linux-kernel - Re: [PATCH 2/3] NFS: lock the readdir page while it is in use

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Message-ID: <1291177753.7694.17.camel@heimdal.trondhjem.org>
Date:	Tue, 30 Nov 2010 23:29:13 -0500
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Bowler <nbowler@...iptictech.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-nfs@...r.kernel.org
Subject: Re: [PATCH 2/3] NFS: lock the readdir page while it is in use

On Tue, 2010-11-30 at 20:10 -0800, Linus Torvalds wrote:
> On Tue, Nov 30, 2010 at 7:47 PM, Trond Myklebust
> <Trond.Myklebust@...app.com> wrote:
> > Otherwise, the VM may end up removing it while we're reading from it.
> 
> I don't think this is valid.
> 
> Maybe it fixes a bug, but the commit description is misleading at
> best.  Since you have a reference count to the page, the page is not
> going away. Locking may hide some other bug (due to serializing with
> other code you care about), but it is _not_ about the "VM may end up
> removing it".
> 
> Even from a serialization angle, I think this patch is a bit suspect,
> since readdir() will always be called under the inode semaphore, so I
> think you'll always be serialized wrt other readdir users. Of course,
> you may have invalidation events etc that are outside of readdir, so
> ...

I'm not worried about other readdir calls invalidating the page. My
concern is rather about the VM memory reclaimers ejecting the page from
the page cache, and calling nfs_readdir_clear_array while we're
referencing the page. This wasn't a problem with the previous readdir
code, but it will be with the new incarnation because the actual
filenames are stored outside the page itself.
As far as I can see, the only way to protect against that is to lock the
page, perform the usual tests and then release the page lock when we're
done...

> Anyway if this patch matters, there's something else going on, and you
> need to describe that.

No problem. I just wanted to get the patches out so that the people who
are reporting regressions can start testing.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com

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