linux-kernel - Re: [GIT PULL] Please pull NFS client bugfixes....

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Date:	Thu, 07 Jan 2010 19:14:42 -0500
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...l.org
Subject: Re: [GIT PULL] Please pull NFS client bugfixes....

On Fri, 2010-01-08 at 00:51 +0100, Andi Kleen wrote: 
> > I'd like to see this fixed, but I do not want to jump onto a solution
> > that changes the behaviour of mmap() w.r.t. revalidation. The current
> > behaviour dates back at least to 2.3.x if not before.
> 
> So do you have a plan to fix it?

Yes. I want to pursue Peter Zijlstra's patches, which split up the mmap
function into a set of parts which require the mmap_sem, and other parts
which don't, and that adds a filesystem callback that allows for
revalidation to occur outside the mmap_sem.

> I don't think it'll be possible to do drastic changes in the
> VFS for 2.6.33, and it seems preserving the current semantics
> would need that.
> 
> > That's why I'm working slowly on this.
> 
> Delaying a fix to after 2.6.33 is not an option imho.
> 
> It hits everyone with LOCKDEP enabled who uses mmap over NFS.
> That's new in 2.6.33, previously LOCKDEP didn't diagnose this.
> 
> I'll keep using my patch, but I suppose once we're going more
> towards a release you'll get more reports of this.

Why should this particular issue require us to rush into a solution?
This has been there for literally _years_, and I've never heard of a
single incident in which a deadlock actually occurred. The only reason
why we've noticed it at all is because lockdep has started to whine.

I agree it should be fixed.

I don't agree that it is urgent enough to warrant kneejerk reactions in
2.6.33 which change long established behaviours that people are actually
relying on.

  Trond
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