Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

["Theodore Tso" <tytso@mit.edu>]

On Sat, Mar 21, 2026 at 07:55:19PM +0800, Anand Jain wrote:
> statfs() currently reports f_fsid derived from the on-disk UUID.
> Cloned block devices share the same UUID, so distinct ext4 instances
> can return identical f_fsid values. This leads to collisions in
> fanotify.
> 
> Encode sb->s_dev into f_fsid instead of using the superblock UUID.
> This provides a per-device identifier and avoids conflicts when
> filesystem is cloned, matching the behavior with xfs.

As I observed in [1] this leads to collisions when for removable block
devices which can be used to mount different file systems.

[1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/

> Place this change behind the new mount option "-o nouuid" for ABI
> compatibility.

I *really* hate this mount option.  It's not at all obvious what it
means for a system administrator who hasn't had the context of reading
the e-mail discussion on this subject.

As I stated in [1], I think the f_fsid is a terrible interface that
was promulgated by history, and future usage should be strongly
discouraged, and the wise programmer won't use it because it has
significant compatibility issues.

As such, my personal preference is that we not try to condition it on
a mount option, which in all likelihood almost no one will use, and
instead just change it so that we hash the file system's UUID and
block device number together and use that for ext4's f_fsid.

Thoughts, comments?

						- Ted