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

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Message-ID: <20260115083131.GA12267@lst.de>
Date: Thu, 15 Jan 2026 09:31:31 +0100
From: Christoph Hellwig <hch@....de>
To: Qu Wenruo <wqu@...e.com>
Cc: Christoph Hellwig <hch@....de>,
	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>,
	Amir Goldstein <amir73il@...il.com>, 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 3/3] ovl: Use real disk UUID for origin file handles

On Thu, Jan 15, 2026 at 06:39:05PM +1030, Qu Wenruo wrote:
>>> Then for those SINGLE_DEV fses, disable any multi-device related features,
>>> and use their dev_t to distinguish different fses just like EXT4/XFS,
>>> without bothering the current tempfsid hack, and just return the same fsid.
>>
>> dev_t is not related to the uuid in any way for XFS, and while I'm not
>> an expert there I'm pretty sure ext4 uses the same not dev related uuid
>> generation.
>
> My bad, by dev_t I mean bdev holder, which is a pointer to the super block 
> of the mounted fs. (And btrfs just recently join this common usage)

How is that related?  That prevents multiple callers from exclusively
using a block device.  Cloned file systems will sit on different devices.

> Yeah, although it's possible to mount different devices with same fsid 
> separately, I don't think it's really that a good idea either.
>
> Thus I really prefer to have special flags for those "uncommon" use cases, 
> other than the current automatically enabled tempfsid feature.

For XFS we require the nouuid option to allows additional mounts of
file systems with the same uuid.  I think that is the equivalent to
what btrfs calls the fsid.