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, 1 insertions, 2 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 0f2f915e42b0..ec18e2210602 100644
--- a/fs/btrfs/super.c
+++ b/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;
 	}