Return-Path: Received: from mx2.netapp.com ([216.240.18.37]:21717 "EHLO mx2.netapp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754642Ab0LAE3a convert rfc822-to-8bit (ORCPT ); Tue, 30 Nov 2010 23:29:30 -0500 Subject: Re: [PATCH 2/3] NFS: lock the readdir page while it is in use From: Trond Myklebust To: Linus Torvalds Cc: Nick Bowler , Linux Kernel Mailing List , linux-nfs@vger.kernel.org In-Reply-To: References: <1291175234-23824-1-git-send-email-Trond.Myklebust@netapp.com> <1291175234-23824-2-git-send-email-Trond.Myklebust@netapp.com> <1291175234-23824-3-git-send-email-Trond.Myklebust@netapp.com> Content-Type: text/plain; charset="UTF-8" Date: Tue, 30 Nov 2010 23:29:13 -0500 Message-ID: <1291177753.7694.17.camel@heimdal.trondhjem.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Tue, 2010-11-30 at 20:10 -0800, Linus Torvalds wrote: > On Tue, Nov 30, 2010 at 7:47 PM, Trond Myklebust > 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@netapp.com www.netapp.com