs_inode, rb_node);
+			if (objectid <= btrfs_ino(entry)) {
+				node = prev;
+				break;
+			}
+			prev = rb_next(prev);
+		}
+	}
+	while (node) {
+		entry = rb_entry(node, struct btrfs_inode, rb_node);
+		inode = igrab(&entry->vfs_inode);
+		if (inode) {
+			spin_unlock(&root->inode_lock);
+			return inode;
+		}
+
+		objectid = btrfs_ino(entry) + 1;
+		if (cond_resched_lock(&root->inode_lock))
+			goto again;
+
+		node = rb_next(node);
+	}
+	spin_unlock(&root->inode_lock);
+	return NULL;
+}
+
+/*
+ * get new location of data
+ */
+static int get_new_location(struct inode *reloc_inode, u64 *new_bytenr,
+			    u64 bytenr, u64 num_bytes)
+{
+	struct btrfs_root *root = BTRFS_I(reloc_inode)->root;
+	struct btrfs_path *path;
+	struct btrfs_file_extent_item *fi;
+	struct extent_buffer *leaf;
+	int ret;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	bytenr -= BTRFS_I(reloc_inode)->index_cnt;
+	ret = btrfs_lookup_file_extent(NULL, root, path,
+			btrfs_ino(BTRFS_I(reloc_inode)), bytenr, 0);
+	if (ret < 0)
+		goto out;
+	if (ret > 0) {
+		ret = -ENOENT;
+		goto out;
+	}
+
+	leaf = path->nodes[0];
+	fi = btrfs_item_ptr(leaf, path->slots[0],
+			    struct btrfs_file_extent_item);
+
+	BUG_ON(btrfs_file_extent_offset(leaf, fi) ||
+	       btrfs_file_extent_compression(leaf, fi) ||
+	       btrfs_file_extent_encryption(leaf, fi) ||
+	       btrfs_file_extent_other_encoding(leaf, fi));
+
+	if (num_bytes != btrfs_file_extent_disk_num_bytes(leaf, fi)) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	*new_bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
+	ret = 0;
+out:
+	btrfs_free_path(path);
+	return ret;
+}
+
+/*
+ * update file extent items in the tree leaf to point to
+ * the new locations.
+ */
+static noinline_for_stack
+int replace_file_extents(struct btrfs_trans_handle *trans,
+			 struct reloc_control *rc,
+			 struct btrfs_root *root,
+			 struct extent_buffer *leaf)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_key key;
+	struct btrfs_file_extent_item *fi;
+	struct inode *inode = NULL;
+	u64 parent;
+	u64 bytenr;
+	u64 new_bytenr = 0;
+	u64 num_bytes;
+	u64 end;
+	u32 nritems;
+	u32 i;
+	int ret = 0;
+	int first = 1;
+	int dirty = 0;
+
+	if (rc->stage != UPDATE_DATA_PTRS)
+		return 0;
+
+	/* reloc trees always use full backref */
+	if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID)
+		parent = leaf->start;
+	else
+		parent = 0;
+
+	nritems = btrfs_header_nritems(leaf);
+	for (i = 0; i < nritems; i++) {
+		struct btrfs_ref ref = { 0 };
+
+		cond_resched();
+		btrfs_item_key_to_cpu(leaf, &key, i);
+		if (key.type != BTRFS_EXTENT_DATA_KEY)
+			continue;
+		fi = btrfs_item_ptr(leaf, i, struct btrfs_file_extent_item);
+		if (btrfs_file_extent_type(leaf, fi) ==
+		    BTRFS_FILE_EXTENT_INLINE)
+			continue;
+		bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
+		num_bytes = btrfs_file_extent_disk_num_bytes(leaf, fi);
+		if (bytenr == 0)
+			continue;
+		if (!in_range(bytenr, rc->block_group->start,
+			      rc->block_group->length))
+			continue;
+
+		/*
+		 * if we are modifying block in fs tree, wait for read_folio
+		 * to complete and drop the extent cache
+		 */
+		if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID) {
+			if (first) {
+				inode = find_next_inode(root, key.objectid);
+				first = 0;
+			} else if (inode && btrfs_ino(BTRFS_I(inode)) < key.objectid) {
+				btrfs_add_delayed_iput(inode);
+				inode = find_next_inode(root, key.objectid);
+			}
+			if (inode && btrfs_ino(BTRFS_I(inode)) == key.objectid) {
+				end = key.offset +
+				      btrfs_file_extent_num_bytes(leaf, fi);
+				WARN_ON(!IS_ALIGNED(key.offset,
+						    fs_info->sectorsize));
+				WARN_ON(!IS_ALIGNED(end, fs_info->sectorsize));
+				end--;
+				ret = try_lock_extent(&BTRFS_I(inode)->io_tree,
+						      key.offset, end);
+				if (!ret)
+					continue;
+
+				btrfs_drop_extent_map_range(BTRFS_I(inode),
+							    key.offset, end, true);
+				unlock_extent(&BTRFS_I(inode)->io_tree,
+					      key.offset, end, NULL);
+			}
+		}
+
+		ret = get_new_location(rc->data_inode, &new_bytenr,
+				       bytenr, num_bytes);
+		if (ret) {
+			/*
+			 * Don't have to abort since we've not changed anything
+			 * in the file extent yet.
+			 */
+			break;
+		}
+
+		btrfs_set_file_extent_disk_bytenr(leaf, fi, new_bytenr);
+		dirty = 1;
+
+		key.offset -= btrfs_file_extent_offset(leaf, fi);
+		btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF, new_bytenr,
+				       num_bytes, parent);
+		btrfs_init_data_ref(&ref, btrfs_header_owner(leaf),
+				    key.objectid, key.offset,
+				    root->root_key.objectid, false);
+		ret = btrfs_inc_extent_ref(trans, &ref);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			break;
+		}
+
+		btrfs_init_generic_ref(&ref, BTRFS_DROP_DELAYED_REF, bytenr,
+				       num_bytes, parent);
+		btrfs_init_data_ref(&ref, btrfs_header_owner(leaf),
+				    key.objectid, key.offset,
+				    root->root_key.objectid, false);
+		ret = btrfs_free_extent(trans, &ref);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			break;
+		}
+	}
+	if (dirty)
+		btrfs_mark_buffer_dirty(leaf);
+	if (inode)
+		btrfs_add_delayed_iput(inode);
+	return ret;
+}
+
+static noinline_for_stack
+int memcmp_node_keys(struct extent_buffer *eb, int slot,
+		     struct btrfs_path *path, int level)
+{
+	struct btrfs_disk_key key1;
+	struct btrfs_disk_key key2;
+	btrfs_node_key(eb, &key1, slot);
+	btrfs_node_key(path->nodes[level], &key2, path->slots[level]);
+	return memcmp(&key1, &key2, sizeof(key1));
+}
+
+/*
+ * try to replace tree blocks in fs tree with the new blocks
+ * in reloc tree. tree blocks haven't been modified since the
+ * reloc tree was create can be replaced.
+ *
+ * if a block was replaced, level of the block + 1 is returned.
+ * if no block got replaced, 0 is returned. if there are other
+ * errors, a negative error number is returned.
+ */
+static noinline_for_stack
+int replace_path(struct btrfs_trans_handle *trans, struct reloc_control *rc,
+		 struct btrfs_root *dest, struct btrfs_root *src,
+		 struct btrfs_path *path, struct btrfs_key *next_key,
+		 int lowest_level, int max_level)
+{
+	struct btrfs_fs_info *fs_info = dest->fs_info;
+	struct extent_buffer *eb;
+	struct extent_buffer *parent;
+	struct btrfs_ref ref = { 0 };
+	struct btrfs_key key;
+	u64 old_bytenr;
+	u64 new_bytenr;
+	u64 old_ptr_gen;
+	u64 new_ptr_gen;
+	u64 last_snapshot;
+	u32 blocksize;
+	int cow = 0;
+	int level;
+	int ret;
+	int slot;
+
+	ASSERT(src->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID);
+	ASSERT(dest->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID);
+
+	last_snapshot = btrfs_root_last_snapshot(&src->root_item);
+again:
+	slot = path->slots[lowest_level];
+	btrfs_node_key_to_cpu(path->nodes[lowest_level], &key, slot);
+
+	eb = btrfs_lock_root_node(dest);
+	level = btrfs_header_level(eb);
+
+	if (level < lowest_level) {
+		btrfs_tree_unlock(eb);
+		free_extent_buffer(eb);
+		return 0;
+	}
+
+	if (cow) {
+		ret = btrfs_cow_block(trans, dest, eb, NULL, 0, &eb,
+				      BTRFS_NESTING_COW);
+		if (ret) {
+			btrfs_tree_unlock(eb);
+			free_extent_buffer(eb);
+			return ret;
+		}
+	}
+
+	if (next_key) {
+		next_key->objectid = (u64)-1;
+		next_key->type = (u8)-1;
+		next_key->offset = (u64)-1;
+	}
+
+	parent = eb;
+	while (1) {
+		level = btrfs_header_level(parent);
+		ASSERT(level >= lowest_level);
+
+		ret = btrfs_bin_search(parent, &key, &slot);
+		if (ret < 0)
+			break;
+		if (ret && slot > 0)
+			slot--;
+
+		if (next_key && slot + 1 < btrfs_header_nritems(parent))
+			btrfs_node_key_to_cpu(parent, next_key, slot + 1);
+
+		old_bytenr = btrfs_node_blockptr(parent, slot);
+		blocksize = fs_info->nodesize;
+		old_ptr_gen = btrfs_node_ptr_generation(parent, slot);
+
+		if (level <= max_level) {
+			eb = path->nodes[level];
+			new_bytenr = btrfs_node_blockptr(eb,
+							path->slots[level]);
+			new_ptr_gen = btrfs_node_ptr_generation(eb,
+							path->slots[level]);
+		} else {
+			new_bytenr = 0;
+			new_ptr_gen = 0;
+		}
+
+		if (WARN_ON(new_bytenr > 0 && new_bytenr == old_bytenr)) {
+			ret = level;
+			break;
+		}
+
+		if (new_bytenr == 0 || old_ptr_gen > last_snapshot ||
+		    memcmp_node_keys(parent, slot, path, level)) {
+			if (level <= lowest_level) {
+				ret = 0;
+				break;
+			}
+
+			eb = btrfs_read_node_slot(parent, slot);
+			if (IS_ERR(eb)) {
+				ret = PTR_ERR(eb);
+				break;
+			}
+			btrfs_tree_lock(eb);
+			if (cow) {
+				ret = btrfs_cow_block(trans, dest, eb, parent,
+						      slot, &eb,
+						      BTRFS_NESTING_COW);
+				if (ret) {
+					btrfs_tree_unlock(eb);
+					free_extent_buffer(eb);
+					break;
+				}
+			}
+
+			btrfs_tree_unlock(parent);
+			free_extent_buffer(parent);
+
+			parent = eb;
+			continue;
+		}
+
+		if (!cow) {
+			btrfs_tree_unlock(parent);
+			free_extent_buffer(parent);
+			cow = 1;
+			goto again;
+		}
+
+		btrfs_node_key_to_cpu(path->nodes[level], &key,
+				      path->slots[level]);
+		btrfs_release_path(path);
+
+		path->lowest_level = level;
+		set_bit(BTRFS_ROOT_RESET_LOCKDEP_CLASS, &src->state);
+		ret = btrfs_search_slot(trans, src, &key, path, 0, 1);
+		clear_bit(BTRFS_ROOT_RESET_LOCKDEP_CLASS, &src->state);
+		path->lowest_level = 0;
+		if (ret) {
+			if (ret > 0)
+				ret = -ENOENT;
+			break;
+		}
+
+		/*
+		 * Info qgroup to trace both subtrees.
+		 *
+		 * We must trace both trees.
+		 * 1) Tree reloc subtree
+		 *    If not traced, we will leak data numbers
+		 * 2) Fs subtree
+		 *    If not traced, we will double count old data
+		 *
+		 * We don't scan the subtree right now, but only record
+		 * the swapped tree blocks.
+		 * The real subtree rescan is delayed until we have new
+		 * CoW on the subtree root node before transaction commit.
+		 */
+		ret = btrfs_qgroup_add_swapped_blocks(trans, dest,
+				rc->block_group, parent, slot,
+				path->nodes[level], path->slots[level],
+				last_snapshot);
+		if (ret < 0)
+			break;
+		/*
+		 * swap blocks in fs tree and reloc tree.
+		 */
+		btrfs_set_node_blockptr(parent, slot, new_bytenr);
+		btrfs_set_node_ptr_generation(parent, slot, new_ptr_gen);
+		btrfs_mark_buffer_dirty(parent);
+
+		btrfs_set_node_blockptr(path->nodes[level],
+					path->slots[level], old_bytenr);
+		btrfs_set_node_ptr_generation(path->nodes[level],
+					      path->slots[level], old_ptr_gen);
+		btrfs_mark_buffer_dirty(path->nodes[level]);
+
+		btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF, old_bytenr,
+				       blocksize, path->nodes[level]->start);
+		btrfs_init_tree_ref(&ref, level - 1, src->root_key.objectid,
+				    0, true);
+		ret = btrfs_inc_extent_ref(trans, &ref);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			break;
+		}
+		btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF, new_bytenr,
+				       blocksize, 0);
+		btrfs_init_tree_ref(&ref, level - 1, dest->root_key.objectid, 0,
+				    true);
+		ret = btrfs_inc_extent_ref(trans, &ref);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			break;
+		}
+
+		btrfs_init_generic_ref(&ref, BTRFS_DROP_DELAYED_REF, new_bytenr,
+				       blocksize, path->nodes[level]->start);
+		btrfs_init_tree_ref(&ref, level - 1, src->root_key.objectid,
+				    0, true);
+		ret = btrfs_free_extent(trans, &ref);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			break;
+		}
+
+		btrfs_init_generic_ref(&ref, BTRFS_DROP_DELAYED_REF, old_bytenr,
+				       blocksize, 0);
+		btrfs_init_tree_ref(&ref, level - 1, dest->root_key.objectid,
+				    0, true);
+		ret = btrfs_free_extent(trans, &ref);
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			break;
+		}
+
+		btrfs_unlock_up_safe(path, 0);
+
+		ret = level;
+		break;
+	}
+	btrfs_tree_unlock(parent);
+	free_extent_buffer(parent);
+	return ret;
+}
+
+/*
+ * helper to find next relocated block in reloc tree
+ */
+static noinline_for_stack
+int walk_up_reloc_tree(struct btrfs_root *root, struct btrfs_path *path,
+		       int *level)
+{
+	struct extent_buffer *eb;
+	int i;
+	u64 last_snapshot;
+	u32 nritems;
+
+	last_snapshot = btrfs_root_last_snapshot(&root->root_item);
+
+	for (i = 0; i < *level; i++) {
+		free_extent_buffer(path->nodes[i]);
+		path->nodes[i] = NULL;
+	}
+
+	for (i = *level; i < BTRFS_MAX_LEVEL && path->nodes[i]; i++) {
+		eb = path->nodes[i];
+		nritems = btrfs_header_nritems(eb);
+		while (path->slots[i] + 1 < nritems) {
+			path->slots[i]++;
+			if (btrfs_node_ptr_generation(eb, path->slots[i]) <=
+			    last_snapshot)
+				continue;
+
+			*level = i;
+			return 0;
+		}
+		free_extent_buffer(path->nodes[i]);
+		path->nodes[i] = NULL;
+	}
+	return 1;
+}
+
+/*
+ * walk down reloc tree to find relocated block of lowest level
+ */
+static noinline_for_stack
+int walk_down_reloc_tree(struct btrfs_root *root, struct btrfs_path *path,
+			 int *level)
+{
+	struct extent_buffer *eb = NULL;
+	int i;
+	u64 ptr_gen = 0;
+	u64 last_snapshot;
+	u32 nritems;
+
+	last_snapshot = btrfs_root_last_snapshot(&root->root_item);
+
+	for (i = *level; i > 0; i--) {
+		eb = path->nodes[i];
+		nritems = btrfs_header_nritems(eb);
+		while (path->slots[i] < nritems) {
+			ptr_gen = btrfs_node_ptr_generation(eb, path->slots[i]);
+			if (ptr_gen > last_snapshot)
+				break;
+			path->slots[i]++;
+		}
+		if (path->slots[i] >= nritems) {
+			if (i == *level)
+				break;
+			*level = i + 1;
+			return 0;
+		}
+		if (i == 1) {
+			*level = i;
+			return 0;
+		}
+
+		eb = btrfs_read_node_slot(eb, path->slots[i]);
+		if (IS_ERR(eb))
+			return PTR_ERR(eb);
+		BUG_ON(btrfs_header_level(eb) != i - 1);
+		path->nodes[i - 1] = eb;
+		path->slots[i - 1] = 0;
+	}
+	return 1;
+}
+
+/*
+ * invalidate extent cache for file extents whose key in range of
+ * [min_key, max_key)
+ */
+static int invalidate_extent_cache(struct btrfs_root *root,
+				   struct btrfs_key *min_key,
+				   struct btrfs_key *max_key)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct inode *inode = NULL;
+	u64 objectid;
+	u64 start, end;
+	u64 ino;
+
+	objectid = min_key->objectid;
+	while (1) {
+		cond_resched();
+		iput(inode);
+
+		if (objectid > max_key->objectid)
+			break;
+
+		inode = find_next_inode(root, objectid);
+		if (!inode)
+			break;
+		ino = btrfs_ino(BTRFS_I(inode));
+
+		if (ino > max_key->objectid) {
+			iput(inode);
+			break;
+		}
+
+		objectid = ino + 1;
+		if (!S_ISREG(inode->i_mode))
+			continue;
+
+		if (unlikely(min_key->objectid == ino)) {
+			if (min_key->type > BTRFS_EXTENT_DATA_KEY)
+				continue;
+			if (min_key->type < BTRFS_EXTENT_DATA_KEY)
+				start = 0;
+			else {
+				start = min_key->offset;
+				WARN_ON(!IS_ALIGNED(start, fs_info->sectorsize));
+			}
+		} else {
+			start = 0;
+		}
+
+		if (unlikely(max_key->objectid == ino)) {
+			if (max_key->type < BTRFS_EXTENT_DATA_KEY)
+				continue;
+			if (max_key->type > BTRFS_EXTENT_DATA_KEY) {
+				end = (u64)-1;
+			} else {
+				if (max_key->offset == 0)
+					continue;
+				end = max_key->offset;
+				WARN_ON(!IS_ALIGNED(end, fs_info->sectorsize));
+				end--;
+			}
+		} else {
+			end = (u64)-1;
+		}
+
+		/* the lock_extent waits for read_folio to complete */
+		lock_extent(&BTRFS_I(inode)->io_tree, start, end, NULL);
+		btrfs_drop_extent_map_range(BTRFS_I(inode), start, end, true);
+		unlock_extent(&BTRFS_I(inode)->io_tree, start, end, NULL);
+	}
+	return 0;
+}
+
+static int find_next_key(struct btrfs_path *path, int level,
+			 struct btrfs_key *key)
+
+{
+	while (level < BTRFS_MAX_LEVEL) {
+		if (!path->nodes[level])
+			break;
+		if (path->slots[level] + 1 <
+		    btrfs_header_nritems(path->nodes[level])) {
+			btrfs_node_key_to_cpu(path->nodes[level], key,
+					      path->slots[level] + 1);
+			return 0;
+		}
+		level++;
+	}
+	return 1;
+}
+
+/*
+ * Insert current subvolume into reloc_control::dirty_subvol_roots
+ */
+static int insert_dirty_subvol(struct btrfs_trans_handle *trans,
+			       struct reloc_control *rc,
+			       struct btrfs_root *root)
+{
+	struct btrfs_root *reloc_root = root->reloc_root;
+	struct btrfs_root_item *reloc_root_item;
+	int ret;
+
+	/* @root must be a subvolume tree root with a valid reloc tree */
+	ASSERT(root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID);
+	ASSERT(reloc_root);
+
+	reloc_root_item = &reloc_root->root_item;
+	memset(&reloc_root_item->drop_progress, 0,
+		sizeof(reloc_root_item->drop_progress));
+	btrfs_set_root_drop_level(reloc_root_item, 0);
+	btrfs_set_root_refs(reloc_root_item, 0);
+	ret = btrfs_update_reloc_root(trans, root);
+	if (ret)
+		return ret;
+
+	if (list_empty(&root->reloc_dirty_list)) {
+		btrfs_grab_root(root);
+		list_add_tail(&root->reloc_dirty_list, &rc->dirty_subvol_roots);
+	}
+
+	return 0;
+}
+
+static int clean_dirty_subvols(struct reloc_control *rc)
+{
+	struct btrfs_root *root;
+	struct btrfs_root *next;
+	int ret = 0;
+	int ret2;
+
+	list_for_each_entry_safe(root, next, &rc->dirty_subvol_roots,
+				 reloc_dirty_list) {
+		if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID) {
+			/* Merged subvolume, cleanup its reloc root */
+			struct btrfs_root *reloc_root = root->reloc_root;
+
+			list_del_init(&root->reloc_dirty_list);
+			root->reloc_root = NULL;
+			/*
+			 * Need barrier to ensure clear_bit() only happens after
+			 * root->reloc_root = NULL. Pairs with have_reloc_root.
+			 */
+			smp_wmb();
+			clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
+			if (reloc_root) {
+				/*
+				 * btrfs_drop_snapshot drops our ref we hold for
+				 * ->reloc_root.  If it fails however we must
+				 * drop the ref ourselves.
+				 */
+				ret2 = btrfs_drop_snapshot(reloc_root, 0, 1);
+				if (ret2 < 0) {
+					btrfs_put_root(reloc_root);
+					if (!ret)
+						ret = ret2;
+				}
+			}
+			btrfs_put_root(root);
+		} else {
+			/* Orphan reloc tree, just clean it up */
+			ret2 = btrfs_drop_snapshot(root, 0, 1);
+			if (ret2 < 0) {
+				btrfs_put_root(root);
+				if (!ret)
+					ret = ret2;
+			}
+		}
+	}
+	return ret;
+}
+
+/*
+ * merge the relocated tree blocks in reloc tree with corresponding
+ * fs tree.
+ */
+static noinline_for_stack int merge_reloc_root(struct reloc_control *rc,
+					       struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct btrfs_key key;
+	struct btrfs_key next_key;
+	struct btrfs_trans_handle *trans = NULL;
+	struct btrfs_root *reloc_root;
+	struct btrfs_root_item *root_item;
+	struct btrfs_path *path;
+	struct extent_buffer *leaf;
+	int reserve_level;
+	int level;
+	int max_level;
+	int replaced = 0;
+	int ret = 0;
+	u32 min_reserved;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+	path->reada = READA_FORWARD;
+
+	reloc_root = root->reloc_root;
+	root_item = &reloc_root->root_item;
+
+	if (btrfs_disk_key_objectid(&root_item->drop_progress) == 0) {
+		level = btrfs_root_level(root_item);
+		atomic_inc(&reloc_root->node->refs);
+		path->nodes[level] = reloc_root->node;
+		path->slots[level] = 0;
+	} else {
+		btrfs_disk_key_to_cpu(&key, &root_item->drop_progress);
+
+		level = btrfs_root_drop_level(root_item);
+		BUG_ON(level == 0);
+		path->lowest_level = level;
+		ret = btrfs_search_slot(NULL, reloc_root, &key, path, 0, 0);
+		path->lowest_level = 0;
+		if (ret < 0) {
+			btrfs_free_path(path);
+			return ret;
+		}
+
+		btrfs_node_key_to_cpu(path->nodes[level], &next_key,
+				      path->slots[level]);
+		WARN_ON(memcmp(&key, &next_key, sizeof(key)));
+
+		btrfs_unlock_up_safe(path, 0);
+	}
+
+	/*
+	 * In merge_reloc_root(), we modify the upper level pointer to swap the
+	 * tree blocks between reloc tree and subvolume tree.  Thus for tree
+	 * block COW, we COW at most from level 1 to root level for each tree.
+	 *
+	 * Thus the needed metadata size is at most root_level * nodesize,
+	 * and * 2 since we have two trees to COW.
+	 */
+	reserve_level = max_t(int, 1, btrfs_root_level(root_item));
+	min_reserved = fs_info->nodesize * reserve_level * 2;
+	memset(&next_key, 0, sizeof(next_key));
+
+	while (1) {
+		ret = btrfs_block_rsv_refill(fs_info, rc->block_rsv,
+					     min_reserved,
+					     BTRFS_RESERVE_FLUSH_LIMIT);
+		if (ret)
+			goto out;
+		trans = btrfs_start_transaction(root, 0);
+		if (IS_ERR(trans)) {
+			ret = PTR_ERR(trans);
+			trans = NULL;
+			goto out;
+		}
+
+		/*
+		 * At this point we no longer have a reloc_control, so we can't
+		 * depend on btrfs_init_reloc_root to update our last_trans.
+		 *
+		 * But that's ok, we started the trans handle on our
+		 * corresponding fs_root, which means it's been added to the
+		 * dirty list.  At commit time we'll still call
+		 * btrfs_update_reloc_root() and update our root item
+		 * appropriately.
+		 */
+		reloc_root->last_trans = trans->transid;
+		trans->block_rsv = rc->block_rsv;
+
+		replaced = 0;
+		max_level = level;
+
+		ret = walk_down_reloc_tree(reloc_root, path, &level);
+		if (ret < 0)
+			goto out;
+		if (ret > 0)
+			break;
+
+		if (!find_next_key(path, level, &key) &&
+		    btrfs_comp_cpu_keys(&next_key, &key) >= 0) {
+			ret = 0;
+		} else {
+			ret = replace_path(trans, rc, root, reloc_root, path,
+					   &next_key, level, max_level);
+		}
+		if (ret < 0)
+			goto out;
+		if (ret > 0) {
+			level = ret;
+			btrfs_node_key_to_cpu(path->nodes[level], &key,
+					      path->slots[level]);
+			replaced = 1;
+		}
+
+		ret = walk_up_reloc_tree(reloc_root, path, &level);
+		if (ret > 0)
+			break;
+
+		BUG_ON(level == 0);
+		/*
+		 * save the merging progress in the drop_progress.
+		 * this is OK since root refs == 1 in this case.
+		 */
+		btrfs_node_key(path->nodes[level], &root_item->drop_progress,
+			       path->slots[level]);
+		btrfs_set_root_drop_level(root_item, level);
+
+		btrfs_end_transaction_throttle(trans);
+		trans = NULL;
+
+		btrfs_btree_balance_dirty(fs_info);
+
+		if (replaced && rc->stage == UPDATE_DATA_PTRS)
+			invalidate_extent_cache(root, &key, &next_key);
+	}
+
+	/*
+	 * handle the case only one block in the fs tree need to be
+	 * relocated and the block is tree root.
+	 */
+	leaf = btrfs_lock_root_node(root);
+	ret = btrfs_cow_block(trans, root, leaf, NULL, 0, &leaf,
+			      BTRFS_NESTING_COW);
+	btrfs_tree_unlock(leaf);
+	free_extent_buffer(leaf);
+out:
+	btrfs_free_path(path);
+
+	if (ret == 0) {
+		ret = insert_dirty_subvol(trans, rc, root);
+		if (ret)
+			btrfs_abort_transaction(trans, ret);
+	}
+
+	if (trans)
+		btrfs_end_transaction_throttle(trans);
+
+	btrfs_btree_balance_dirty(fs_info);
+
+	if (replaced && rc->stage == UPDATE_DATA_PTRS)
+		invalidate_extent_cache(root, &key, &next_key);
+
+	return ret;
+}
+
+static noinline_for_stack
+int prepare_to_merge(struct reloc_control *rc, int err)
+{
+	struct btrfs_root *root = rc->extent_root;
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_root *reloc_root;
+	struct btrfs_trans_handle *trans;
+	LIST_HEAD(reloc_roots);
+	u64 num_bytes = 0;
+	int ret;
+
+	mutex_lock(&fs_info->reloc_mutex);
+	rc->merging_rsv_size += fs_info->nodesize * (BTRFS_MAX_LEVEL - 1) * 2;
+	rc->merging_rsv_size += rc->nodes_relocated * 2;
+	mutex_unlock(&fs_info->reloc_mutex);
+
+again:
+	if (!err) {
+		num_bytes = rc->merging_rsv_size;
+		ret = btrfs_block_rsv_add(fs_info, rc->block_rsv, num_bytes,
+					  BTRFS_RESERVE_FLUSH_ALL);
+		if (ret)
+			err = ret;
+	}
+
+	trans = btrfs_join_transaction(rc->extent_root);
+	if (IS_ERR(trans)) {
+		if (!err)
+			btrfs_block_rsv_release(fs_info, rc->block_rsv,
+						num_bytes, NULL);
+		return PTR_ERR(trans);
+	}
+
+	if (!err) {
+		if (num_bytes != rc->merging_rsv_size) {
+			btrfs_end_transaction(trans);
+			btrfs_block_rsv_release(fs_info, rc->block_rsv,
+						num_bytes, NULL);
+			goto again;
+		}
+	}
+
+	rc->merge_reloc_tree = 1;
+
+	while (!list_empty(&rc->reloc_roots)) {
+		reloc_root = list_entry(rc->reloc_roots.next,
+					struct btrfs_root, root_list);
+		list_del_init(&reloc_root->root_list);
+
+		root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
+				false);
+		if (IS_ERR(root)) {
+			/*
+			 * Even if we have an error we need this reloc root
+			 * back on our list so we can clean up properly.
+			 */
+			list_add(&reloc_root->root_list, &reloc_roots);
+			btrfs_abort_transaction(trans, (int)PTR_ERR(root));
+			if (!err)
+				err = PTR_ERR(root);
+			break;
+		}
+
+		if (unlikely(root->reloc_root != reloc_root)) {
+			if (root->reloc_root) {
+				btrfs_err(fs_info,
+"reloc tree mismatch, root %lld has reloc root key (%lld %u %llu) gen %llu, expect reloc root key (%lld %u %llu) gen %llu",
+					  root->root_key.objectid,
+					  root->reloc_root->root_key.objectid,
+					  root->reloc_root->root_key.type,
+					  root->reloc_root->root_key.offset,
+					  btrfs_root_generation(
+						  &root->reloc_root->root_item),
+					  reloc_root->root_key.objectid,
+					  reloc_root->root_key.type,
+					  reloc_root->root_key.offset,
+					  btrfs_root_generation(
+						  &reloc_root->root_item));
+			} else {
+				btrfs_err(fs_info,
+"reloc tree mismatch, root %lld has no reloc root, expect reloc root key (%lld %u %llu) gen %llu",
+					  root->root_key.objectid,
+					  reloc_root->root_key.objectid,
+					  reloc_root->root_key.type,
+					  reloc_root->root_key.offset,
+					  btrfs_root_generation(
+						  &reloc_root->root_item));
+			}
+			list_add(&reloc_root->root_list, &reloc_roots);
+			btrfs_put_root(root);
+			btrfs_abort_transaction(trans, -EUCLEAN);
+			if (!err)
+				err = -EUCLEAN;
+			break;
+		}
+
+		/*
+		 * set reference count to 1, so btrfs_recover_relocation
+		 * knows it should resumes merging
+		 */
+		if (!err)
+			btrfs_set_root_refs(&reloc_root->root_item, 1);
+		ret = btrfs_update_reloc_root(trans, root);
+
+		/*
+		 * Even if we have an error we need this reloc root back on our
+		 * list so we can clean up properly.
+		 */
+		list_add(&reloc_root->root_list, &reloc_roots);
+		btrfs_put_root(root);
+
+		if (ret) {
+			btrfs_abort_transaction(trans, ret);
+			if (!err)
+				err = ret;
+			break;
+		}
+	}
+
+	list_splice(&reloc_roots, &rc->reloc_roots);
+
+	if (!err)
+		err = btrfs_commit_transaction(trans);
+	else
+		btrfs_end_transaction(trans);
+	return err;
+}
+
+static noinline_for_stack
+void free_reloc_roots(struct list_head *list)
+{
+	struct btrfs_root *reloc_root, *tmp;
+
+	list_for_each_entry_safe(reloc_root, tmp, list, root_list)
+		__del_reloc_root(reloc_root);
+}
+
+static noinline_for_stack
+void merge_reloc_roots(struct reloc_control *rc)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct btrfs_root *root;
+	struct btrfs_root *reloc_root;
+	LIST_HEAD(reloc_roots);
+	int found = 0;
+	int ret = 0;
+again:
+	root = rc->extent_root;
+
+	/*
+	 * this serializes us with btrfs_record_root_in_transaction,
+	 * we have to make sure nobody is in the middle of
+	 * adding their roots to the list while we are
+	 * doing this splice
+	 */
+	mutex_lock(&fs_info->reloc_mutex);
+	list_splice_init(&rc->reloc_roots, &reloc_roots);
+	mutex_unlock(&fs_info->reloc_mutex);
+
+	while (!list_empty(&reloc_roots)) {
+		found = 1;
+		reloc_root = list_entry(reloc_roots.next,
+					struct btrfs_root, root_list);
+
+		root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
+					 false);
+		if (btrfs_root_refs(&reloc_root->root_item) > 0) {
+			if (WARN_ON(IS_ERR(root))) {
+				/*
+				 * For recovery we read the fs roots on mount,
+				 * and if we didn't find the root then we marked
+				 * the reloc root as a garbage root.  For normal
+				 * relocation obviously the root should exist in
+				 * memory.  However there's no reason we can't
+				 * handle the error properly here just in case.
+				 */
+				ret = PTR_ERR(root);
+				goto out;
+			}
+			if (WARN_ON(root->reloc_root != reloc_root)) {
+				/*
+				 * This can happen if on-disk metadata has some
+				 * corruption, e.g. bad reloc tree key offset.
+				 */
+				ret = -EINVAL;
+				goto out;
+			}
+			ret = merge_reloc_root(rc, root);
+			btrfs_put_root(root);
+			if (ret) {
+				if (list_empty(&reloc_root->root_list))
+					list_add_tail(&reloc_root->root_list,
+						      &reloc_roots);
+				goto out;
+			}
+		} else {
+			if (!IS_ERR(root)) {
+				if (root->reloc_root == reloc_root) {
+					root->reloc_root = NULL;
+					btrfs_put_root(reloc_root);
+				}
+				clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE,
+					  &root->state);
+				btrfs_put_root(root);
+			}
+
+			list_del_init(&reloc_root->root_list);
+			/* Don't forget to queue this reloc root for cleanup */
+			list_add_tail(&reloc_root->reloc_dirty_list,
+				      &rc->dirty_subvol_roots);
+		}
+	}
+
+	if (found) {
+		found = 0;
+		goto again;
+	}
+out:
+	if (ret) {
+		btrfs_handle_fs_error(fs_info, ret, NULL);
+		free_reloc_roots(&reloc_roots);
+
+		/* new reloc root may be added */
+		mutex_lock(&fs_info->reloc_mutex);
+		list_splice_init(&rc->reloc_roots, &reloc_roots);
+		mutex_unlock(&fs_info->reloc_mutex);
+		free_reloc_roots(&reloc_roots);
+	}
+
+	/*
+	 * We used to have
+	 *
+	 * BUG_ON(!RB_EMPTY_ROOT(&rc->reloc_root_tree.rb_root));
+	 *
+	 * here, but it's wrong.  If we fail to start the transaction in
+	 * prepare_to_merge() we will have only 0 ref reloc roots, none of which
+	 * have actually been removed from the reloc_root_tree rb tree.  This is
+	 * fine because we're bailing here, and we hold a reference on the root
+	 * for the list that holds it, so these roots will be cleaned up when we
+	 * do the reloc_dirty_list afterwards.  Meanwhile the root->reloc_root
+	 * will be cleaned up on unmount.
+	 *
+	 * The remaining nodes will be cleaned up by free_reloc_control.
+	 */
+}
+
+static void free_block_list(struct rb_root *blocks)
+{
+	struct tree_block *block;
+	struct rb_node *rb_node;
+	while ((rb_node = rb_first(blocks))) {
+		block = rb_entry(rb_node, struct tree_block, rb_node);
+		rb_erase(rb_node, blocks);
+		kfree(block);
+	}
+}
+
+static int record_reloc_root_in_trans(struct btrfs_trans_handle *trans,
+				      struct btrfs_root *reloc_root)
+{
+	struct btrfs_fs_info *fs_info = reloc_root->fs_info;
+	struct btrfs_root *root;
+	int ret;
+
+	if (reloc_root->last_trans == trans->transid)
+		return 0;
+
+	root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset, false);
+
+	/*
+	 * This should succeed, since we can't have a reloc root without having
+	 * already looked up the actual root and created the reloc root for this
+	 * root.
+	 *
+	 * However if there's some sort of corruption where we have a ref to a
+	 * reloc root without a corresponding root this could return ENOENT.
+	 */
+	if (IS_ERR(root)) {
+		ASSERT(0);
+		return PTR_ERR(root);
+	}
+	if (root->reloc_root != reloc_root) {
+		ASSERT(0);
+		btrfs_err(fs_info,
+			  "root %llu has two reloc roots associated with it",
+			  reloc_root->root_key.offset);
+		btrfs_put_root(root);
+		return -EUCLEAN;
+	}
+	ret = btrfs_record_root_in_trans(trans, root);
+	btrfs_put_root(root);
+
+	return ret;
+}
+
+static noinline_for_stack
+struct btrfs_root *select_reloc_root(struct btrfs_trans_handle *trans,
+				     struct reloc_control *rc,
+				     struct btrfs_backref_node *node,
+				     struct btrfs_backref_edge *edges[])
+{
+	struct btrfs_backref_node *next;
+	struct btrfs_root *root;
+	int index = 0;
+	int ret;
+
+	next = node;
+	while (1) {
+		cond_resched();
+		next = walk_up_backref(next, edges, &index);
+		root = next->root;
+
+		/*
+		 * If there is no root, then our references for this block are
+		 * incomplete, as we should be able to walk all the way up to a
+		 * block that is owned by a root.
+		 *
+		 * This path is only for SHAREABLE roots, so if we come upon a
+		 * non-SHAREABLE root then we have backrefs that resolve
+		 * improperly.
+		 *
+		 * Both of these cases indicate file system corruption, or a bug
+		 * in the backref walking code.
+		 */
+		if (!root) {
+			ASSERT(0);
+			btrfs_err(trans->fs_info,
+		"bytenr %llu doesn't have a backref path ending in a root",
+				  node->bytenr);
+			return ERR_PTR(-EUCLEAN);
+		}
+		if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state)) {
+			ASSERT(0);
+			btrfs_err(trans->fs_info,
+	"bytenr %llu has multiple refs with one ending in a non-shareable root",
+				  node->bytenr);
+			return ERR_PTR(-EUCLEAN);
+		}
+
+		if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID) {
+			ret = record_reloc_root_in_trans(trans, root);
+			if (ret)
+				return ERR_PTR(ret);
+			break;
+		}
+
+		ret = btrfs_record_root_in_trans(trans, root);
+		if (ret)
+			return ERR_PTR(ret);
+		root = root->reloc_root;
+
+		/*
+		 * We could have raced with another thread which failed, so
+		 * root->reloc_root may not be set, return ENOENT in this case.
+		 */
+		if (!root)
+			return ERR_PTR(-ENOENT);
+
+		if (next->new_bytenr != root->node->start) {
+			/*
+			 * We just created the reloc root, so we shouldn't have
+			 * ->new_bytenr set and this shouldn't be in the changed
+			 *  list.  If it is then we have multiple roots pointing
+			 *  at the same bytenr which indicates corruption, or
+			 *  we've made a mistake in the backref walking code.
+			 */
+			ASSERT(next->new_bytenr == 0);
+			ASSERT(list_empty(&next->list));
+			if (next->new_bytenr || !list_empty(&next->list)) {
+				btrfs_err(trans->fs_info,
+	"bytenr %llu possibly has multiple roots pointing at the same bytenr %llu",
+					  node->bytenr, next->bytenr);
+				return ERR_PTR(-EUCLEAN);
+			}
+
+			next->new_bytenr = root->node->start;
+			btrfs_put_root(next->root);
+			next->root = btrfs_grab_root(root);
+			ASSERT(next->root);
+			list_add_tail(&next->list,
+				      &rc->backref_cache.changed);
+			mark_block_processed(rc, next);
+			break;
+		}
+
+		WARN_ON(1);
+		root = NULL;
+		next = walk_down_backref(edges, &index);
+		if (!next || next->level <= node->level)
+			break;
+	}
+	if (!root) {
+		/*
+		 * This can happen if there's fs corruption or if there's a bug
+		 * in the backref lookup code.
+		 */
+		ASSERT(0);
+		return ERR_PTR(-ENOENT);
+	}
+
+	next = node;
+	/* setup backref node path for btrfs_reloc_cow_block */
+	while (1) {
+		rc->backref_cache.path[next->level] = next;
+		if (--index < 0)
+			break;
+		next = edges[index]->node[UPPER];
+	}
+	return root;
+}
+
+/*
+ * Select a tree root for relocation.
+ *
+ * Return NULL if the block is not shareable. We should use do_relocation() in
+ * this case.
+ *
+ * Return a tree root pointer if the block is shareable.
+ * Return -ENOENT if the block is root of reloc tree.
+ */
+static noinline_for_stack
+struct btrfs_root *select_one_root(struct btrfs_backref_node *node)
+{
+	struct btrfs_backref_node *next;
+	struct btrfs_root *root;
+	struct btrfs_root *fs_root = NULL;
+	struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
+	int index = 0;
+
+	next = node;
+	while (1) {
+		cond_resched();
+		next = walk_up_backref(next, edges, &index);
+		root = next->root;
+
+		/*
+		 * This can occur if we have incomplete extent refs leading all
+		 * the way up a particular path, in this case return -EUCLEAN.
+		 */
+		if (!root)
+			return ERR_PTR(-EUCLEAN);
+
+		/* No other choice for non-shareable tree */
+		if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
+			return root;
+
+		if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID)
+			fs_root = root;
+
+		if (next != node)
+			return NULL;
+
+		next = walk_down_backref(edges, &index);
+		if (!next || next->level <= node->level)
+			break;
+	}
+
+	if (!fs_root)
+		return ERR_PTR(-ENOENT);
+	return fs_root;
+}
+
+static noinline_for_stack
+u64 calcu_metadata_size(struct reloc_control *rc,
+			struct btrfs_backref_node *node, int reserve)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct btrfs_backref_node *next = node;
+	struct btrfs_backref_edge *edge;
+	struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
+	u64 num_bytes = 0;
+	int index = 0;
+
+	BUG_ON(reserve && node->processed);
+
+	while (next) {
+		cond_resched();
+		while (1) {
+			if (next->processed && (reserve || next != node))
+				break;
+
+			num_bytes += fs_info->nodesize;
+
+			if (list_empty(&next->upper))
+				break;
+
+			edge = list_entry(next->upper.next,
+					struct btrfs_backref_edge, list[LOWER]);
+			edges[index++] = edge;
+			next = edge->node[UPPER];
+		}
+		next = walk_down_backref(edges, &index);
+	}
+	return num_bytes;
+}
+
+static int reserve_metadata_space(struct btrfs_trans_handle *trans,
+				  struct reloc_control *rc,
+				  struct btrfs_backref_node *node)
+{
+	struct btrfs_root *root = rc->extent_root;
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	u64 num_bytes;
+	int ret;
+	u64 tmp;
+
+	num_bytes = calcu_metadata_size(rc, node, 1) * 2;
+
+	trans->block_rsv = rc->block_rsv;
+	rc->reserved_bytes += num_bytes;
+
+	/*
+	 * We are under a transaction here so we can only do limited flushing.
+	 * If we get an enospc just kick back -EAGAIN so we know to drop the
+	 * transaction and try to refill when we can flush all the things.
+	 */
+	ret = btrfs_block_rsv_refill(fs_info, rc->block_rsv, num_bytes,
+				     BTRFS_RESERVE_FLUSH_LIMIT);
+	if (ret) {
+		tmp = fs_info->nodesize * RELOCATION_RESERVED_NODES;
+		while (tmp <= rc->reserved_bytes)
+			tmp <<= 1;
+		/*
+		 * only one thread can access block_rsv at this point,
+		 * so we don't need hold lock to protect block_rsv.
+		 * we expand more reservation size here to allow enough
+		 * space for relocation and we will return earlier in
+		 * enospc case.
+		 */
+		rc->block_rsv->size = tmp + fs_info->nodesize *
+				      RELOCATION_RESERVED_NODES;
+		return -EAGAIN;
+	}
+
+	return 0;
+}
+
+/*
+ * relocate a block tree, and then update pointers in upper level
+ * blocks that reference the block to point to the new location.
+ *
+ * if called by link_to_upper, the block has already been relocated.
+ * in that case this function just updates pointers.
+ */
+static int do_relocation(struct btrfs_trans_handle *trans,
+			 struct reloc_control *rc,
+			 struct btrfs_backref_node *node,
+			 struct btrfs_key *key,
+			 struct btrfs_path *path, int lowest)
+{
+	struct btrfs_backref_node *upper;
+	struct btrfs_backref_edge *edge;
+	struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
+	struct btrfs_root *root;
+	struct extent_buffer *eb;
+	u32 blocksize;
+	u64 bytenr;
+	int slot;
+	int ret = 0;
+
+	/*
+	 * If we are lowest then this is the first time we're processing this
+	 * block, and thus shouldn't have an eb associated with it yet.
+	 */
+	ASSERT(!lowest || !node->eb);
+
+	path->lowest_level = node->level + 1;
+	rc->backref_cache.path[node->level] = node;
+	list_for_each_entry(edge, &node->upper, list[LOWER]) {
+		struct btrfs_ref ref = { 0 };
+
+		cond_resched();
+
+		upper = edge->node[UPPER];
+		root = select_reloc_root(trans, rc, upper, edges);
+		if (IS_ERR(root)) {
+			ret = PTR_ERR(root);
+			goto next;
+		}
+
+		if (upper->eb && !upper->locked) {
+			if (!lowest) {
+				ret = btrfs_bin_search(upper->eb, key, &slot);
+				if (ret < 0)
+					goto next;
+				BUG_ON(ret);
+				bytenr = btrfs_node_blockptr(upper->eb, slot);
+				if (node->eb->start == bytenr)
+					goto next;
+			}
+			btrfs_backref_drop_node_buffer(upper);
+		}
+
+		if (!upper->eb) {
+			ret = btrfs_search_slot(trans, root, key, path, 0, 1);
+			if (ret) {
+				if (ret > 0)
+					ret = -ENOENT;
+
+				btrfs_release_path(path);
+				break;
+			}
+
+			if (!upper->eb) {
+				upper->eb = path->nodes[upper->level];
+				path->nodes[upper->level] = NULL;
+			} else {
+				BUG_ON(upper->eb != path->nodes[upper->level]);
+			}
+
+			upper->locked = 1;
+			path->locks[upper->level] = 0;
+
+			slot = path->slots[upper->level];
+			btrfs_release_path(path);
+		} else {
+			ret = btrfs_bin_search(upper->eb, key, &slot);
+			if (ret < 0)
+				goto next;
+			BUG_ON(ret);
+		}
+
+		bytenr = btrfs_node_blockptr(upper->eb, slot);
+		if (lowest) {
+			if (bytenr != node->bytenr) {
+				btrfs_err(root->fs_info,
+		"lowest leaf/node mismatch: bytenr %llu node->bytenr %llu slot %d upper %llu",
+					  bytenr, node->bytenr, slot,
+					  upper->eb->start);
+				ret = -EIO;
+				goto next;
+			}
+		} else {
+			if (node->eb->start == bytenr)
+				goto next;
+		}
+
+		blocksize = root->fs_info->nodesize;
+		eb = btrfs_read_node_slot(upper->eb, slot);
+		if (IS_ERR(eb)) {
+			ret = PTR_ERR(eb);
+			goto next;
+		}
+		btrfs_tree_lock(eb);
+
+		if (!node->eb) {
+			ret = btrfs_cow_block(trans, root, eb, upper->eb,
+					      slot, &eb, BTRFS_NESTING_COW);
+			btrfs_tree_unlock(eb);
+			free_extent_buffer(eb);
+			if (ret < 0)
+				goto next;
+			/*
+			 * We've just COWed this block, it should have updated
+			 * the correct backref node entry.
+			 */
+			ASSERT(node->eb == eb);
+		} else {
+			btrfs_set_node_blockptr(upper->eb, slot,
+						node->eb->start);
+			btrfs_set_node_ptr_generation(upper->eb, slot,
+						      trans->transid);
+			btrfs_mark_buffer_dirty(upper->eb);
+
+			btrfs_init_generic_ref(&ref, BTRFS_ADD_DELAYED_REF,
+					       node->eb->start, blocksize,
+					       upper->eb->start);
+			btrfs_init_tree_ref(&ref, node->level,
+					    btrfs_header_owner(upper->eb),
+					    root->root_key.objectid, false);
+			ret = btrfs_inc_extent_ref(trans, &ref);
+			if (!ret)
+				ret = btrfs_drop_subtree(trans, root, eb,
+							 upper->eb);
+			if (ret)
+				btrfs_abort_transaction(trans, ret);
+		}
+next:
+		if (!upper->pending)
+			btrfs_backref_drop_node_buffer(upper);
+		else
+			btrfs_backref_unlock_node_buffer(upper);
+		if (ret)
+			break;
+	}
+
+	if (!ret && node->pending) {
+		btrfs_backref_drop_node_buffer(node);
+		list_move_tail(&node->list, &rc->backref_cache.changed);
+		node->pending = 0;
+	}
+
+	path->lowest_level = 0;
+
+	/*
+	 * We should have allocated all of our space in the block rsv and thus
+	 * shouldn't ENOSPC.
+	 */
+	ASSERT(ret != -ENOSPC);
+	return ret;
+}
+
+static int link_to_upper(struct btrfs_trans_handle *trans,
+			 struct reloc_control *rc,
+			 struct btrfs_backref_node *node,
+			 struct btrfs_path *path)
+{
+	struct btrfs_key key;
+
+	btrfs_node_key_to_cpu(node->eb, &key, 0);
+	return do_relocation(trans, rc, node, &key, path, 0);
+}
+
+static int finish_pending_nodes(struct btrfs_trans_handle *trans,
+				struct reloc_control *rc,
+				struct btrfs_path *path, int err)
+{
+	LIST_HEAD(list);
+	struct btrfs_backref_cache *cache = &rc->backref_cache;
+	struct btrfs_backref_node *node;
+	int level;
+	int ret;
+
+	for (level = 0; level < BTRFS_MAX_LEVEL; level++) {
+		while (!list_empty(&cache->pending[level])) {
+			node = list_entry(cache->pending[level].next,
+					  struct btrfs_backref_node, list);
+			list_move_tail(&node->list, &list);
+			BUG_ON(!node->pending);
+
+			if (!err) {
+				ret = link_to_upper(trans, rc, node, path);
+				if (ret < 0)
+					err = ret;
+			}
+		}
+		list_splice_init(&list, &cache->pending[level]);
+	}
+	return err;
+}
+
+/*
+ * mark a block and all blocks directly/indirectly reference the block
+ * as processed.
+ */
+static void update_processed_blocks(struct reloc_control *rc,
+				    struct btrfs_backref_node *node)
+{
+	struct btrfs_backref_node *next = node;
+	struct btrfs_backref_edge *edge;
+	struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
+	int index = 0;
+
+	while (next) {
+		cond_resched();
+		while (1) {
+			if (next->processed)
+				break;
+
+			mark_block_processed(rc, next);
+
+			if (list_empty(&next->upper))
+				break;
+
+			edge = list_entry(next->upper.next,
+					struct btrfs_backref_edge, list[LOWER]);
+			edges[index++] = edge;
+			next = edge->node[UPPER];
+		}
+		next = walk_down_backref(edges, &index);
+	}
+}
+
+static int tree_block_processed(u64 bytenr, struct reloc_control *rc)
+{
+	u32 blocksize = rc->extent_root->fs_info->nodesize;
+
+	if (test_range_bit(&rc->processed_blocks, bytenr,
+			   bytenr + blocksize - 1, EXTENT_DIRTY, 1, NULL))
+		return 1;
+	return 0;
+}
+
+static int get_tree_block_key(struct btrfs_fs_info *fs_info,
+			      struct tree_block *block)
+{
+	struct extent_buffer *eb;
+
+	eb = read_tree_block(fs_info, block->bytenr, block->owner,
+			     block->key.offset, block->level, NULL);
+	if (IS_ERR(eb))
+		return PTR_ERR(eb);
+	if (!extent_buffer_uptodate(eb)) {
+		free_extent_buffer(eb);
+		return -EIO;
+	}
+	if (block->level == 0)
+		btrfs_item_key_to_cpu(eb, &block->key, 0);
+	else
+		btrfs_node_key_to_cpu(eb, &block->key, 0);
+	free_extent_buffer(eb);
+	block->key_ready = 1;
+	return 0;
+}
+
+/*
+ * helper function to relocate a tree block
+ */
+static int relocate_tree_block(struct btrfs_trans_handle *trans,
+				struct reloc_control *rc,
+				struct btrfs_backref_node *node,
+				struct btrfs_key *key,
+				struct btrfs_path *path)
+{
+	struct btrfs_root *root;
+	int ret = 0;
+
+	if (!node)
+		return 0;
+
+	/*
+	 * If we fail here we want to drop our backref_node because we are going
+	 * to start over and regenerate the tree for it.
+	 */
+	ret = reserve_metadata_space(trans, rc, node);
+	if (ret)
+		goto out;
+
+	BUG_ON(node->processed);
+	root = select_one_root(node);
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
+
+		/* See explanation in select_one_root for the -EUCLEAN case. */
+		ASSERT(ret == -ENOENT);
+		if (ret == -ENOENT) {
+			ret = 0;
+			update_processed_blocks(rc, node);
+		}
+		goto out;
+	}
+
+	if (root) {
+		if (test_bit(BTRFS_ROOT_SHAREABLE, &root->state)) {
+			/*
+			 * This block was the root block of a root, and this is
+			 * the first time we're processing the block and thus it
+			 * should not have had the ->new_bytenr modified and
+			 * should have not been included on the changed list.
+			 *
+			 * However in the case of corruption we could have
+			 * multiple refs pointing to the same block improperly,
+			 * and thus we would trip over these checks.  ASSERT()
+			 * for the developer case, because it could indicate a
+			 * bug in the backref code, however error out for a
+			 * normal user in the case of corruption.
+			 */
+			ASSERT(node->new_bytenr == 0);
+			ASSERT(list_empty(&node->list));
+			if (node->new_bytenr || !list_empty(&node->list)) {
+				btrfs_err(root->fs_info,
+				  "bytenr %llu has improper references to it",
+					  node->bytenr);
+				ret = -EUCLEAN;
+				goto out;
+			}
+			ret = btrfs_record_root_in_trans(trans, root);
+			if (ret)
+				goto out;
+			/*
+			 * Another thread could have failed, need to check if we
+			 * have reloc_root actually set.
+			 */
+			if (!root->reloc_root) {
+				ret = -ENOENT;
+				goto out;
+			}
+			root = root->reloc_root;
+			node->new_bytenr = root->node->start;
+			btrfs_put_root(node->root);
+			node->root = btrfs_grab_root(root);
+			ASSERT(node->root);
+			list_add_tail(&node->list, &rc->backref_cache.changed);
+		} else {
+			path->lowest_level = node->level;
+			if (root == root->fs_info->chunk_root)
+				btrfs_reserve_chunk_metadata(trans, false);
+			ret = btrfs_search_slot(trans, root, key, path, 0, 1);
+			btrfs_release_path(path);
+			if (root == root->fs_info->chunk_root)
+				btrfs_trans_release_chunk_metadata(trans);
+			if (ret > 0)
+				ret = 0;
+		}
+		if (!ret)
+			update_processed_blocks(rc, node);
+	} else {
+		ret = do_relocation(trans, rc, node, key, path, 1);
+	}
+out:
+	if (ret || node->level == 0 || node->cowonly)
+		btrfs_backref_cleanup_node(&rc->backref_cache, node);
+	return ret;
+}
+
+/*
+ * relocate a list of blocks
+ */
+static noinline_for_stack
+int relocate_tree_blocks(struct btrfs_trans_handle *trans,
+			 struct reloc_control *rc, struct rb_root *blocks)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct btrfs_backref_node *node;
+	struct btrfs_path *path;
+	struct tree_block *block;
+	struct tree_block *next;
+	int ret;
+	int err = 0;
+
+	path = btrfs_alloc_path();
+	if (!path) {
+		err = -ENOMEM;
+		goto out_free_blocks;
+	}
+
+	/* Kick in readahead for tree blocks with missing keys */
+	rbtree_postorder_for_each_entry_safe(block, next, blocks, rb_node) {
+		if (!block->key_ready)
+			btrfs_readahead_tree_block(fs_info, block->bytenr,
+						   block->owner, 0,
+						   block->level);
+	}
+
+	/* Get first keys */
+	rbtree_postorder_for_each_entry_safe(block, next, blocks, rb_node) {
+		if (!block->key_ready) {
+			err = get_tree_block_key(fs_info, block);
+			if (err)
+				goto out_free_path;
+		}
+	}
+
+	/* Do tree relocation */
+	rbtree_postorder_for_each_entry_safe(block, next, blocks, rb_node) {
+		node = build_backref_tree(rc, &block->key,
+					  block->level, block->bytenr);
+		if (IS_ERR(node)) {
+			err = PTR_ERR(node);
+			goto out;
+		}
+
+		ret = relocate_tree_block(trans, rc, node, &block->key,
+					  path);
+		if (ret < 0) {
+			err = ret;
+			break;
+		}
+	}
+out:
+	err = finish_pending_nodes(trans, rc, path, err);
+
+out_free_path:
+	btrfs_free_path(path);
+out_free_blocks:
+	free_block_list(blocks);
+	return err;
+}
+
+static noinline_for_stack int prealloc_file_extent_cluster(
+				struct btrfs_inode *inode,
+				struct file_extent_cluster *cluster)
+{
+	u64 alloc_hint = 0;
+	u64 start;
+	u64 end;
+	u64 offset = inode->index_cnt;
+	u64 num_bytes;
+	int nr;
+	int ret = 0;
+	u64 i_size = i_size_read(&inode->vfs_inode);
+	u64 prealloc_start = cluster->start - offset;
+	u64 prealloc_end = cluster->end - offset;
+	u64 cur_offset = prealloc_start;
+
+	/*
+	 * For subpage case, previous i_size may not be aligned to PAGE_SIZE.
+	 * This means the range [i_size, PAGE_END + 1) is filled with zeros by
+	 * btrfs_do_readpage() call of previously relocated file cluster.
+	 *
+	 * If the current cluster starts in the above range, btrfs_do_readpage()
+	 * will skip the read, and relocate_one_page() will later writeback
+	 * the padding zeros as new data, causing data corruption.
+	 *
+	 * Here we have to manually invalidate the range (i_size, PAGE_END + 1).
+	 */
+	if (!IS_ALIGNED(i_size, PAGE_SIZE)) {
+		struct address_space *mapping = inode->vfs_inode.i_mapping;
+		struct btrfs_fs_info *fs_info = inode->root->fs_info;
+		const u32 sectorsize = fs_info->sectorsize;
+		struct page *page;
+
+		ASSERT(sectorsize < PAGE_SIZE);
+		ASSERT(IS_ALIGNED(i_size, sectorsize));
+
+		/*
+		 * Subpage can't handle page with DIRTY but without UPTODATE
+		 * bit as it can lead to the following deadlock:
+		 *
+		 * btrfs_read_folio()
+		 * | Page already *locked*
+		 * |- btrfs_lock_and_flush_ordered_range()
+		 *    |- btrfs_start_ordered_extent()
+		 *       |- extent_write_cache_pages()
+		 *          |- lock_page()
+		 *             We try to lock the page we already hold.
+		 *
+		 * Here we just writeback the whole data reloc inode, so that
+		 * we will be ensured to have no dirty range in the page, and
+		 * are safe to clear the uptodate bits.
+		 *
+		 * This shouldn't cause too much overhead, as we need to write
+		 * the data back anyway.
+		 */
+		ret = filemap_write_and_wait(mapping);
+		if (ret < 0)
+			return ret;
+
+		clear_extent_bits(&inode->io_tree, i_size,
+				  round_up(i_size, PAGE_SIZE) - 1,
+				  EXTENT_UPTODATE);
+		page = find_lock_page(mapping, i_size >> PAGE_SHIFT);
+		/*
+		 * If page is freed we don't need to do anything then, as we
+		 * will re-read the whole page anyway.
+		 */
+		if (page) {
+			btrfs_subpage_clear_uptodate(fs_info, page, i_size,
+					round_up(i_size, PAGE_SIZE) - i_size);
+			unlock_page(page);
+			put_page(page);
+		}
+	}
+
+	BUG_ON(cluster->start != cluster->boundary[0]);
+	ret = btrfs_alloc_data_chunk_ondemand(inode,
+					      prealloc_end + 1 - prealloc_start);
+	if (ret)
+		return ret;
+
+	btrfs_inode_lock(&inode->vfs_inode, 0);
+	for (nr = 0; nr < cluster->nr; nr++) {
+		start = cluster->boundary[nr] - offset;
+		if (nr + 1 < cluster->nr)
+			end = cluster->boundary[nr + 1] - 1 - offset;
+		else
+			end = cluster->end - offset;
+
+		lock_extent(&inode->io_tree, start, end, NULL);
+		num_bytes = end + 1 - start;
+		ret = btrfs_prealloc_file_range(&inode->vfs_inode, 0, start,
+						num_bytes, num_bytes,
+						end + 1, &alloc_hint);
+		cur_offset = end + 1;
+		unlock_extent(&inode->io_tree, start, end, NULL);
+		if (ret)
+			break;
+	}
+	btrfs_inode_unlock(&inode->vfs_inode, 0);
+
+	if (cur_offset < prealloc_end)
+		btrfs_free_reserved_data_space_noquota(inode->root->fs_info,
+					       prealloc_end + 1 - cur_offset);
+	return ret;
+}
+
+static noinline_for_stack int setup_relocation_extent_mapping(struct inode *inode,
+				u64 start, u64 end, u64 block_start)
+{
+	struct extent_map *em;
+	int ret = 0;
+
+	em = alloc_extent_map();
+	if (!em)
+		return -ENOMEM;
+
+	em->start = start;
+	em->len = end + 1 - start;
+	em->block_len = em->len;
+	em->block_start = block_start;
+	set_bit(EXTENT_FLAG_PINNED, &em->flags);
+
+	lock_extent(&BTRFS_I(inode)->io_tree, start, end, NULL);
+	ret = btrfs_replace_extent_map_range(BTRFS_I(inode), em, false);
+	unlock_extent(&BTRFS_I(inode)->io_tree, start, end, NULL);
+	free_extent_map(em);
+
+	return ret;
+}
+
+/*
+ * Allow error injection to test balance/relocation cancellation
+ */
+noinline int btrfs_should_cancel_balance(struct btrfs_fs_info *fs_info)
+{
+	return atomic_read(&fs_info->balance_cancel_req) ||
+		atomic_read(&fs_info->reloc_cancel_req) ||
+		fatal_signal_pending(current);
+}
+ALLOW_ERROR_INJECTION(btrfs_should_cancel_balance, TRUE);
+
+static u64 get_cluster_boundary_end(struct file_extent_cluster *cluster,
+				    int cluster_nr)
+{
+	/* Last extent, use cluster end directly */
+	if (cluster_nr >= cluster->nr - 1)
+		return cluster->end;
+
+	/* Use next boundary start*/
+	return cluster->boundary[cluster_nr + 1] - 1;
+}
+
+static int relocate_one_page(struct inode *inode, struct file_ra_state *ra,
+			     struct file_extent_cluster *cluster,
+			     int *cluster_nr, unsigned long page_index)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
+	u64 offset = BTRFS_I(inode)->index_cnt;
+	const unsigned long last_index = (cluster->end - offset) >> PAGE_SHIFT;
+	gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
+	struct page *page;
+	u64 page_start;
+	u64 page_end;
+	u64 cur;
+	int ret;
+
+	ASSERT(page_index <= last_index);
+	page = find_lock_page(inode->i_mapping, page_index);
+	if (!page) {
+		page_cache_sync_readahead(inode->i_mapping, ra, NULL,
+				page_index, last_index + 1 - page_index);
+		page = find_or_create_page(inode->i_mapping, page_index, mask);
+		if (!page)
+			return -ENOMEM;
+	}
+
+	if (PageReadahead(page))
+		page_cache_async_readahead(inode->i_mapping, ra, NULL,
+				page_folio(page), page_index,
+				last_index + 1 - page_index);
+
+	if (!PageUptodate(page)) {
+		btrfs_read_folio(NULL, page_folio(page));
+		lock_page(page);
+		if (!PageUptodate(page)) {
+			ret = -EIO;
+			goto release_page;
+		}
+	}
+
+	/*
+	 * We could have lost page private when we dropped the lock to read the
+	 * page above, make sure we set_page_extent_mapped here so we have any
+	 * of the subpage blocksize stuff we need in place.
+	 */
+	ret = set_page_extent_mapped(page);
+	if (ret < 0)
+		goto release_page;
+
+	page_start = page_offset(page);
+	page_end = page_start + PAGE_SIZE - 1;
+
+	/*
+	 * Start from the cluster, as for subpage case, the cluster can start
+	 * inside the page.
+	 */
+	cur = max(page_start, cluster->boundary[*cluster_nr] - offset);
+	while (cur <= page_end) {
+		u64 extent_start = cluster->boundary[*cluster_nr] - offset;
+		u64 extent_end = get_cluster_boundary_end(cluster,
+						*cluster_nr) - offset;
+		u64 clamped_start = max(page_start, extent_start);
+		u64 clamped_end = min(page_end, extent_end);
+		u32 clamped_len = clamped_end + 1 - clamped_start;
+
+		/* Reserve metadata for this range */
+		ret = btrfs_delalloc_reserve_metadata(BTRFS_I(inode),
+						      clamped_len, clamped_len,
+						      false);
+		if (ret)
+			goto release_page;
+
+		/* Mark the range delalloc and dirty for later writeback */
+		lock_extent(&BTRFS_I(inode)->io_tree, clamped_start, clamped_end, NULL);
+		ret = btrfs_set_extent_delalloc(BTRFS_I(inode), clamped_start,
+						clamped_end, 0, NULL);
+		if (ret) {
+			clear_extent_bits(&BTRFS_I(inode)->io_tree,
+					clamped_start, clamped_end,
+					EXTENT_LOCKED | EXTENT_BOUNDARY);
+			btrfs_delalloc_release_metadata(BTRFS_I(inode),
+							clamped_len, true);
+			btrfs_delalloc_release_extents(BTRFS_I(inode),
+						       clamped_len);
+			goto release_page;
+		}
+		btrfs_page_set_dirty(fs_info, page, clamped_start, clamped_len);
+
+		/*
+		 * Set the boundary if it's inside the page.
+		 * Data relocation requires the destination extents to have the
+		 * same size as the source.
+		 * EXTENT_BOUNDARY bit prevents current extent from being merged
+		 * with previous extent.
+		 */
+		if (in_range(cluster->boundary[*cluster_nr] - offset,
+			     page_start, PAGE_SIZE)) {
+			u64 boundary_start = cluster->boundary[*cluster_nr] -
+						offset;
+			u64 boundary_end = boundary_start +
+					   fs_info->sectorsize - 1;
+
+			set_extent_bits(&BTRFS_I(inode)->io_tree,
+					boundary_start, boundary_end,
+					EXTENT_BOUNDARY);
+		}
+		unlock_extent(&BTRFS_I(inode)->io_tree, clamped_start, clamped_end, NULL);
+		btrfs_delalloc_release_extents(BTRFS_I(inode), clamped_len);
+		cur += clamped_len;
+
+		/* Crossed extent end, go to next extent */
+		if (cur >= extent_end) {
+			(*cluster_nr)++;
+			/* Just finished the last extent of the cluster, exit. */
+			if (*cluster_nr >= cluster->nr)
+				break;
+		}
+	}
+	unlock_page(page);
+	put_page(page);
+
+	balance_dirty_pages_ratelimited(inode->i_mapping);
+	btrfs_throttle(fs_info);
+	if (btrfs_should_cancel_balance(fs_info))
+		ret = -ECANCELED;
+	return ret;
+
+release_page:
+	unlock_page(page);
+	put_page(page);
+	return ret;
+}
+
+static int relocate_file_extent_cluster(struct inode *inode,
+					struct file_extent_cluster *cluster)
+{
+	u64 offset = BTRFS_I(inode)->index_cnt;
+	unsigned long index;
+	unsigned long last_index;
+	struct file_ra_state *ra;
+	int cluster_nr = 0;
+	int ret = 0;
+
+	if (!cluster->nr)
+		return 0;
+
+	ra = kzalloc(sizeof(*ra), GFP_NOFS);
+	if (!ra)
+		return -ENOMEM;
+
+	ret = prealloc_file_extent_cluster(BTRFS_I(inode), cluster);
+	if (ret)
+		goto out;
+
+	file_ra_state_init(ra, inode->i_mapping);
+
+	ret = setup_relocation_extent_mapping(inode, cluster->start - offset,
+				   cluster->end - offset, cluster->start);
+	if (ret)
+		goto out;
+
+	last_index = (cluster->end - offset) >> PAGE_SHIFT;
+	for (index = (cluster->start - offset) >> PAGE_SHIFT;
+	     index <= last_index && !ret; index++)
+		ret = relocate_one_page(inode, ra, cluster, &cluster_nr, index);
+	if (ret == 0)
+		WARN_ON(cluster_nr != cluster->nr);
+out:
+	kfree(ra);
+	return ret;
+}
+
+static noinline_for_stack
+int relocate_data_extent(struct inode *inode, struct btrfs_key *extent_key,
+			 struct file_extent_cluster *cluster)
+{
+	int ret;
+
+	if (cluster->nr > 0 && extent_key->objectid != cluster->end + 1) {
+		ret = relocate_file_extent_cluster(inode, cluster);
+		if (ret)
+			return ret;
+		cluster->nr = 0;
+	}
+
+	if (!cluster->nr)
+		cluster->start = extent_key->objectid;
+	else
+		BUG_ON(cluster->nr >= MAX_EXTENTS);
+	cluster->end = extent_key->objectid + extent_key->offset - 1;
+	cluster->boundary[cluster->nr] = extent_key->objectid;
+	cluster->nr++;
+
+	if (cluster->nr >= MAX_EXTENTS) {
+		ret = relocate_file_extent_cluster(inode, cluster);
+		if (ret)
+			return ret;
+		cluster->nr = 0;
+	}
+	return 0;
+}
+
+/*
+ * helper to add a tree block to the list.
+ * the major work is getting the generation and level of the block
+ */
+static int add_tree_block(struct reloc_control *rc,
+			  struct btrfs_key *extent_key,
+			  struct btrfs_path *path,
+			  struct rb_root *blocks)
+{
+	struct extent_buffer *eb;
+	struct btrfs_extent_item *ei;
+	struct btrfs_tree_block_info *bi;
+	struct tree_block *block;
+	struct rb_node *rb_node;
+	u32 item_size;
+	int level = -1;
+	u64 generation;
+	u64 owner = 0;
+
+	eb =  path->nodes[0];
+	item_size = btrfs_item_size(eb, path->slots[0]);
+
+	if (extent_key->type == BTRFS_METADATA_ITEM_KEY ||
+	    item_size >= sizeof(*ei) + sizeof(*bi)) {
+		unsigned long ptr = 0, end;
+
+		ei = btrfs_item_ptr(eb, path->slots[0],
+				struct btrfs_extent_item);
+		end = (unsigned long)ei + item_size;
+		if (extent_key->type == BTRFS_EXTENT_ITEM_KEY) {
+			bi = (struct btrfs_tree_block_info *)(ei + 1);
+			level = btrfs_tree_block_level(eb, bi);
+			ptr = (unsigned long)(bi + 1);
+		} else {
+			level = (int)extent_key->offset;
+			ptr = (unsigned long)(ei + 1);
+		}
+		generation = btrfs_extent_generation(eb, ei);
+
+		/*
+		 * We're reading random blocks without knowing their owner ahead
+		 * of time.  This is ok most of the time, as all reloc roots and
+		 * fs roots have the same lock type.  However normal trees do
+		 * not, and the only way to know ahead of time is to read the
+		 * inline ref offset.  We know it's an fs root if
+		 *
+		 * 1. There's more than one ref.
+		 * 2. There's a SHARED_DATA_REF_KEY set.
+		 * 3. FULL_BACKREF is set on the flags.
+		 *
+		 * Otherwise it's safe to assume that the ref offset == the
+		 * owner of this block, so we can use that when calling
+		 * read_tree_block.
+		 */
+		if (btrfs_extent_refs(eb, ei) == 1 &&
+		    !(btrfs_extent_flags(eb, ei) &
+		      BTRFS_BLOCK_FLAG_FULL_BACKREF) &&
+		    ptr < end) {
+			struct btrfs_extent_inline_ref *iref;
+			int type;
+
+			iref = (struct btrfs_extent_inline_ref *)ptr;
+			type = btrfs_get_extent_inline_ref_type(eb, iref,
+							BTRFS_REF_TYPE_BLOCK);
+			if (type == BTRFS_REF_TYPE_INVALID)
+				return -EINVAL;
+			if (type == BTRFS_TREE_BLOCK_REF_KEY)
+				owner = btrfs_extent_inline_ref_offset(eb, iref);
+		}
+	} else if (unlikely(item_size == sizeof(struct btrfs_extent_item_v0))) {
+		btrfs_print_v0_err(eb->fs_info);
+		btrfs_handle_fs_error(eb->fs_info, -EINVAL, NULL);
+		return -EINVAL;
+	} else {
+		BUG();
+	}
+
+	btrfs_release_path(path);
+
+	BUG_ON(level == -1);
+
+	block = kmalloc(sizeof(*block), GFP_NOFS);
+	if (!block)
+		return -ENOMEM;
+
+	block->bytenr = extent_key->objectid;
+	block->key.objectid = rc->extent_root->fs_info->nodesize;
+	block->key.offset = generation;
+	block->level = level;
+	block->key_ready = 0;
+	block->owner = owner;
+
+	rb_node = rb_simple_insert(blocks, block->bytenr, &block->rb_node);
+	if (rb_node)
+		btrfs_backref_panic(rc->extent_root->fs_info, block->bytenr,
+				    -EEXIST);
+
+	return 0;
+}
+
+/*
+ * helper to add tree blocks for backref of type BTRFS_SHARED_DATA_REF_KEY
+ */
+static int __add_tree_block(struct reloc_control *rc,
+			    u64 bytenr, u32 blocksize,
+			    struct rb_root *blocks)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct btrfs_path *path;
+	struct btrfs_key key;
+	int ret;
+	bool skinny = btrfs_fs_incompat(fs_info, SKINNY_METADATA);
+
+	if (tree_block_processed(bytenr, rc))
+		return 0;
+
+	if (rb_simple_search(blocks, bytenr))
+		return 0;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+again:
+	key.objectid = bytenr;
+	if (skinny) {
+		key.type = BTRFS_METADATA_ITEM_KEY;
+		key.offset = (u64)-1;
+	} else {
+		key.type = BTRFS_EXTENT_ITEM_KEY;
+		key.offset = blocksize;
+	}
+
+	path->search_commit_root = 1;
+	path->skip_locking = 1;
+	ret = btrfs_search_slot(NULL, rc->extent_root, &key, path, 0, 0);
+	if (ret < 0)
+		goto out;
+
+	if (ret > 0 && skinny) {
+		if (path->slots[0]) {
+			path->slots[0]--;
+			btrfs_item_key_to_cpu(path->nodes[0], &key,
+					      path->slots[0]);
+			if (key.objectid == bytenr &&
+			    (key.type == BTRFS_METADATA_ITEM_KEY ||
+			     (key.type == BTRFS_EXTENT_ITEM_KEY &&
+			      key.offset == blocksize)))
+				ret = 0;
+		}
+
+		if (ret) {
+			skinny = false;
+			btrfs_release_path(path);
+			goto again;
+		}
+	}
+	if (ret) {
+		ASSERT(ret == 1);
+		btrfs_print_leaf(path->nodes[0]);
+		btrfs_err(fs_info,
+	     "tree block extent item (%llu) is not found in extent tree",
+		     bytenr);
+		WARN_ON(1);
+		ret = -EINVAL;
+		goto out;
+	}
+
+	ret = add_tree_block(rc, &key, path, blocks);
+out:
+	btrfs_free_path(path);
+	return ret;
+}
+
+static int delete_block_group_cache(struct btrfs_fs_info *fs_info,
+				    struct btrfs_block_group *block_group,
+				    struct inode *inode,
+				    u64 ino)
+{
+	struct btrfs_root *root = fs_info->tree_root;
+	struct btrfs_trans_handle *trans;
+	int ret = 0;
+
+	if (inode)
+		goto truncate;
+
+	inode = btrfs_iget(fs_info->sb, ino, root);
+	if (IS_ERR(inode))
+		return -ENOENT;
+
+truncate:
+	ret = btrfs_check_trunc_cache_free_space(fs_info,
+						 &fs_info->global_block_rsv);
+	if (ret)
+		goto out;
+
+	trans = btrfs_join_transaction(root);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		goto out;
+	}
+
+	ret = btrfs_truncate_free_space_cache(trans, block_group, inode);
+
+	btrfs_end_transaction(trans);
+	btrfs_btree_balance_dirty(fs_info);
+out:
+	iput(inode);
+	return ret;
+}
+
+/*
+ * Locate the free space cache EXTENT_DATA in root tree leaf and delete the
+ * cache inode, to avoid free space cache data extent blocking data relocation.
+ */
+static int delete_v1_space_cache(struct extent_buffer *leaf,
+				 struct btrfs_block_group *block_group,
+				 u64 data_bytenr)
+{
+	u64 space_cache_ino;
+	struct btrfs_file_extent_item *ei;
+	struct btrfs_key key;
+	bool found = false;
+	int i;
+	int ret;
+
+	if (btrfs_header_owner(leaf) != BTRFS_ROOT_TREE_OBJECTID)
+		return 0;
+
+	for (i = 0; i < btrfs_header_nritems(leaf); i++) {
+		u8 type;
+
+		btrfs_item_key_to_cpu(leaf, &key, i);
+		if (key.type != BTRFS_EXTENT_DATA_KEY)
+			continue;
+		ei = btrfs_item_ptr(leaf, i, struct btrfs_file_extent_item);
+		type = btrfs_file_extent_type(leaf, ei);
+
+		if ((type == BTRFS_FILE_EXTENT_REG ||
+		     type == BTRFS_FILE_EXTENT_PREALLOC) &&
+		    btrfs_file_extent_disk_bytenr(leaf, ei) == data_bytenr) {
+			found = true;
+			space_cache_ino = key.objectid;
+			break;
+		}
+	}
+	if (!found)
+		return -ENOENT;
+	ret = delete_block_group_cache(leaf->fs_info, block_group, NULL,
+					space_cache_ino);
+	return ret;
+}
+
+/*
+ * helper to find all tree blocks that reference a given data extent
+ */
+static noinline_for_stack
+int add_data_references(struct reloc_control *rc,
+			struct btrfs_key *extent_key,
+			struct btrfs_path *path,
+			struct rb_root *blocks)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct ulist *leaves = NULL;
+	struct ulist_iterator leaf_uiter;
+	struct ulist_node *ref_node = NULL;
+	const u32 blocksize = fs_info->nodesize;
+	int ret = 0;
+
+	btrfs_release_path(path);
+	ret = btrfs_find_all_leafs(NULL, fs_info, extent_key->objectid,
+				   0, &leaves, NULL, true);
+	if (ret < 0)
+		return ret;
+
+	ULIST_ITER_INIT(&leaf_uiter);
+	while ((ref_node = ulist_next(leaves, &leaf_uiter))) {
+		struct extent_buffer *eb;
+
+		eb = read_tree_block(fs_info, ref_node->val, 0, 0, 0, NULL);
+		if (IS_ERR(eb)) {
+			ret = PTR_ERR(eb);
+			break;
+		}
+		ret = delete_v1_space_cache(eb, rc->block_group,
+					    extent_key->objectid);
+		free_extent_buffer(eb);
+		if (ret < 0)
+			break;
+		ret = __add_tree_block(rc, ref_node->val, blocksize, blocks);
+		if (ret < 0)
+			break;
+	}
+	if (ret < 0)
+		free_block_list(blocks);
+	ulist_free(leaves);
+	return ret;
+}
+
+/*
+ * helper to find next unprocessed extent
+ */
+static noinline_for_stack
+int find_next_extent(struct reloc_control *rc, struct btrfs_path *path,
+		     struct btrfs_key *extent_key)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct btrfs_key key;
+	struct extent_buffer *leaf;
+	u64 start, end, last;
+	int ret;
+
+	last = rc->block_group->start + rc->block_group->length;
+	while (1) {
+		cond_resched();
+		if (rc->search_start >= last) {
+			ret = 1;
+			break;
+		}
+
+		key.objectid = rc->search_start;
+		key.type = BTRFS_EXTENT_ITEM_KEY;
+		key.offset = 0;
+
+		path->search_commit_root = 1;
+		path->skip_locking = 1;
+		ret = btrfs_search_slot(NULL, rc->extent_root, &key, path,
+					0, 0);
+		if (ret < 0)
+			break;
+next:
+		leaf = path->nodes[0];
+		if (path->slots[0] >= btrfs_header_nritems(leaf)) {
+			ret = btrfs_next_leaf(rc->extent_root, path);
+			if (ret != 0)
+				break;
+			leaf = path->nodes[0];
+		}
+
+		btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
+		if (key.objectid >= last) {
+			ret = 1;
+			break;
+		}
+
+		if (key.type != BTRFS_EXTENT_ITEM_KEY &&
+		    key.type != BTRFS_METADATA_ITEM_KEY) {
+			path->slots[0]++;
+			goto next;
+		}
+
+		if (key.type == BTRFS_EXTENT_ITEM_KEY &&
+		    key.objectid + key.offset <= rc->search_start) {
+			path->slots[0]++;
+			goto next;
+		}
+
+		if (key.type == BTRFS_METADATA_ITEM_KEY &&
+		    key.objectid + fs_info->nodesize <=
+		    rc->search_start) {
+			path->slots[0]++;
+			goto next;
+		}
+
+		ret = find_first_extent_bit(&rc->processed_blocks,
+					    key.objectid, &start, &end,
+					    EXTENT_DIRTY, NULL);
+
+		if (ret == 0 && start <= key.objectid) {
+			btrfs_release_path(path);
+			rc->search_start = end + 1;
+		} else {
+			if (key.type == BTRFS_EXTENT_ITEM_KEY)
+				rc->search_start = key.objectid + key.offset;
+			else
+				rc->search_start = key.objectid +
+					fs_info->nodesize;
+			memcpy(extent_key, &key, sizeof(key));
+			return 0;
+		}
+	}
+	btrfs_release_path(path);
+	return ret;
+}
+
+static void set_reloc_control(struct reloc_control *rc)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+
+	mutex_lock(&fs_info->reloc_mutex);
+	fs_info->reloc_ctl = rc;
+	mutex_unlock(&fs_info->reloc_mutex);
+}
+
+static void unset_reloc_control(struct reloc_control *rc)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+
+	mutex_lock(&fs_info->reloc_mutex);
+	fs_info->reloc_ctl = NULL;
+	mutex_unlock(&fs_info->reloc_mutex);
+}
+
+static noinline_for_stack
+int prepare_to_relocate(struct reloc_control *rc)
+{
+	struct btrfs_trans_handle *trans;
+	int ret;
+
+	rc->block_rsv = btrfs_alloc_block_rsv(rc->extent_root->fs_info,
+					      BTRFS_BLOCK_RSV_TEMP);
+	if (!rc->block_rsv)
+		return -ENOMEM;
+
+	memset(&rc->cluster, 0, sizeof(rc->cluster));
+	rc->search_start = rc->block_group->start;
+	rc->extents_found = 0;
+	rc->nodes_relocated = 0;
+	rc->merging_rsv_size = 0;
+	rc->reserved_bytes = 0;
+	rc->block_rsv->size = rc->extent_root->fs_info->nodesize *
+			      RELOCATION_RESERVED_NODES;
+	ret = btrfs_block_rsv_refill(rc->extent_root->fs_info,
+				     rc->block_rsv, rc->block_rsv->size,
+				     BTRFS_RESERVE_FLUSH_ALL);
+	if (ret)
+		return ret;
+
+	rc->create_reloc_tree = 1;
+	set_reloc_control(rc);
+
+	trans = btrfs_join_transaction(rc->extent_root);
+	if (IS_ERR(trans)) {
+		unset_reloc_control(rc);
+		/*
+		 * extent tree is not a ref_cow tree and has no reloc_root to
+		 * cleanup.  And callers are responsible to free the above
+		 * block rsv.
+		 */
+		return PTR_ERR(trans);
+	}
+
+	ret = btrfs_commit_transaction(trans);
+	if (ret)
+		unset_reloc_control(rc);
+
+	return ret;
+}
+
+static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
+{
+	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
+	struct rb_root blocks = RB_ROOT;
+	struct btrfs_key key;
+	struct btrfs_trans_handle *trans = NULL;
+	struct btrfs_path *path;
+	struct btrfs_extent_item *ei;
+	u64 flags;
+	int ret;
+	int err = 0;
+	int progress = 0;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+	path->reada = READA_FORWARD;
+
+	ret = prepare_to_relocate(rc);
+	if (ret) {
+		err = ret;
+		goto out_free;
+	}
+
+	while (1) {
+		rc->reserved_bytes = 0;
+		ret = btrfs_block_rsv_refill(fs_info, rc->block_rsv,
+					     rc->block_rsv->size,
+					     BTRFS_RESERVE_FLUSH_ALL);
+		if (ret) {
+			err = ret;
+			break;
+		}
+		progress++;
+		trans = btrfs_start_transaction(rc->extent_root, 0);
+		if (IS_ERR(trans)) {
+			err = PTR_ERR(trans);
+			trans = NULL;
+			break;
+		}
+restart:
+		if (update_backref_cache(trans, &rc->backref_cache)) {
+			btrfs_end_transaction(trans);
+			trans = NULL;
+			continue;
+		}
+
+		ret = find_next_extent(rc, path, &key);
+		if (ret < 0)
+			err = ret;
+		if (ret != 0)
+			break;
+
+		rc->extents_found++;
+
+		ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
+				    struct btrfs_extent_item);
+		flags = btrfs_extent_flags(path->nodes[0], ei);
+
+		if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
+			ret = add_tree_block(rc, &key, path, &blocks);
+		} else if (rc->stage == UPDATE_DATA_PTRS &&
+			   (flags & BTRFS_EXTENT_FLAG_DATA)) {
+			ret = add_data_references(rc, &key, path, &blocks);
+		} else {
+			btrfs_release_path(path);
+			ret = 0;
+		}
+		if (ret < 0) {
+			err = ret;
+			break;
+		}
+
+		if (!RB_EMPTY_ROOT(&blocks)) {
+			ret = relocate_tree_blocks(trans, rc, &blocks);
+			if (ret < 0) {
+				if (ret != -EAGAIN) {
+					err = ret;
+					break;
+				}
+				rc->extents_found--;
+				rc->search_start = key.objectid;
+			}
+		}
+
+		btrfs_end_transaction_throttle(trans);
+		btrfs_btree_balance_dirty(fs_info);
+		trans = NULL;
+
+		if (rc->stage == MOVE_DATA_EXTENTS &&
+		    (flags & BTRFS_EXTENT_FLAG_DATA)) {
+			rc->found_file_extent = 1;
+			ret = relocate_data_extent(rc->data_inode,
+						   &key, &rc->cluster);
+			if (ret < 0) {
+				err = ret;
+				break;
+			}
+		}
+		if (btrfs_should_cancel_balance(fs_info)) {
+			err = -ECANCELED;
+			break;
+		}
+	}
+	if (trans && progress && err == -ENOSPC) {
+		ret = btrfs_force_chunk_alloc(trans, rc->block_group->flags);
+		if (ret == 1) {
+			err = 0;
+			progress = 0;
+			goto restart;
+		}
+	}
+
+	btrfs_release_path(path);
+	clear_extent_bits(&rc->processed_blocks, 0, (u64)-1, EXTENT_DIRTY);
+
+	if (trans) {
+		btrfs_end_transaction_throttle(trans);
+		btrfs_btree_balance_dirty(fs_info);
+	}
+
+	if (!err) {
+		ret = relocate_file_extent_cluster(rc->data_inode,
+						   &rc->cluster);
+		if (ret < 0)
+			err = ret;
+	}
+
+	rc->create_reloc_tree = 0;
+	set_reloc_control(rc);
+
+	btrfs_backref_release_cache(&rc->backref_cache);
+	btrfs_block_rsv_release(fs_info, rc->block_rsv, (u64)-1, NULL);
+
+	/*
+	 * Even in the case when the relocation is cancelled, we should all go
+	 * through prepare_to_merge() and merge_reloc_roots().
+	 *
+	 * For error (including cancelled balance), prepare_to_merge() will
+	 * mark all reloc trees orphan, then queue them for cleanup in
+	 * merge_reloc_roots()
+	 */
+	err = prepare_to_merge(rc, err);
+
+	merge_reloc_roots(rc);
+
+	rc->merge_reloc_tree = 0;
+	unset_reloc_control(rc);
+	btrfs_block_rsv_release(fs_info, rc->block_rsv, (u64)-1, NULL);
+
+	/* get rid of pinned extents */
+	trans = btrfs_join_transaction(rc->extent_root);
+	if (IS_ERR(trans)) {
+		err = PTR_ERR(trans);
+		goto out_free;
+	}
+	ret = btrfs_commit_transaction(trans);
+	if (ret && !err)
+		err = ret;
+out_free:
+	ret = clean_dirty_subvols(rc);
+	if (ret < 0 && !err)
+		err = ret;
+	btrfs_free_block_rsv(fs_info, rc->block_rsv);
+	btrfs_free_path(path);
+	return err;
+}
+
+static int __insert_orphan_inode(struct btrfs_trans_handle *trans,
+				 struct btrfs_root *root, u64 objectid)
+{
+	struct btrfs_path *path;
+	struct btrfs_inode_item *item;
+	struct extent_buffer *leaf;
+	int ret;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	ret = btrfs_insert_empty_inode(trans, root, path, objectid);
+	if (ret)
+		goto out;
+
+	leaf = path->nodes[0];
+	item = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_inode_item);
+	memzero_extent_buffer(leaf, (unsigned long)item, sizeof(*item));
+	btrfs_set_inode_generation(leaf, item, 1);
+	btrfs_set_inode_size(leaf, item, 0);
+	btrfs_set_inode_mode(leaf, item, S_IFREG | 0600);
+	btrfs_set_inode_flags(leaf, item, BTRFS_INODE_NOCOMPRESS |
+					  BTRFS_INODE_PREALLOC);
+	btrfs_mark_buffer_dirty(leaf);
+out:
+	btrfs_free_path(path);
+	return ret;
+}
+
+static void delete_orphan_inode(struct btrfs_trans_handle *trans,
+				struct btrfs_root *root, u64 objectid)
+{
+	struct btrfs_path *path;
+	struct btrfs_key key;
+	int ret = 0;
+
+	path = btrfs_alloc_path();
+	if (!path) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	key.objectid = objectid;
+	key.type = BTRFS_INODE_ITEM_KEY;
+	key.offset = 0;
+	ret = btrfs_search_slot(trans, root, &key, path, -1, 1);
+	if (ret) {
+		if (ret > 0)
+			ret = -ENOENT;
+		goto out;
+	}
+	ret = btrfs_del_item(trans, root, path);
+out:
+	if (ret)
+		btrfs_abort_transaction(trans, ret);
+	btrfs_free_path(path);
+}
+
+/*
+ * helper to create inode for data relocation.
+ * the inode is in data relocation tree and its link count is 0
+ */
+static noinline_for_stack
+struct inode *create_reloc_inode(struct btrfs_fs_info *fs_info,
+				 struct btrfs_block_group *group)
+{
+	struct inode *inode = NULL;
+	struct btrfs_trans_handle *trans;
+	struct btrfs_root *root;
+	u64 objectid;
+	int err = 0;
+
+	root = btrfs_grab_root(fs_info->data_reloc_root);
+	trans = btrfs_start_transaction(root, 6);
+	if (IS_ERR(trans)) {
+		btrfs_put_root(root);
+		return ERR_CAST(trans);
+	}
+
+	err = btrfs_get_free_objectid(root, &objectid);
+	if (err)
+		goto out;
+
+	err = __insert_orphan_inode(trans, root, objectid);
+	if (err)
+		goto out;
+
+	inode = btrfs_iget(fs_info->sb, objectid, root);
+	if (IS_ERR(inode)) {
+		delete_orphan_inode(trans, root, objectid);
+		err = PTR_ERR(inode);
+		inode = NULL;
+		goto out;
+	}
+	BTRFS_I(inode)->index_cnt = group->start;
+
+	err = btrfs_orphan_add(trans, BTRFS_I(inode));
+out:
+	btrfs_put_root(root);
+	btrfs_end_transaction(trans);
+	btrfs_btree_balance_dirty(fs_info);
+	if (err) {
+		iput(inode);
+		inode = ERR_PTR(err);
+	}
+	return inode;
+}
+
+/*
+ * Mark start of chunk relocation that is cancellable. Check if the cancellation
+ * has been requested meanwhile and don't start in that case.
+ *
+ * Return:
+ *   0             success
+ *   -EINPROGRESS  operation is already in progress, that's probably a bug
+ *   -ECANCELED    cancellation request was set before the operation started
+ */
+static int reloc_chunk_start(struct btrfs_fs_info *fs_info)
+{
+	if (test_and_set_bit(BTRFS_FS_RELOC_RUNNING, &fs_info->flags)) {
+		/* This should not happen */
+		btrfs_err(fs_info, "reloc already running, cannot start");
+		return -EINPROGRESS;
+	}
+
+	if (atomic_read(&fs_info->reloc_cancel_req) > 0) {
+		btrfs_info(fs_info, "chunk relocation canceled on start");
+		/*
+		 * On cancel, clear all requests but let the caller mark
+		 * the end after cleanup operations.
+		 */
+		atomic_set(&fs_info->reloc_cancel_req, 0);
+		return -ECANCELED;
+	}
+	return 0;
+}
+
+/*
+ * Mark end of chunk relocation that is cancellable and wake any waiters.
+ */
+static void reloc_chunk_end(struct btrfs_fs_info *fs_info)
+{
+	/* Requested after start, clear bit first so any waiters can continue */
+	if (atomic_read(&fs_info->reloc_cancel_req) > 0)
+		btrfs_info(fs_info, "chunk relocation canceled during operation");
+	clear_and_wake_up_bit(BTRFS_FS_RELOC_RUNNING, &fs_info->flags);
+	atomic_set(&fs_info->reloc_cancel_req, 0);
+}
+
+static struct reloc_control *alloc_reloc_control(struct btrfs_fs_info *fs_info)
+{
+	struct reloc_control *rc;
+
+	rc = kzalloc(sizeof(*rc), GFP_NOFS);
+	if (!rc)
+		return NULL;
+
+	INIT_LIST_HEAD(&rc->reloc_roots);
+	INIT_LIST_HEAD(&rc->dirty_subvol_roots);
+	btrfs_backref_init_cache(fs_info, &rc->backref_cache, 1);
+	mapping_tree_init(&rc->reloc_root_tree);
+	extent_io_tree_init(fs_info, &rc->processed_blocks,
+			    IO_TREE_RELOC_BLOCKS, NULL);
+	return rc;
+}
+
+static void free_reloc_control(struct reloc_control *rc)
+{
+	struct mapping_node *node, *tmp;
+
+	free_reloc_roots(&rc->reloc_roots);
+	rbtree_postorder_for_each_entry_safe(node, tmp,
+			&rc->reloc_root_tree.rb_root, rb_node)
+		kfree(node);
+
+	kfree(rc);
+}
+
+/*
+ * Print the block group being relocated
+ */
+static void describe_relocation(struct btrfs_fs_info *fs_info,
+				struct btrfs_block_group *block_group)
+{
+	char buf[128] = {'\0'};
+
+	btrfs_describe_block_groups(block_group->flags, buf, sizeof(buf));
+
+	btrfs_info(fs_info,
+		   "relocating block group %llu flags %s",
+		   block_group->start, buf);
+}
+
+static const char *stage_to_string(int stage)
+{
+	if (stage == MOVE_DATA_EXTENTS)
+		return "move data extents";
+	if (stage == UPDATE_DATA_PTRS)
+		return "update data pointers";
+	return "unknown";
+}
+
+/*
+ * function to relocate all extents in a block group.
+ */
+int btrfs_relocate_block_group(struct btrfs_fs_info *fs_info, u64 group_start)
+{
+	struct btrfs_block_group *bg;
+	struct btrfs_root *extent_root = btrfs_extent_root(fs_info, group_start);
+	struct reloc_control *rc;
+	struct inode *inode;
+	struct btrfs_path *path;
+	int ret;
+	int rw = 0;
+	int err = 0;
+
+	/*
+	 * This only gets set if we had a half-deleted snapshot on mount.  We
+	 * cannot allow relocation to start while we're still trying to clean up
+	 * these pending deletions.
+	 */
+	ret = wait_on_bit(&fs_info->flags, BTRFS_FS_UNFINISHED_DROPS, TASK_INTERRUPTIBLE);
+	if (ret)
+		return ret;
+
+	/* We may have been woken up by close_ctree, so bail if we're closing. */
+	if (btrfs_fs_closing(fs_info))
+		return -EINTR;
+
+	bg = btrfs_lookup_block_group(fs_info, group_start);
+	if (!bg)
+		return -ENOENT;
+
+	/*
+	 * Relocation of a data block group creates ordered extents.  Without
+	 * sb_start_write(), we can freeze the filesystem while unfinished
+	 * ordered extents are left. Such ordered extents can cause a deadlock
+	 * e.g. when syncfs() is waiting for their completion but they can't
+	 * finish because they block when joining a transaction, due to the
+	 * fact that the freeze locks are being held in write mode.
+	 */
+	if (bg->flags & BTRFS_BLOCK_GROUP_DATA)
+		ASSERT(sb_write_started(fs_info->sb));
+
+	if (btrfs_pinned_by_swapfile(fs_info, bg)) {
+		btrfs_put_block_group(bg);
+		return -ETXTBSY;
+	}
+
+	rc = alloc_reloc_control(fs_info);
+	if (!rc) {
+		btrfs_put_block_group(bg);
+		return -ENOMEM;
+	}
+
+	ret = reloc_chunk_start(fs_info);
+	if (ret < 0) {
+		err = ret;
+		goto out_put_bg;
+	}
+
+	rc->extent_root = extent_root;
+	rc->block_group = bg;
+
+	ret = btrfs_inc_block_group_ro(rc->block_group, true);
+	if (ret) {
+		err = ret;
+		goto out;
+	}
+	rw = 1;
+
+	path = btrfs_alloc_path();
+	if (!path) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	inode = lookup_free_space_inode(rc->block_group, path);
+	btrfs_free_path(path);
+
+	if (!IS_ERR(inode))
+		ret = delete_block_group_cache(fs_info, rc->block_group, inode, 0);
+	else
+		ret = PTR_ERR(inode);
+
+	if (ret && ret != -ENOENT) {
+		err = ret;
+		goto out;
+	}
+
+	rc->data_inode = create_reloc_inode(fs_info, rc->block_group);
+	if (IS_ERR(rc->data_inode)) {
+		err = PTR_ERR(rc->data_inode);
+		rc->data_inode = NULL;
+		goto out;
+	}
+
+	describe_relocation(fs_info, rc->block_group);
+
+	btrfs_wait_block_group_reservations(rc->block_group);
+	btrfs_wait_nocow_writers(rc->block_group);
+	btrfs_wait_ordered_roots(fs_info, U64_MAX,
+				 rc->block_group->start,
+				 rc->block_group->length);
+
+	ret = btrfs_zone_finish(rc->block_group);
+	WARN_ON(ret && ret != -EAGAIN);
+
+	while (1) {
+		int finishes_stage;
+
+		mutex_lock(&fs_info->cleaner_mutex);
+		ret = relocate_block_group(rc);
+		mutex_unlock(&fs_info->cleaner_mutex);
+		if (ret < 0)
+			err = ret;
+
+		finishes_stage = rc->stage;
+		/*
+		 * We may have gotten ENOSPC after we already dirtied some
+		 * extents.  If writeout happens while we're relocating a
+		 * different block group we could end up hitting the
+		 * BUG_ON(rc->stage == UPDATE_DATA_PTRS) in
+		 * btrfs_reloc_cow_block.  Make sure we write everything out
+		 * properly so we don't trip over this problem, and then break
+		 * out of the loop if we hit an error.
+		 */
+		if (rc->stage == MOVE_DATA_EXTENTS && rc->found_file_extent) {
+			ret = btrfs_wait_ordered_range(rc->data_inode, 0,
+						       (u64)-1);
+			if (ret)
+				err = ret;
+			invalidate_mapping_pages(rc->data_inode->i_mapping,
+						 0, -1);
+			rc->stage = UPDATE_DATA_PTRS;
+		}
+
+		if (err < 0)
+			goto out;
+
+		if (rc->extents_found == 0)
+			break;
+
+		btrfs_info(fs_info, "found %llu extents, stage: %s",
+			   rc->extents_found, stage_to_string(finishes_stage));
+	}
+
+	WARN_ON(rc->block_group->pinned > 0);
+	WARN_ON(rc->block_group->reserved > 0);
+	WARN_ON(rc->block_group->used > 0);
+out:
+	if (err && rw)
+		btrfs_dec_block_group_ro(rc->block_group);
+	iput(rc->data_inode);
+out_put_bg:
+	btrfs_put_block_group(bg);
+	reloc_chunk_end(fs_info);
+	free_reloc_control(rc);
+	return err;
+}
+
+static noinline_for_stack int mark_garbage_root(struct btrfs_root *root)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_trans_handle *trans;
+	int ret, err;
+
+	trans = btrfs_start_transaction(fs_info->tree_root, 0);
+	if (IS_ERR(trans))
+		return PTR_ERR(trans);
+
+	memset(&root->root_item.drop_progress, 0,
+		sizeof(root->root_item.drop_progress));
+	btrfs_set_root_drop_level(&root->root_item, 0);
+	btrfs_set_root_refs(&root->root_item, 0);
+	ret = btrfs_update_root(trans, fs_info->tree_root,
+				&root->root_key, &root->root_item);
+
+	err = btrfs_end_transaction(trans);
+	if (err)
+		return err;
+	return ret;
+}
+
+/*
+ * recover relocation interrupted by system crash.
+ *
+ * this function resumes merging reloc trees with corresponding fs trees.
+ * this is important for keeping the sharing of tree blocks
+ */
+int btrfs_recover_relocation(struct btrfs_fs_info *fs_info)
+{
+	LIST_HEAD(reloc_roots);
+	struct btrfs_key key;
+	struct btrfs_root *fs_root;
+	struct btrfs_root *reloc_root;
+	struct btrfs_path *path;
+	struct extent_buffer *leaf;
+	struct reloc_control *rc = NULL;
+	struct btrfs_trans_handle *trans;
+	int ret;
+	int err = 0;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+	path->reada = READA_BACK;
+
+	key.objectid = BTRFS_TREE_RELOC_OBJECTID;
+	key.type = BTRFS_ROOT_ITEM_KEY;
+	key.offset = (u64)-1;
+
+	while (1) {
+		ret = btrfs_search_slot(NULL, fs_info->tree_root, &key,
+					path, 0, 0);
+		if (ret < 0) {
+			err = ret;
+			goto out;
+		}
+		if (ret > 0) {
+			if (path->slots[0] == 0)
+				break;
+			path->slots[0]--;
+		}
+		leaf = path->nodes[0];
+		btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
+		btrfs_release_path(path);
+
+		if (key.objectid != BTRFS_TREE_RELOC_OBJECTID ||
+		    key.type != BTRFS_ROOT_ITEM_KEY)
+			break;
+
+		reloc_root = btrfs_read_tree_root(fs_info->tree_root, &key);
+		if (IS_ERR(reloc_root)) {
+			err = PTR_ERR(reloc_root);
+			goto out;
+		}
+
+		set_bit(BTRFS_ROOT_SHAREABLE, &reloc_root->state);
+		list_add(&reloc_root->root_list, &reloc_roots);
+
+		if (btrfs_root_refs(&reloc_root->root_item) > 0) {
+			fs_root = btrfs_get_fs_root(fs_info,
+					reloc_root->root_key.offset, false);
+			if (IS_ERR(fs_root)) {
+				ret = PTR_ERR(fs_root);
+				if (ret != -ENOENT) {
+					err = ret;
+					goto out;
+				}
+				ret = mark_garbage_root(reloc_root);
+				if (ret < 0) {
+					err = ret;
+					goto out;
+				}
+			} else {
+				btrfs_put_root(fs_root);
+			}
+		}
+
+		if (key.offset == 0)
+			break;
+
+		key.offset--;
+	}
+	btrfs_release_path(path);
+
+	if (list_empty(&reloc_roots))
+		goto out;
+
+	rc = alloc_reloc_control(fs_info);
+	if (!rc) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	ret = reloc_chunk_start(fs_info);
+	if (ret < 0) {
+		err = ret;
+		goto out_end;
+	}
+
+	rc->extent_root = btrfs_extent_root(fs_info, 0);
+
+	set_reloc_control(rc);
+
+	trans = btrfs_join_transaction(rc->extent_root);
+	if (IS_ERR(trans)) {
+		err = PTR_ERR(trans);
+		goto out_unset;
+	}
+
+	rc->merge_reloc_tree = 1;
+
+	while (!list_empty(&reloc_roots)) {
+		reloc_root = list_entry(reloc_roots.next,
+					struct btrfs_root, root_list);
+		list_del(&reloc_root->root_list);
+
+		if (btrfs_root_refs(&reloc_root->root_item) == 0) {
+			list_add_tail(&reloc_root->root_list,
+				      &rc->reloc_roots);
+			continue;
+		}
+
+		fs_root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
+					    false);
+		if (IS_ERR(fs_root)) {
+			err = PTR_ERR(fs_root);
+			list_add_tail(&reloc_root->root_list, &reloc_roots);
+			btrfs_end_transaction(trans);
+			goto out_unset;
+		}
+
+		err = __add_reloc_root(reloc_root);
+		ASSERT(err != -EEXIST);
+		if (err) {
+			list_add_tail(&reloc_root->root_list, &reloc_roots);
+			btrfs_put_root(fs_root);
+			btrfs_end_transaction(trans);
+			goto out_unset;
+		}
+		fs_root->reloc_root = btrfs_grab_root(reloc_root);
+		btrfs_put_root(fs_root);
+	}
+
+	err = btrfs_commit_transaction(trans);
+	if (err)
+		goto out_unset;
+
+	merge_reloc_roots(rc);
+
+	unset_reloc_control(rc);
+
+	trans = btrfs_join_transaction(rc->extent_root);
+	if (IS_ERR(trans)) {
+		err = PTR_ERR(trans);
+		goto out_clean;
+	}
+	err = btrfs_commit_transaction(trans);
+out_clean:
+	ret = clean_dirty_subvols(rc);
+	if (ret < 0 && !err)
+		err = ret;
+out_unset:
+	unset_reloc_control(rc);
+out_end:
+	reloc_chunk_end(fs_info);
+	free_reloc_control(rc);
+out:
+	free_reloc_roots(&reloc_roots);
+
+	btrfs_free_path(path);
+
+	if (err == 0) {
+		/* cleanup orphan inode in data relocation tree */
+		fs_root = btrfs_grab_root(fs_info->data_reloc_root);
+		ASSERT(fs_root);
+		err = btrfs_orphan_cleanup(fs_root);
+		btrfs_put_root(fs_root);
+	}
+	return err;
+}
+
+/*
+ * helper to add ordered checksum for data relocation.
+ *
+ * cloning checksum properly handles the nodatasum extents.
+ * it also saves CPU time to re-calculate the checksum.
+ */
+int btrfs_reloc_clone_csums(struct btrfs_inode *inode, u64 file_pos, u64 len)
+{
+	struct btrfs_fs_info *fs_info = inode->root->fs_info;
+	struct btrfs_root *csum_root;
+	struct btrfs_ordered_sum *sums;
+	struct btrfs_ordered_extent *ordered;
+	int ret;
+	u64 disk_bytenr;
+	u64 new_bytenr;
+	LIST_HEAD(list);
+
+	ordered = btrfs_lookup_ordered_extent(inode, file_pos);
+	BUG_ON(ordered->file_offset != file_pos || ordered->num_bytes != len);
+
+	disk_bytenr = file_pos + inode->index_cnt;
+	csum_root = btrfs_csum_root(fs_info, disk_bytenr);
+	ret = btrfs_lookup_csums_range(csum_root, disk_bytenr,
+				       disk_bytenr + len - 1, &list, 0, false);
+	if (ret)
+		goto out;
+
+	while (!list_empty(&list)) {
+		sums = list_entry(list.next, struct btrfs_ordered_sum, list);
+		list_del_init(&sums->list);
+
+		/*
+		 * We need to offset the new_bytenr based on where the csum is.
+		 * We need to do this because we will read in entire prealloc
+		 * extents but we may have written to say the middle of the
+		 * prealloc extent, so we need to make sure the csum goes with
+		 * the right disk offset.
+		 *
+		 * We can do this because the data reloc inode refers strictly
+		 * to the on disk bytes, so we don't have to worry about
+		 * disk_len vs real len like with real inodes since it's all
+		 * disk length.
+		 */
+		new_bytenr = ordered->disk_bytenr + sums->bytenr - disk_bytenr;
+		sums->bytenr = new_bytenr;
+
+		btrfs_add_ordered_sum(ordered, sums);
+	}
+out:
+	btrfs_put_ordered_extent(ordered);
+	return ret;
+}
+
+int btrfs_reloc_cow_block(struct btrfs_trans_handle *trans,
+			  struct btrfs_root *root, struct extent_buffer *buf,
+			  struct extent_buffer *cow)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct reloc_control *rc;
+	struct btrfs_backref_node *node;
+	int first_cow = 0;
+	int level;
+	int ret = 0;
+
+	rc = fs_info->reloc_ctl;
+	if (!rc)
+		return 0;
+
+	BUG_ON(rc->stage == UPDATE_DATA_PTRS && btrfs_is_data_reloc_root(root));
+
+	level = btrfs_header_level(buf);
+	if (btrfs_header_generation(buf) <=
+	    btrfs_root_last_snapshot(&root->root_item))
+		first_cow = 1;
+
+	if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID &&
+	    rc->create_reloc_tree) {
+		WARN_ON(!first_cow && level == 0);
+
+		node = rc->backref_cache.path[level];
+		BUG_ON(node->bytenr != buf->start &&
+		       node->new_bytenr != buf->start);
+
+		btrfs_backref_drop_node_buffer(node);
+		atomic_inc(&cow->refs);
+		node->eb = cow;
+		node->new_bytenr = cow->start;
+
+		if (!node->pending) {
+			list_move_tail(&node->list,
+				       &rc->backref_cache.pending[level]);
+			node->pending = 1;
+		}
+
+		if (first_cow)
+			mark_block_processed(rc, node);
+
+		if (first_cow && level > 0)
+			rc->nodes_relocated += buf->len;
+	}
+
+	if (level == 0 && first_cow && rc->stage == UPDATE_DATA_PTRS)
+		ret = replace_file_extents(trans, rc, root, cow);
+	return ret;
+}
+
+/*
+ * called before creating snapshot. it calculates metadata reservation
+ * required for relocating tree blocks in the snapshot
+ */
+void btrfs_reloc_pre_snapshot(struct btrfs_pending_snapshot *pending,
+			      u64 *bytes_to_reserve)
+{
+	struct btrfs_root *root = pending->root;
+	struct reloc_control *rc = root->fs_info->reloc_ctl;
+
+	if (!rc || !have_reloc_root(root))
+		return;
+
+	if (!rc->merge_reloc_tree)
+		return;
+
+	root = root->reloc_root;
+	BUG_ON(btrfs_root_refs(&root->root_item) == 0);
+	/*
+	 * relocation is in the stage of merging trees. the space
+	 * used by merging a reloc tree is twice the size of
+	 * relocated tree nodes in the worst case. half for cowing
+	 * the reloc tree, half for cowing the fs tree. the space
+	 * used by cowing the reloc tree will be freed after the
+	 * tree is dropped. if we create snapshot, cowing the fs
+	 * tree may use more space than it frees. so we need
+	 * reserve extra space.
+	 */
+	*bytes_to_reserve += rc->nodes_relocated;
+}
+
+/*
+ * called after snapshot is created. migrate block reservation
+ * and create reloc root for the newly created snapshot
+ *
+ * This is similar to btrfs_init_reloc_root(), we come out of here with two
+ * references held on the reloc_root, one for root->reloc_root and one for
+ * rc->reloc_roots.
+ */
+int btrfs_reloc_post_snapshot(struct btrfs_trans_handle *trans,
+			       struct btrfs_pending_snapshot *pending)
+{
+	struct btrfs_root *root = pending->root;
+	struct btrfs_root *reloc_root;
+	struct btrfs_root *new_root;
+	struct reloc_control *rc = root->fs_info->reloc_ctl;
+	int ret;
+
+	if (!rc || !have_reloc_root(root))
+		return 0;
+
+	rc = root->fs_info->reloc_ctl;
+	rc->merging_rsv_size += rc->nodes_relocated;
+
+	if (rc->merge_reloc_tree) {
+		ret = btrfs_block_rsv_migrate(&pending->block_rsv,
+					      rc->block_rsv,
+					      rc->nodes_relocated, true);
+		if (ret)
+			return ret;
+	}
+
+	new_root = pending->snap;
+	reloc_root = create_reloc_root(trans, root->reloc_root,
+				       new_root->root_key.objectid);
+	if (IS_ERR(reloc_root))
+		return PTR_ERR(reloc_root);
+
+	ret = __add_reloc_root(reloc_root);
+	ASSERT(ret != -EEXIST);
+	if (ret) {
+		/* Pairs with create_reloc_root */
+		btrfs_put_root(reloc_root);
+		return ret;
+	}
+	new_root->reloc_root = btrfs_grab_root(reloc_root);
+
+	if (rc->create_reloc_tree)
+		ret = clone_backref_node(trans, rc, root, reloc_root);
+	return ret;
+}