linux-kernel - Re: [PATCH 2/2] NFSv4: Return the delegation if the server returns NFS4ERR

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Date:	Thu, 8 Mar 2012 15:23:34 -0500
From:	Olga Kornievskaia <aglo@...i.umich.edu>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] NFSv4: Return the delegation if the server returns NFS4ERR_OPENMODE

On Thu, Mar 8, 2012 at 1:15 PM, Myklebust, Trond
<Trond.Myklebust@...app.com> wrote:
> On Thu, 2012-03-08 at 12:52 -0500, Olga Kornievskaia wrote:
>> wouldn't it be better for you to proactively return a read delegation
>> then unnecessarily erroring?
>
> If nobody else holds a delegation, then the NFS client is actually
> allowed to keep its read delegation while writing to the file. It does
> admittedly need to request an OPEN stateid for write in that case...
> (See section 10.4 of RFC3530bis draft 16)

If we both agree that there has to be a request for an open stateid for
a write, then instead of returning the read delegation if the client receives
err_openmode (when it send the request with read delegation stateid
as you said per 3560bis), can't the client resend the setattr with the open
stateid? The ordering of the stateid usage is a "should" and not a "must".

In rfc5661, it really doesn't make sense to ever send a setattr with
a read delegation stateid. According to 9.1.2, the server "MUST" return
err_open_mode" error in that case.

I gather you are in this case dealing with 4.0 delegations. But I wonder
if you'll do something else for 4.1 delegation then?

Another comment is that I was suggesting a return of delegation only
because (from what i recall) the 4.1 servers out there will recall the
previously
given read delegation in this case.

> That said, in either case we do need to deal with the fact that a new
> delegation may be granted after we return the old one. There is no
> locking around the setattr call to prevent this.

Can that really happen since we agreed that the client also has an open
stateid for write in this case?

>> i also don't understand how this error occurs. doing a setattr in this
>> case you must have used a non-special stateid. the server would only
>> return an err_openmode if you sent the setattr with a read delegation
>> stateid. i guess my question is what stateid would you use that from
>> client's perspective represent a write-type state id but yet a server
>> would flag as having wrong access type?
>
> The read delegation stateid is being sent as per the prescription in
> section 9.1.3.6 of RFC3530bis.

>> also i'm curious why would a server, instead of returning
>> err_openmode, would not first recall your read delegation?
>
> It could, but why do so when it can just return an error value? The
> presence of the delegation stateid in the SETATTR request allows it to
> communicate directly to the client what the problem is without the need
> for any callbacks.

> Trond Myklebust
> Linux NFS client maintainer
>
> NetApp
> Trond.Myklebust@...app.com
> www.netapp.com
>
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