linux-kernel - Re: [PATCH 1/3] exportfs: Rename get_uuid() to get_disk

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Message-ID: <20260114131130.GA6967@lst.de>
Date: Wed, 14 Jan 2026 14:11:30 +0100
From: Christoph Hellwig <hch@....de>
To: Amir Goldstein <amir73il@...il.com>
Cc: Christoph Hellwig <hch@....de>, "Darrick J. Wong" <djwong@...nel.org>,
	André Almeida <andrealmeid@...lia.com>,
	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,
	linux-btrfs@...r.kernel.org, linux-unionfs@...r.kernel.org,
	kernel-dev@...lia.com
Subject: Re: [PATCH 1/3] exportfs: Rename get_uuid() to get_disk_uuid()

On Wed, Jan 14, 2026 at 11:12:17AM +0100, Amir Goldstein wrote:
> In the context of overlayfs index and "origin" xattr, this is exactly what is
> needed - to validate that the object's copy up source is reliable for
> the generation of a unique overlayfs object id.

And that's what is in sb->s_uuid.  And it better be persistent.

> TBH, I am not sure if the file handle domain is invariant to XFS admin
> change of uuid. How likely it is to get an identical file handles for two
> different objects, with XFS fs which have diverged by an LVM clone?
> I think it's quite likely.

Of course it is, unlike you explicitly change it using xfs_admin.  Note
that to even mount two clones/snapshots you need to mount with nouuid,
so it doesn't happen accidentally.

> Whether or not we should repurpose the existing get_uuid() I don't
> know - that depends whether pNFS expects the same UUID from an
> "xfs clone" as overlayfs would.

That method does not just return an uuid, but in fact a uniqueue
identifier of the file systems choice and the offset/len where to
look for it on disk, as that is how pnfs/block finds the matching
device.  It is a dangerous concept and should not spread further.