Re: [PATCH v6 04/11] fstests: add _require_unique_f_fsid() helper

On 29/5/26 12:30, Darrick J. Wong wrote:
> On Thu, May 28, 2026 at 12:05:35PM +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 <[email protected]>
>> ---
>>  common/rc | 21 +++++++++++++++++++++
>>  1 file changed, 21 insertions(+)
>>
>> diff --git a/common/rc b/common/rc
>> index 937f478963b4..5446552aed92 100644
>> --- a/common/rc
>> +++ b/common/rc
>> @@ -6314,6 +6314,27 @@ _require_fanotify_ioerrors()
>>  	_notrun "$FSTYP does not support fanotify ioerrors"
>>  }
>>  
>> +# Ext4 derives f_fsid from the superblock UUID, meaning clones share the
>> +# same f_fsid until their UUIDs diverge. Conversely, XFS, Btrfs,
>> +# and F2FS ensure f_fsid remains unique per filesystem instance (often by
>> +# deriving it from the UUID and underlying block device.)
>> +#
>> +# Across all filesystems, a UUID collision causes libblkid tools to return
>> +# non-deterministic device mappings. It is ultimately the responsibility
> 
> "device mappings", as in /dev/disk/by-id/$UUID ?
> 

Correct.. I'll make it specific.

>> +# of the userspace utility or use-case to enforce uniqueness when a clone
>> +# diverges. For details, see mailing list thread discussions titled:
>> +#      "ext4: derive f_fsid from block device to avoid collisions".
> 
> How about providing a direct lore link?
> 

Sure, that will be..

Link:
https://lore.kernel.org/linux-ext4/[email protected]/

instead of the title.

Thanks, Anand


> --D
> 




>> +_require_unique_f_fsid()
>> +{
>> +	# 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.
>> +	if [ "$FSTYP" == "ext4" ]; then
>> +		_notrun "Target filesystem ($FSTYP) does not guarantee unique f_fsid on clones."
>> +	fi
>> +}
>> +
>> +
>>  # 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
>>
>>