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

[Qu Wenruo <wqu@suse.com>]

在 2025/10/15 04:54, David Sterba 写道:
> On Mon, Oct 13, 2025 at 10:57:06PM -0300, André Almeida 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.
>> - Have a workaround in ovl for btrfs.
>> - Document this as unsupported, and userland needs to erase overlay.origin each
>> time it wants to remount.
>> - If ovl detects that temp_fsid and index are being used at the same time,
>> refuses to mount.
>>
>> I'm not sure which one would be better here, so I would like to hear some ideas
>> on this.
> 
> I haven't looked deeper if there's a workable solution, but the feature
> combination should be refused. I don't think this will affect many
> users.
> 

I believe the root problem is that we're not fully implementing the 
proper handling just like other single-device fses.

We do not use on-disk flags which means at least one fsid is registered 
into btrfs, thus we have to use different temp-fsid.

If fully single-device feature flag is properly implemented, we should 
be able to return the same uuid without extra hacks thus solve the problem.

Thanks,
Qu