linux-kernel - Re: [PATCH v4] nfsd: allow for up to 32 callback session slots

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Message-ID: <9A8D2B27-AF26-4A5F-A1A2-1C30935A026C@oracle.com>
Date: Thu, 14 Nov 2024 14:19:28 +0000
From: Chuck Lever III <chuck.lever@...cle.com>
To: Neil Brown <neilb@...e.de>
CC: Jeff Layton <jlayton@...nel.org>, Dai Ngo <dai.ngo@...cle.com>,
        Tom Talpey
	<tom@...pey.com>, Olga Kornievskaia <okorniev@...hat.com>,
        Linux NFS Mailing
 List <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] nfsd: allow for up to 32 callback session slots



> On Nov 13, 2024, at 10:19 PM, NeilBrown <neilb@...e.de> wrote:
> 
> On Thu, 14 Nov 2024, Jeff Layton wrote:
>> On Wed, 2024-11-13 at 12:31 +1100, NeilBrown wrote:
>>> 
>>> So initialising them all to 1 when the session is created, as you do in
>>> init_session(), is clearly correct.  Reinitialising them after
>>> target_highest_slot_id has been reduced and then increased is not
>>> justified by the above.
>>> 
>> 
>> But, once the client and server have forgotten about those slots after
>> shrinking the slot table, aren't they effectively new? IOW, once you've
>> shrunk the slot table, the slots are effectively "freed". Growing it
>> means that you have to allocate new ones. The fact that this patch just
>> keeps them around is an implementation detail.
> 
> 
> There is no text in the RFC about shrinking or growing or forgetting.
> The only meaning given to numbers like ca_maxreqs is that the client
> shouldn't use a larger slot number than the given one.
> 
> I think the slot table is conceptually infinite and exists in its
> entirety from the moment CREATE_SESSION completes to the moment
> DESTROY_SESSION completes (or a lease expires or similar).  The client
> can limit how much of that infinitude that it will choose to use, and
> the server can limit how much of it it will allow to be used so neither
> need to store the full infinity.  But it never changes size.
> Implementations can choose how much to store in real memory and can
> discard every except (I think) the last sequence number seen on any slot
> for which a request was sent (client) or accepted (server).

This is, IMO, one possible implementation of a slot table.

As you say, the spec doesn't provide a lot of guidance
about it. Therefore I believe other implementations are
possible.

It would be prudent to survey some of them.


> I agree that this seems less that ideal and it would be good if the
> protocol has a mechanism for the client and server to agree to reset
> the seqid for some slots.  But I cannot find any such mechanism.


--
Chuck Lever