Re: [PATCH 4/4] NFSD: Send a meaningful CB_RECALL_ANY keep count

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2026-08-12 at 08:17 +0200, Cedric Blancher wrote:
> On Tue, 11 Aug 2026 at 21:57, Chuck Lever <cel@kernel.org> wrote:
> > 
> > deleg_reaper() sets craa_objects_to_keep to zero on every
> > CB_RECALL_ANY. Per RFC 8881 Section 20.6.3, that asks the client to
> > retain no read or write delegation at all, whether or not the
> > delegation backs an open file.
> > 
> > The field names a count the client may keep. The client picks which
> > objects to return, because the server cannot read lack of recent
> > use as lack of usefulness. Zero leaves nothing to choose among. A
> > client that complies returns the delegations backing its open files
> > and reopens each one with CLAIM_DELEGATE_CUR, so NFSD trades a
> > delegation for an open stateid and recovers nothing. A client that
> > reads the zero as "unspecified" does nothing instead, and
> > nfsd4_cb_recall_any_done() inspects only the reply status, so NFSD
> > cannot tell the two apart.
> > 
> > Derive the keep count from cl_deleg_count and ask each client for a
> > single delegation. A larger request reaches delegations an
> > application still has open, and both callers re-arm while their
> > condition lasts. Skip a client holding one delegation rather than
> > send the zero again. That gate subsumes the list_empty() test above
> > it, and it belongs above the NFSD4_CALLBACK_RUNNING test_and_set. A
> > continue below that point latches the bit with no callback in
> > flight to clear it.
> > 
> > The Linux client ignores craa_objs_to_keep and returns unused
> > delegations from the type mask alone, so the count changes nothing
> > for it. The gate does. The reaper goes quiet for a client walked
> > down to one delegation.
> > 
> > Fixes: 44df6f439a17 ("NFSD: add delegation reaper to react to low memory condition")
> > Signed-off-by: Chuck Lever <cel@kernel.org>
> 
> ms-nfs41-client hit that bug when implementing CB_RECALL_ANY
> (https://github.com/kofemann/ms-nfs41-client/commit/3abe73c8ab0d924450a1c1c480f0b14f963f9531)
> with Linux 7.0 nfsd.
> What should existing NFSv4.1 clients do if they encounter a
> objects_to_keep value of 0? Right now it recalls ALL delegations,
> which basically is a "reset" of all delegations.
> 
> 

Like Chuck quotes above:

"Per RFC 8881 Section 20.6.3, that asks the client to retain no read or
write delegation at all, whether or not the delegation backs an open
file."

I think a compliant client would need to continue to return everything.

We should note that the Linux client apparently ignores
craa_objects_to_keep and just returns a single delegation, so you could
follow suit (but know that it's not following the spec).
-- 
Jeff Layton <jlayton@kernel.org>