On Wed, Apr 14, 2021 at 09:47:36PM +0800, Zhang Yi wrote:

Commit <87d8fe1ee6b8> ("add releasepage hooks to block devices which can
be used by file systems") introduce a hook that used by ext4 filesystem
to release journal buffers, but it doesn't add corresponding concurrency
protection that ->bdev_try_to_free_page() could be raced by umount
filesystem concurrently. This patch add a usage count on superblock that
filesystem can use it to prevent above race and make invoke
->bdev_try_to_free_page() safe.

We already have two refcounts in the superblock: s_active which counts
the active refernce, and s_count which prevents the data structures
from beeing freed.  I don't think we need yet another one.