btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs

Currently, btrfs_relocate_chunk() is removing relocated BG by itself. But
the work can be done by btrfs_delete_unused_bgs() (and it's better since it
trim the BG). Let's dedupe the code.

While btrfs_delete_unused_bgs() is already hitting the relocated BG, it
skip the BG since the BG has "ro" flag set (to keep balancing BG intact).
On the other hand, btrfs cannot drop "ro" flag here to prevent additional
writes. So this patch make use of "removed" flag.
btrfs_delete_unused_bgs() now detect the flag to distinguish whether a
read-only BG is relocating or not.

Signed-off-by: Naohiro Aota <naohiro.aota@hgst.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -10833,7 +10833,7 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
 		spin_lock(&block_group->lock);
 		if (block_group->reserved ||
 		    btrfs_block_group_used(&block_group->item) ||
-		    block_group->ro ||
+		    (block_group->ro && !block_group->removed) ||
 		    list_is_singular(&block_group->list)) {
 			/*
 			 * We want to bail if we made new allocations or have

fs/btrfs/volumes.c

@@ -2908,8 +2908,8 @@ out:
 static int btrfs_relocate_chunk(struct btrfs_root *root, u64 chunk_offset)
 {
 	struct btrfs_root *extent_root;
-	struct btrfs_trans_handle *trans;
 	int ret;
+	struct btrfs_block_group_cache *block_group;
 
 	root = root->fs_info->chunk_root;
 	extent_root = root->fs_info->extent_root;
@@ -2939,21 +2939,17 @@ static int btrfs_relocate_chunk(struct btrfs_root *root, u64 chunk_offset)
 	if (ret)
 		return ret;
 
-	trans = btrfs_start_trans_remove_block_group(root->fs_info,
-						     chunk_offset);
-	if (IS_ERR(trans)) {
-		ret = PTR_ERR(trans);
-		btrfs_handle_fs_error(root->fs_info, ret, NULL);
-		return ret;
-	}
-
 	/*
-	 * step two, delete the device extents and the
-	 * chunk tree entries
+	 * step two, flag the chunk as removed and let
+	 * btrfs_delete_unused_bgs() remove it.
 	 */
-	ret = btrfs_remove_chunk(trans, root, chunk_offset);
-	btrfs_end_transaction(trans, extent_root);
-	return ret;
+	block_group = btrfs_lookup_block_group(root->fs_info, chunk_offset);
+	spin_lock(&block_group->lock);
+	block_group->removed = 1;
+	spin_unlock(&block_group->lock);
+	btrfs_put_block_group(block_group);
+
+	return 0;
 }
 
 static int btrfs_relocate_sys_chunks(struct btrfs_root *root)