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

[Trond Myklebust <[e-mail]>]

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.
…. 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.


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