btrfs: make clear_cache mount option to rebuild FST without disabling it

Previously clear_cache mount option would simply disable free-space-tree
feature temporarily then re-enable it to rebuild the whole free space
tree.

But this is problematic for block-group-tree feature, as we have an
artificial dependency on free-space-tree feature.

If we go the existing method, after clearing the free-space-tree
feature, we would flip the filesystem to read-only mode, as we detect a
super block write with block-group-tree but no free-space-tree feature.

This patch would change the behavior by properly rebuilding the free
space tree without disabling this feature, thus allowing clear_cache
mount option to work with block group tree.

Now we can mount a filesystem with block-group-tree feature and
clear_mount option:

  $ mkfs.btrfs  -O block-group-tree /dev/test/scratch1  -f
  $ sudo mount /dev/test/scratch1 /mnt/btrfs -o clear_cache
  $ sudo dmesg -t | head -n 5
  BTRFS info (device dm-1): force clearing of disk cache
  BTRFS info (device dm-1): using free space tree
  BTRFS info (device dm-1): auto enabling async discard
  BTRFS info (device dm-1): rebuilding free space tree
  BTRFS info (device dm-1): checking UUID tree

CC: stable@vger.kernel.org # 6.1+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3121,23 +3121,34 @@ int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info)
 {
 	int ret;
 	const bool cache_opt = btrfs_test_opt(fs_info, SPACE_CACHE);
-	bool clear_free_space_tree = false;
+	bool rebuild_free_space_tree = false;
 
 	if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
 	    btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
-		clear_free_space_tree = true;
+		rebuild_free_space_tree = true;
 	} else if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
 		   !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID)) {
 		btrfs_warn(fs_info, "free space tree is invalid");
-		clear_free_space_tree = true;
+		rebuild_free_space_tree = true;
 	}
 
-	if (clear_free_space_tree) {
-		btrfs_info(fs_info, "clearing free space tree");
-		ret = btrfs_clear_free_space_tree(fs_info);
+	if (rebuild_free_space_tree) {
+		btrfs_info(fs_info, "rebuilding free space tree");
+		ret = btrfs_rebuild_free_space_tree(fs_info);
 		if (ret) {
 			btrfs_warn(fs_info,
-				   "failed to clear free space tree: %d", ret);
+				   "failed to rebuild free space tree: %d", ret);
+			goto out;
+		}
+	}
+
+	if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
+	    !btrfs_test_opt(fs_info, FREE_SPACE_TREE)) {
+		btrfs_info(fs_info, "disabling free space tree");
+		ret = btrfs_delete_free_space_tree(fs_info);
+		if (ret) {
+			btrfs_warn(fs_info,
+				   "failed to disable free space tree: %d", ret);
 			goto out;
 		}
 	}

fs/btrfs/free-space-tree.c

@@ -1252,7 +1252,7 @@ out:
 	return ret;
 }
 
-int btrfs_clear_free_space_tree(struct btrfs_fs_info *fs_info)
+int btrfs_delete_free_space_tree(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_trans_handle *trans;
 	struct btrfs_root *tree_root = fs_info->tree_root;
@@ -1298,6 +1298,54 @@ abort:
 	return ret;
 }
 
+int btrfs_rebuild_free_space_tree(struct btrfs_fs_info *fs_info)
+{
+	struct btrfs_trans_handle *trans;
+	struct btrfs_key key = {
+		.objectid = BTRFS_FREE_SPACE_TREE_OBJECTID,
+		.type = BTRFS_ROOT_ITEM_KEY,
+		.offset = 0,
+	};
+	struct btrfs_root *free_space_root = btrfs_global_root(fs_info, &key);
+	struct rb_node *node;
+	int ret;
+
+	trans = btrfs_start_transaction(free_space_root, 1);
+	if (IS_ERR(trans))
+		return PTR_ERR(trans);
+
+	set_bit(BTRFS_FS_CREATING_FREE_SPACE_TREE, &fs_info->flags);
+	set_bit(BTRFS_FS_FREE_SPACE_TREE_UNTRUSTED, &fs_info->flags);
+
+	ret = clear_free_space_tree(trans, free_space_root);
+	if (ret)
+		goto abort;
+
+	node = rb_first_cached(&fs_info->block_group_cache_tree);
+	while (node) {
+		struct btrfs_block_group *block_group;
+
+		block_group = rb_entry(node, struct btrfs_block_group,
+				       cache_node);
+		ret = populate_free_space_tree(trans, block_group);
+		if (ret)
+			goto abort;
+		node = rb_next(node);
+	}
+
+	btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE);
+	btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
+	clear_bit(BTRFS_FS_CREATING_FREE_SPACE_TREE, &fs_info->flags);
+
+	ret = btrfs_commit_transaction(trans);
+	clear_bit(BTRFS_FS_FREE_SPACE_TREE_UNTRUSTED, &fs_info->flags);
+	return ret;
+abort:
+	btrfs_abort_transaction(trans, ret);
+	btrfs_end_transaction(trans);
+	return ret;
+}
+
 static int __add_block_group_free_space(struct btrfs_trans_handle *trans,
 					struct btrfs_block_group *block_group,
 					struct btrfs_path *path)

fs/btrfs/free-space-tree.h

@@ -18,7 +18,8 @@ struct btrfs_caching_control;
 
 void set_free_space_tree_thresholds(struct btrfs_block_group *block_group);
 int btrfs_create_free_space_tree(struct btrfs_fs_info *fs_info);
-int btrfs_clear_free_space_tree(struct btrfs_fs_info *fs_info);
+int btrfs_delete_free_space_tree(struct btrfs_fs_info *fs_info);
+int btrfs_rebuild_free_space_tree(struct btrfs_fs_info *fs_info);
 int load_free_space_tree(struct btrfs_caching_control *caching_ctl);
 int add_block_group_free_space(struct btrfs_trans_handle *trans,
 			       struct btrfs_block_group *block_group);

fs/btrfs/super.c

@@ -828,8 +828,7 @@ out:
 		ret = -EINVAL;
 	}
 	if (btrfs_fs_compat_ro(info, BLOCK_GROUP_TREE) &&
-	    (btrfs_test_opt(info, CLEAR_CACHE) ||
-	     !btrfs_test_opt(info, FREE_SPACE_TREE))) {
+	     !btrfs_test_opt(info, FREE_SPACE_TREE)) {
 		btrfs_err(info, "cannot disable free space tree with block-group-tree feature");
 		ret = -EINVAL;
 	}