diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 1fe82cfc1d93..ac606f076eb7 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -203,6 +203,8 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 			em->block_start = merge->block_start;
 			merge->in_tree = 0;
 			if (merge->generation > em->generation) {
+				em->mod_start = em->start;
+				em->mod_len = em->len;
 				em->generation = merge->generation;
 				list_move(&em->list, &tree->modified_extents);
 			}
@@ -222,6 +224,7 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 		rb_erase(&merge->rb_node, &tree->map);
 		merge->in_tree = 0;
 		if (merge->generation > em->generation) {
+			em->mod_len = em->len;
 			em->generation = merge->generation;
 			list_move(&em->list, &tree->modified_extents);
 		}
@@ -247,6 +250,7 @@ int unpin_extent_cache(struct extent_map_tree *tree, u64 start, u64 len,
 {
 	int ret = 0;
 	struct extent_map *em;
+	bool prealloc = false;
 
 	write_lock(&tree->lock);
 	em = lookup_extent_mapping(tree, start, len);
@@ -259,8 +263,21 @@ int unpin_extent_cache(struct extent_map_tree *tree, u64 start, u64 len,
 	list_move(&em->list, &tree->modified_extents);
 	em->generation = gen;
 	clear_bit(EXTENT_FLAG_PINNED, &em->flags);
+	em->mod_start = em->start;
+	em->mod_len = em->len;
+
+	if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) {
+		prealloc = true;
+		clear_bit(EXTENT_FLAG_PREALLOC, &em->flags);
+	}
 
 	try_merge_map(tree, em);
+
+	if (prealloc) {
+		em->mod_start = em->start;
+		em->mod_len = em->len;
+	}
+
 	free_extent_map(em);
 out:
 	write_unlock(&tree->lock);
@@ -298,6 +315,9 @@ int add_extent_mapping(struct extent_map_tree *tree,
 	}
 	atomic_inc(&em->refs);
 
+	em->mod_start = em->start;
+	em->mod_len = em->len;
+
 	try_merge_map(tree, em);
 out:
 	return ret;
diff --git a/fs/btrfs/extent_map.h b/fs/btrfs/extent_map.h
index 2388a60bd6e3..8e6294b51357 100644
--- a/fs/btrfs/extent_map.h
+++ b/fs/btrfs/extent_map.h
@@ -20,6 +20,8 @@ struct extent_map {
 	/* all of these are in bytes */
 	u64 start;
 	u64 len;
+	u64 mod_start;
+	u64 mod_len;
 	u64 orig_start;
 	u64 block_start;
 	u64 block_len;
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index ca4fa05171ab..878116d9625d 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1308,6 +1308,7 @@ out_check:
 			em->block_start = disk_bytenr;
 			em->bdev = root->fs_info->fs_devices->latest_bdev;
 			set_bit(EXTENT_FLAG_PINNED, &em->flags);
+			set_bit(EXTENT_FLAG_PREALLOC, &em->flags);
 			while (1) {
 				write_lock(&em_tree->lock);
 				ret = add_extent_mapping(em_tree, em);
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 58075d711d24..71e71539ffb7 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2833,8 +2833,8 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
 	struct btrfs_root *log = root->log_root;
 	struct btrfs_file_extent_item *fi;
 	struct btrfs_key key;
-	u64 start = em->start;
-	u64 len = em->len;
+	u64 start = em->mod_start;
+	u64 len = em->mod_len;
 	u64 num_bytes;
 	int nritems;
 	int ret;
@@ -2970,7 +2970,7 @@ static int btrfs_log_changed_extents(struct btrfs_trans_handle *trans,
 		 * sequential then we need to copy the items we have and redo
 		 * our search
 		 */
-		if (args.nr && em->start != args.next_offset) {
+		if (args.nr && em->mod_start != args.next_offset) {
 			ret = copy_items(trans, log, dst_path, args.src,
 					 args.start_slot, args.nr,
 					 LOG_INODE_ALL);