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>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/btrfs/free-space-tree.h b/fs/btrfs/free-space-tree.h
index dc2463e4cfe3..6d5551d0ced8 100644
--- a/fs/btrfs/free-space-tree.h
+++ b/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);