From: "J. Bruce Fields" <bfields@fieldses.org>
Subject: Re: [PATCH] Fix nfsd rewrite performance
Date: Tue, 2 Aug 2005 06:02:22 -0400
Message-ID: <20050802100222.GA1940@fieldses.org>
References: <20050801113954.GA8698@suse.de> <20050801115319.GD6497@fieldses.org> <20050801115922.GE8698@suse.de> <20050801121053.GA8238@fieldses.org> <20050801121422.GH8698@suse.de> <20050801125832.GB8598@fieldses.org> <20050802094950.GB29054@suse.de>
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Cc: nfs@lists.sourceforge.net
To: Olaf Kirch <okir@suse.de>
In-Reply-To: <20050802094950.GB29054@suse.de>
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On Tue, Aug 02, 2005 at 11:49:50AM +0200, Olaf Kirch wrote:
> I just looked at the code, and there are two types of deferrals.
> One is for authentication stuff - this will actually use svc_defer and
> svc_revisit, which goes back to look at the entire request.
> 
> The other type is what hte nfs4idmap stuff does, which uses its own
> defer/revisit routines, which just make the nfsd thread wait.
> 
> So neither poses a problem to munging the iovec in nfsd_write:

It can also happen whenever we do fh_verify().  (See the exp_find() call in
fh_verify().)

> The authentication stuff will happen when we parse the RPC header,
> which is way before we decide to call nfsd_write.  The idmap stuff may
> be called after processing a write call, but it will not go back to
> revisit the entire request, it will just stall briefly while idmapd is
> doing its job.  Correct?
> 
> In general, I believe it does not make sense to call svc_defer once
> we've started to process a request, because the request may not be
> idempotent.  It sounds like a reasonable assumption to me that
> deferring and revisiting an entire requests is only done before we hit
> the RPC program handler.

Yeah.  So the simplest solution might be to make the other upcalls use
the same sort of deferral as the idmap upcalls, at least in the v4 case.
In the v3 case fh_verify always happens early enough not to be a
problem, I believe.

Though I still think it's unfortunate even for v3 that it will try to
copy (and allocate space for) the entire write request in the case of an
authentication- or export- related upcall.

--b.


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