[PATCH] btrfs: fix spurious free_space_tree remount warning

[Boris Burkov <boris@bur.io>]

The intended logic of the check is to catch cases where the desired
free_space_tree setting doesn't match the mounted setting, and the
remount is anything but ro->rw. However, it makes the mistake of
checking equality on a masked integer (btrfs_test_opt) against a boolean
(btrfs_fs_compat_ro).

If you run the reproducer:
mount -o space_cache=v2 dev mnt
mount -o remount,ro mnt

you would expect no warning, because the remount is not attempting to
change the free space tree setting, but we do see the warning.

To fix this, convert the option test to a boolean.

I tested a variety of transitions:
sudo mount -o space_cache=v2 /dev/vg0/lv0 mnt/lol
(fst enabled)
mount -o remount,ro mnt/lol
(no warning, no fst change)
sudo mount -o remount,rw,space_cache=v1,clear_cache
(no warning, ro->rw)
sudo mount -o remount,rw,space_cache=v2 mnt
(warning, rw->rw with change)
sudo mount -o remount,ro mnt
(no warning, no fst change)
sudo mount -o remount,rw,space_cache=v2 mnt
(no warning, no fst change)

Reported-by: Chris Murphy <lists@colorremedies.com>
Signed-off-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index f8435641b912..d4992ceab5ea 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1918,7 +1918,7 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 	btrfs_resize_thread_pool(fs_info,
 		fs_info->thread_pool_size, old_thread_pool_size);
 
-	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE) !=
+	if (!!btrfs_test_opt(fs_info, FREE_SPACE_TREE) !=
 	    btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
 	    (!sb_rdonly(sb) || (*flags & SB_RDONLY))) {
 		btrfs_warn(fs_info,
-- 
2.24.1