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

["Darrick J. Wong" <djwong@kernel.org>]

On Sun, Mar 22, 2026 at 11:16:24PM -0500, Theodore Tso wrote:
> 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.

I don't love 'nouuid' either, because it means something completely
different in XFS.  'fsid_from_dev' or something would at least be
clearer about what it's doing...

> 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.

...but why not just set fsid to some approximation of the dev_t like
XFS and be done with it?

	st->f_fsid = u64_to_fsid(huge_encode_dev(mp->m_ddev_targp->bt_dev))

There are a few other single-bdev filesystems that do this.

--D

> Thoughts, comments?
> 
> 						- Ted
>