Re: [PATCH v8 05/13] fstests: add _require_unique_f_fsid() helper

[Zorro Lang <zlang@kernel.org>]

On Sat, Jul 25, 2026 at 03:39:02PM +0800, Anand Jain wrote:
> Add a helper to check if the target filesystem supports unique f_fsid
> tracking across cloned or snapshot instances.
> 
> Certain filesystems like XFS, Btrfs, and F2FS ensure unique f_fsid
> identifiers per filesystem instance. However, Ext4 derives its f_fsid
> directly from its superblock UUID, which leads to identical f_fsid
> values on cloned images until the UUID is manually modified by userspace.
> 
> Introduce _require_unique_f_fsid() to allow test cases requiring strict
> f_fsid uniqueness to skip gracefully on unsupported filesystems.
> 
> Signed-off-by: Anand Jain <asj@kernel.org>
> ---
>  common/rc | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/common/rc b/common/rc
> index 21b516accdf7..f73251365a60 100644
> --- a/common/rc
> +++ b/common/rc
> @@ -6356,6 +6356,33 @@ _require_fanotify_ioerrors()
>  	_notrun "$FSTYP does not support fanotify ioerrors"
>  }
>  
> +# Skip the test if the filesystem does not enforce unique f_fsids
> +# natively. Checking this dynamically requires recreating a clone
> +# layout, so we use a static lookup based on FSTYP.
> +#
> +# Across all filesystems, a UUID collision causes libblkid tools to return
> +# non-deterministic device mappings. It is ultimately the responsibility
> +# of the userspace utility or use-case to enforce uniqueness when a clone
> +# diverges. For details, see mailing list thread discussions:
> +#   Link: https://lore.kernel.org/linux-ext4/20260409131238.GC18443@macsyma-wired.lan/
> +_require_unique_f_fsid()
> +{
> +	case "$FSTYP" in
> +	ext*)
> +		# Ext4 derives f_fsid from the superblock UUID, meaning
> +		# clones share the same f_fsid until their UUIDs
> +		# diverge.
> +		_notrun "Target filesystem ($FSTYP) does not guarantee unique f_fsid on clones."
> +		;;
> +	*)
> +		# Conversely, XFS, Btrfs, and F2FS ensure f_fsid remains
> +		# unique per filesystem instance (often by deriving it
> +		# from the UUID and underlying block device.)
> +		;;
> +	esac

I would prefer to let the experts of each filesystem confirm whether their
fs guarantees a unique f_fsid. How about switching this logic to an *allowlist*
*(whitelist)* approach, where we _notrun by default and only allow filesystems
that explicitly guarantee unique f_fsid support?

  case "$FSTYP" in
  btrfs|xfs|f2fs)
        ;;
  *)
        _notrun "Target filesystem ($FSTYP) does not guarantee unique f_fsid on clones."
        ;;
  esac

Thanks,
Zorro

> +}
> +
> +
>  # Computes a percentage of the available space in a filesystem and
>  # returns that quantity in MB. The percentage must not contain a percent
>  # sign ("%").
> -- 
> 2.43.0
>