Re: [f2fs-dev] [PATCH v8 05/13] fstests: add _require_unique_f_fsid() helper

[Anand Suveer Jain via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On 2/9/26 00:09, Zorro Lang wrote:
> 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.

Theoretically, whether f_fsid is unique per clone depends on
how it is derived. If it includes the device major:minor,
it provides a unique ID per clone.

Btrfs does it since commit
 c2a74ed0494c btrfs: derive f_fsid from on-disk fsid and dev_t

> 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

So _notrun() unless confirmed to have unique f_fsid for the cloned
filesystem. Works for me.

Thanks, Anand

> 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
>>



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