Re: [nfsv4] RFC: Is Open/Claim_Delegate_Prev done before/after Reclaim_complete?

[Rick Macklem <[e-mail]>]

On Fri, Apr 5, 2024 at 8:24 PM Trond Myklebust <[e-mail]> wrote:
>
>
>
> On Apr 5, 2024, at 23:02, Trond Myklebust <[e-mail]> wrote:
>
> On Fri, 2024-04-05 at 16:13 -0700, Rick Macklem wrote:
>
> On Thu, Apr 4, 2024 at 9:18 PM Trond Myklebust
> <[e-mail]> wrote:
>
>
> On Thu, 2024-04-04 at 16:23 -0700, Rick Macklem wrote:
>
> Hi,
>
> I am in the process of implementing Open/Claim_Delegate_Prev and
> have
> run
> into a question I cannot seem to answer from reading RFC8881.
>
> There is this snippet:
>    enum open_claim_type4 {
>            /*
>             * Not a reclaim.
>             */
>            CLAIM_NULL              = 0,
>
>            CLAIM_PREVIOUS          = 1,
>            CLAIM_DELEGATE_CUR      = 2,
>            CLAIM_DELEGATE_PREV     = 3,
>
>            /*
>             * Not a reclaim.
>             *
>             * Like CLAIM_NULL, but object identified
>             * by the current filehandle.
>             */
>            CLAIM_FH                = 4, /* new to v4.1 */
> The comments in the above seem to "hint" that CLAIM_DELEGATE_PREV
> is
> a
> reclaim type operation.
> If that is the case, I'd assume it/should be performed by the
> client
> before it does a
> Reclaim_Complete?
>
> Then there is this snippet:
>    For OPEN requests that reach the server during the grace
> period,
> the
>    server returns an error of NFS4ERR_GRACE.  The following claim
> types
>    are exceptions:
>
>    *  OPEN requests specifying the claim type CLAIM_PREVIOUS are
> devoted
>       to reclaiming opens after a server restart and are
> typically
> only
>       valid during the grace period.
>
>    *  OPEN requests specifying the claim types CLAIM_DELEGATE_CUR
> and
>       CLAIM_DELEG_CUR_FH are valid both during and after the
> grace
>       period.  Since the granting of the delegation that they are
>       subordinate to assures that there is no conflict with locks
> to
> be
>       reclaimed by other clients, the server need not return
>       NFS4ERR_GRACE when these are received during the grace
> period.
> This clearly states that CLAIM_DELEGATE_PREV cannot be performed
> during the server's grace period.
>
> The above two statements are not necessarily contradictory but it
> does seem
> that, if the server is in its grace period, the Reclaim_Complete
> needs to be
> done before any Open/Claim_Delegate_Prev, since Reclaim_Complete
> tells
> the server when to end its grace period.
> Note that, usually, a client would not be rebooting when a server
> is
> in its
> grace period, but that could happen.
>
> Bottom line...
> Should a client perform Open/Claim_Delegate_Prev before or after
> Reclaim_Complete?
> And, if the answer is "after", how does the server know when a
> client
> is done recovering opens/delegations via
> Open/Claim_Delegate_Prev?
>
> Thanks for any help with this, rick
>
>
> CLAIM_DELEGATE_PREV has now survived at least 24 years as part of a
> "standard" and has yet to be properly described. I can find
> references
> to it in RFC3010 and all NFSv4 descriptions since then, but I have
> yet
> to meet anybody who can claim to have implemented it in a product.
>
> IMO, it is time to drop it, and start afresh with something that
> has
> actually been road tested before someone started putting pen to
> paper.
>
> Well, I am going to respond to David's post in some more detail, but
> I will
> note here that, with the exception of whether to do it before or
> after
> Reclaim_complete,
> I have not found serious issues with it. (Btw, the FreeBSD server has
> had a "dormant"
> implementation for about 20years, although I am just now trying to
> use it by the
> FreeBSD client. I was wondering if I would find any other server
> implementation at
> the Bakeathon in a couple of weeks.)
>
>
> I see. So how do you plan to deal with the case where the server denies
> the request for a delegation? Will your client just toss away the dirty
> data?
>
>
> My problem with CLAIM_DELEGATE_PREV is that the “spec” has no description whatsoever for what happens in the “unhappy path” case, where the application has done everything right to apply a share lock, or a byte range lock to the parts that it plans to modify, and yet because the server cannot be contacted, the client lets everything proceed because it wants to assert a delegation once the server is back online.

I have assumed that Open/Claim_delegate_prev will only be used by a
client after it has rebooted. I cannot see it being
any more/less useful for recovery from network partitioning than the
rest of the Opens.

I assume that a server will reply to an Open/Claim_delegate_prev with
NFS4ERR_EXPIRED if it does not have such a
delegation for the client, since it was not able to recall it.
I'm not sure if the spec states that, but since delegations are
covered by the lease, it makes
sense to me?

No doubt network partitioning and NFS4ERR_EXPIRED is ugly. But I do
not see anything different for CLAIM_DELEGATE_PREV
than other open/delegation handling. Typically a client mount will
hang pretty quickly during a network partitioning, but applications
can certainly write a lot of dirty data into a client's data cache
before the network partitioning is noticed and a client does not
have any good way to deal with the dirty data.

> …. and yet the assertion of the CLAIM_DELEGATE_PREV delegation can trivially fail if someone has modified or deleted the file during the period when the client was unable to talk to the server, and there is nothing in the spec that advises what to expect next.
If the file has been deleted, all opens will fail (NFS4ERR_NOENT or
NFS4ERR_STALE?).

If the file was modified and the client does a Claim_null open, the
only indication
that it has been modified is the change attribute's value, which it
might not have kept.
At least the open/Claim_delegate_prev fails (presumably with NFS4ERR_EXPIRED)
and the client knows the file might have been modified by someone
else. Not great,
but I do not see how it is worse off than without Claim_delegate_prev?

rick

>
>
> _________________________________
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> [e-mail]
>