[nfsv4] handling GETATTR vs. fh with outstanding write delegation

[Jeff Layton <[e-mail]>]

Dai has started (finally) implementing write delegations in the Linux
kernel NFS server, and we've been looking to the spec for guidance.
This section of RFC8881 has a long treatment on the use of CB_GETATTR:

    https://datatracker.ietf.org/doc/html/rfc8881#section-10.4.3

At the bottom, in particular, it says:

    It should be noted that the server is under no obligation to use
    CB_GETATTR, and therefore the server MAY simply recall the
    delegation to avoid its use.

However, at least one server implementor (Netapp) seems to ignore this.
The filer simply reports the info it has in response to a GETATTR
without using CB_GETATTR and without recalling a delegation.

Given that the client can buffer up writes even when it doesn't have a
write delegation, what's the rationale for requiring a tighter level of
cache coherency just because there is a delegation outstanding?

Note that we have not yet implemented timestamp/attr delegations:

    https://datatracker.ietf.org/doc/draft-ietf-nfsv4-delstid/02/

Will we be any worse off if we were to also ignore this bit of the spec
and allow GETATTRs without CB_GETATTR or CB_RECALL?
-- 
Jeff Layton <[e-mail]>