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>