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

[Christoph Hellwig <hch@infradead.org>]

On Sat, Apr 04, 2026 at 04:59:08PM +0800, Anand Jain wrote:
> Context:
> Currently, ext4's f_fsid is consistent across reboots but fails to be
> unique when dealing with cloned filesystems (sharing the same UUID). Per
> statfs(2) [1], the primary requirement is that the (f_fsid, ino) pair
> uniquely identifies a file. The man page makes no explicit guarantee
> regarding consistency across mount cycles or reboots.
> 
> Proposal:
> With this fix, f_fsid becomes f(uuid, dev_t). This ensures OS-wide
> uniqueness and maintains consistency as long as the underlying dev_t
> remains stable.
> 
> Dilemma:
> While statfs(2) [1] suggests f_fsid is "some random stuff," we know
> userspace (NFS, systemd) often treats it as a persistent handle.
> 
> Do you prefer one of the names above, or is there a more idiomatic ext4
> naming convention I should follow?
> 
> Given the ambiguity in the man page, is gating this behind an -o option
> necessary, or should we consider making uniqueness the default behavior?
> 

My take is that anything that should persist should be an on-disk
feature flag, not a mount option.  But I'm not in charge for ext4.