linux-kernel - Re: [PATCH 3/3] nfs: use ->mmap_prepare() to avoid an AB-BA deadlock

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Date:	Wed, 14 Nov 2007 23:24:48 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: [PATCH 3/3] nfs: use ->mmap_prepare() to avoid an AB-BA deadlock

On Wed, Nov 14, 2007 at 05:18:50PM -0500, Trond Myklebust wrote:
> 
> On Wed, 2007-11-14 at 22:50 +0100, Peter Zijlstra wrote:
> > Right, but I guess what Nick asked is, if pages could be stale to start
> > with, how is that avoided in the future.
> > 
> > The way I understand it, this re-validate is just a best effort at
> > getting a coherent image.
> 
> The normal convention for NFS is to use a close-to-open cache
> consistency model. In that model, applications must agree never to open
> the file for reading or writing if an application on a different NFS
> client already holds it open for writing.
> 
> However there is no standard locking model for _enforcing_ such an
> agreement, so some setups do violate it. One obvious model that we try
> to support is that where the applications are using POSIX locking in
> order to ensure exclusive access to the data when requires.
> 
> Another model is to rely rather on synchronous writes and heavy
> attribute revalidation to detect when a competing application has
> written to the file (the 'noac' mount option). While such a model is
> obviously deficient in that it can never guarantee cache coherency, we
> do attempt to ensure that it works on a per-operation basis (IOW: we
> check cache coherency before each call to read(), to mmap(), etc) since
> it is by far the easiest model to apply if you have applications that
> cannot be rewritten and that satisfy the requirement that they rarely
> conflict.

mmap()s can be different from read in that the syscall may have little
relation to when the data gets used. But I guess it's still a best
effort thing. Fair enough.

Thanks,
Nick
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