Re: [PATCH 1/1] nfs: return a write delegation when a SETATTR drops our write access

[Benjamin Coddington]

On 29 May 2026, at 10:00, Trond Myklebust wrote:
>
> Hmmm... I'd argue that while recalling the delegation in this case is
> mandatory for NFSv4.0, that is certainly not true for NFSv4.1.
>
> Furthermore, I'd argue that if the holder of a write delegation is just
> changing the mode, then that should never result in a delegation recall
> for a well written NFSv4.1 server. The reason is this does not impact
> the client's ability to cache data, metadata or lock state. It only
> impacts its ability to rely on previously cached access data when
> handling new opens.
>
> One can argue whether or not it's needed for a uid or gid change by
> said holder of the delegation, but there too I'd say the right
> behaviour is to err on the side of not recalling.
> The exception might be if this is an attribute delegation, and the
> result will be that the cred attached to the delegation will no longer
> be able to issue a SETATTR to update the atime/mtime on delegation
> return.
>
> So yes to pre-emptive invalidation of the access cache, but I'm very
> sceptical to actually pre-emptively returning the delegation or even
> the layouts.

The latency I was chasing comes from the server electing to recall on these
SETATTRs.

You're right that for v4.1 the mode change doesn't need to trigger a recall,
and that the only client-side exposure is stale cached access on subsequent
opens.  It's already covered in the SETATTR reply path changing mode/uid/gid
where it sets NFS_INO_INVALID_ACCESS in nfs_update_inode(), and a fresh open
over the delegation still goes through nfs_may_open() -> nfs_do_access(), so
it revalidates with the server rather than trusting the cached result.

The server isn't recalling these for cache coherence.  When the change
removes the holder's write, a later SETATTR(size) under the delegation
carries the delegation stateid, not an open stateid
(nfs4_copy_delegation_stateid()), and the client can reopen O_WRONLY from
the delegation with no OPEN on the wire — so the server can't tell whether
the holder still has the file open for write, nor apply the usual
"open-for-write overrides current mode" check for the truncate. The recall
forces a DELEGRETURN + CLAIM_DELEGATE_CUR reclaim that re-establishes a real
(or absent) open stateid, letting the server decide the truncate correctly.

For write-retaining SETATTRs none of that applies and the server shouldn't
recall — which is where the latency I was chasing actually came from, and
the better place to remove it.

Thanks for the review.

Ben