linux-kernel - Re: [RFC PATCH 1/1] ovl: Use fsid as unique identifier for trusted origin

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Message-ID: <aPas60j7AoyLLQK0@dread.disaster.area>
Date: Tue, 21 Oct 2025 08:43:07 +1100
From: Dave Chinner <david@...morbit.com>
To: Anand Jain <anajain.sg@...il.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	André Almeida <andrealmeid@...lia.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kernel-dev@...lia.com, Miklos Szeredi <miklos@...redi.hu>,
	Amir Goldstein <amir73il@...il.com>, Chris Mason <clm@...com>,
	David Sterba <dsterba@...e.com>,
	"Guilherme G . Piccoli" <gpiccoli@...lia.com>
Subject: Re: [RFC PATCH 1/1] ovl: Use fsid as unique identifier for trusted
 origin

On Wed, Oct 15, 2025 at 07:46:34AM +0800, Anand Jain wrote:
> On 14-Oct-25 12:39 PM, Christoph Hellwig wrote:
> > On Mon, Oct 13, 2025 at 10:57:07PM -0300, André Almeida wrote:
> > > Some filesystem have non-persistent UUIDs, that can change
> > > between mounting, even if the filesystem is not modified. To
> > > prevent false-positives when mounting overlayfs with index
> > > enabled, use the fsid reported from statfs that is persistent
> > > across mounts.
> > 
> > Please fix btrfs to not change uuids, as that completely defeats
> > the point of uuids.
> 
> We needed cloned device mount support for an A/B testing use case,
> but changing the on-disk UUID defeats the purpose.
> 
> Right now, ext4 and Btrfs can mount identical devices, but XFS
> can't.

Absolutely not true.

XFS has been able to mount filesystems with duplicate UUIDs on Linux
for almost 25 years. The "-o nouuid" mount option (introduced in
2001) to bypass the duplicate uuid checks done at mount time.

XFS tracks all mounted filesystem UUIDs largely to prevent multiple
mounts of the same filesystem due to multipath storage presenting it
via multiple different block devices.

The nouuid mount option was added back when enterprise storage
arrays started supporting hardware level thinp and LUN
clone/snapshot functionality. Adding "-o nouuid" allowed cloned LUNs
to be mounted for for backup/recovery purposes whilst the main
filesystem was still mounted and in active use.

> How about extending this to the common
> VFS layer and adding a parameter to tell apart a cloned
> device from the same device accessed through multiple
> paths?

Perhaps we should lift the XFS UUID tracking code to the VFS
and intercept "-o nouuid" at the VFS to allow duplicates only when
that mount option is set?

-Dave.
-- 
Dave Chinner
david@...morbit.com