On Thu, Feb 25, 2021 at 03:59:20PM +0100, David Sterba wrote:

On Tue, Feb 23, 2021 at 08:28:10PM +0200, Nikolay Borisov wrote:

quoted

On 23.02.21 г. 20:22 ч., Boris Burkov wrote:

quoted

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 <redacted>
Signed-off-by: Boris Burkov <redacted>
---
 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) !=

I'd rather thave the !! convert to  bool magic in the macro definition i.e : 

#define btrfs_test_opt(fs_info, opt)    !!((fs_info)->mount_opt & \               
                                               BTRFS_MOUNT_##opt)                     

Yeah, that sounds safer and we should convert all predicate functions to
bool eg. __btrfs_fs_compat_ro. The whole value of the macro needs to be
in ( .. ) too.

For the minimal quick fix I'd add (bool) cast to both sides of == so
it's clear and then we can do further cleanups in separate patches.