Re: [RFC PATCH 0/1] ovl: brtfs' temp_fsid doesn't work with ovl index=on

[Amir Goldstein]

On Tue, Oct 14, 2025 at 3:57 AM André Almeida <andrealmeid@xxxxxxxxxx> wrote:
>
> Hi everyone,
>
> When using overlayfs with the mount option index=on, the first time a directory is
> used as upper dir, overlayfs stores in a xattr "overlay.origin" the UUID of the
> filesystem being used in the layers. If the upper dir is reused, overlayfs
> refuses to mount for a different filesystem, by comparing the UUID with what's
> stored at overlay.origin, and it fails with "failed to verify upper root origin"
> on dmesg. Remounting with the very same fs is supported and works fine.
>
> However, btrfs mounts may have volatiles UUIDs. When mounting the exact same
> disk image with btrfs, a random UUID is assigned for the following disks each
> time they are mounted, stored at temp_fsid and used across the kernel as the
> disk UUID. `btrfs filesystem show` presents that. Calling statfs() however shows
> the original (and duplicated) UUID for all disks.
>
> This feature doesn't work well with overlayfs with index=on, as when the image
> is mounted a second time, will get a different UUID and ovl will refuse to
> mount, breaking the user expectation that using the same image should work. A
> small script can be find in the end of this cover letter that illustrates this.
>
> From this, I can think of some options:
>
> - Use statfs() internally to always get the fsid, that is persistent. The patch
> here illustrates that approach, but doesn't fully implement it.
> - Create a new sb op, called get_uuid() so the filesystem returns what's
> appropriated.

FWIW this operation already exists in export_operations.
It is currently only used by pnfs and only implemented by xfs.
I would nor object for overlayfs to use this method if implemented
and fall back to copying uuid directly from s_uuid
(better yet make it a vfs helper)
Note that commit
8f720d9f892e0 xfs: publish UUID in struct super_block
was done for a similar reason.
The xfs mount option nouuid is the poor man's solution for
mounting cloned disk images.

Thanks,
Amir.