linux-kernel - Re: [PATCH 3/3] ovl: Use real disk UUID for origin file handles

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Message-ID: <8ab387b1-c4aa-40a5-946f-f4510d8afd02@igalia.com>
Date: Fri, 23 Jan 2026 17:08:23 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Amir Goldstein <amir73il@...il.com>
Cc: Christoph Hellwig <hch@....de>, Chuck Lever <chuck.lever@...cle.com>,
 Jeff Layton <jlayton@...nel.org>, NeilBrown <neil@...wn.name>,
 Olga Kornievskaia <okorniev@...hat.com>, Dai Ngo <Dai.Ngo@...cle.com>,
 Tom Talpey <tom@...pey.com>, Carlos Maiolino <cem@...nel.org>,
 Chris Mason <clm@...com>, David Sterba <dsterba@...e.com>,
 Miklos Szeredi <miklos@...redi.hu>, Christian Brauner <brauner@...nel.org>,
 Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
 linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Qu Wenruo <wqu@...e.com>, linux-btrfs@...r.kernel.org,
 linux-unionfs@...r.kernel.org, kernel-dev@...lia.com, vivek@...labora.com,
 Ludovico de Nittis <ludovico.denittis@...labora.com>
Subject: Re: [PATCH 3/3] ovl: Use real disk UUID for origin file handles

Em 23/01/2026 10:24, André Almeida escreveu:
> 
> Em 22/01/2026 17:07, Amir Goldstein escreveu:
>> On Tue, Jan 20, 2026 at 4:12 PM Amir Goldstein <amir73il@...il.com> 
>> wrote:
>>>
>>> On Mon, Jan 19, 2026 at 5:56 PM André Almeida 
>>> <andrealmeid@...lia.com> wrote:
>>>>
>> ...
>>>> Actually they are not in the same fs, upper and lower are coming from
>>>> different fs', so when trying to mount I get the fallback to
>>>> `uuid=null`. A quick hack circumventing this check makes the mount 
>>>> work.
>>>>
>>>> If you think this is the best way to solve this issue (rather than
>>>> following the VFS helper path for instance),
>>>
>>> That's up to you if you want to solve the "all lower layers on same fs"
>>> or want to also allow lower layers on different fs.
>>> The former could be solved by relaxing the ovl rules.
>>>
>>>> please let me know how can
>>>> I safely lift this restriction, like maybe adding a new flag for this?
>>>
>>> I think the attached patch should work for you and should not
>>> break anything.
>>>
>>> It's only sanity tested and will need to write tests to verify it.
>>>
>>
>> Andre,
>>
>> I tested the patch and it looks good on my side.
>> If you want me to queue this patch for 7.0,
>> please let me know if it addresses your use case.
>>
> 
> Hi Amir,
> 
> I'm still testing it to make sure it works my case, I will return to you 
> ASAP. Thanks for the help!
> 

So, your patch wasn't initially working in my setup here, and after some 
debugging it turns out that on ovl_verify_fh() *fh would have a NULL 
UUID, but *ofh would have a valid UUID, so the compare would then fail.

Adding this line at ovl_get_fh() fixed the issue for me and made the 
patch work as I was expecting:

+       if (!ovl_origin_uuid(ofs))
+               fh->fb.uuid = uuid_null;
+
         return fh;

Please let me know if that makes sense to you.

Thanks!
	André