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

[Christoph Hellwig <hch@lst.de>]

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.