Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Christoph Hellwig <hch@infradead.org>]

On Thu, Apr 16, 2026 at 11:21:41PM +0800, Anand Jain wrote:
> > But given that you originally stumbled across this with Overlayfs,
> > because it was originally using s_uuid, and that didn't work well for
> > btrfs, why not change overlayfs to just use s_uuid plus kdev_t in its
> > xattr, and just fix the problem for overlayfs?  That has the benefit
> > that it will work for all file system types in Linux, not just for
> > those where we have changed what f_fsid does.
> 
> Using `kdev_t` (or any derivation of it) for persistent storage, such
> as Overlayfs xattrs, is problematic. Since `kdev_t` is transient and
> inconsistent across reboots or device re-discovery, it could lead to
> broken associations.

Yes, using a dev_t in anything persistent is a really bad ida.

> It seems we've reached the functional limits of f_fsid.
> If we want to solve this properly for Overlayfs, NFS handles, or a
> complex system monitoring..etc, we need a new identifier let's call
> it f_fsid_v2, that meets the following requirements:
> 
>   System-wide Uniqueness: Must distinguish between cloned filesystems.
> 
>   Persistence: Must remain consistent across reboots/HW re-enumeration.
> 
>   Non-On-Disk: Must not be stored on-disk.

The third requirement doesn't make much sense to me.  If it is
persistent it. or something it can be derived from must be stored
on-disk.

> 
> 
> One possible implementation for f_fsid_v2 could be:
> 
>    f_fsid_v2 =  hash(s_uuid, block_device_serial, [subvol_id])
> 
> For pseudo block devices (virtio-blk, loop, nbd, brd,..),
> the serial could be derived recursively:
> 
>    serial_number = hash(backing_file.f_fsid_v2, backing_file.ino)

What i the point in this?  All of this seems to be better served
by s_uuid.

> Note on Hardware Serials:
>  Standard storage protocols (T10, NVMe, SAS) mandate unique,
>  persistent serials per LUN. While I've seen T10 protocol
>  violations during my time authoring Solaris HBA drivers, I
>  believe these outliers shouldn't dictate the design.

No, T10 does not actually mandate unique identifiers, NVMe does, but the
implementations are often totally broken.