Return-Path: Received: from mx2.netapp.com ([216.240.18.37]:13168 "EHLO mx2.netapp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756112Ab0LASyq convert rfc822-to-8bit (ORCPT ); Wed, 1 Dec 2010 13:54:46 -0500 Subject: Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir From: Trond Myklebust To: Linus Torvalds Cc: Nick Bowler , Linux Kernel Mailing List , linux-nfs@vger.kernel.org, Andrew Morton , Hugh Dickins , Rik van Riel , Christoph Hellwig , Al Viro In-Reply-To: References: <1291217804-11257-1-git-send-email-Trond.Myklebust@netapp.com> <1291217804-11257-2-git-send-email-Trond.Myklebust@netapp.com> <20101201150428.GA2879@elliptictech.com> <1291217804-11257-3-git-send-email-Trond.Myklebust@netapp.com> <1291217804-11257-4-git-send-email-Trond.Myklebust@netapp.com> Content-Type: text/plain; charset="UTF-8" Date: Wed, 01 Dec 2010 13:54:29 -0500 Message-ID: <1291229669.6609.24.camel@heimdal.trondhjem.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Wed, 2010-12-01 at 08:17 -0800, Linus Torvalds wrote: > On Wed, Dec 1, 2010 at 7:36 AM, Trond Myklebust > wrote: > > > > We need to ensure that the entries in the nfs_cache_array get cleared > > when the page is removed from the page cache. To do so, we use the > > releasepage address_space operation (which also requires us to set > > the Pg_private flag). > > So I really think that the whole "releasepage" use in NFS is simply > overly complicated and was obviously too subtle. > > The whole need for odd return values, for the page lock, and for the > addition of clearing the up-to-date bit comes from the fact that this > wasn't really what releasepage was designed for. > > 'releasepage' was really designed for the filesystem having its own > version of 'try_to_free_buffers()', which is just an optimistic "ok, > we may be releasing this page, so try to get rid of any IO structures > you have cached". It wasn't really a memory management thing. > > And the thing is, it looks trivial to do the memory management > approach by adding a new callback that gets called after the page is > actually removed from the page cache. If we do that, then there are no > races with any other users, since we remove things from the page cache > atomically wrt page cache lookup. So the need for playing games with > page locking and 'uptodate' simply goes away. As does the PG_private > thing or the interaction with invalidatepage() etc. > > So this is a TOTALLY UNTESTED trivial patch that just adds another > callback. Does this work? I dunno. But I get the feeling that instead > of having NFS work around the odd semantics that don't actually match > what NFS wants, introducing a new callback with much simpler semantics > would be simpler for everybody, and avoid the need for subtle code. > > Hmm? > > Linus > include/linux/fs.h | 1 + > mm/vmscan.c | 3 +++ > 2 files changed, 4 insertions(+), 0 deletions(-) > > diff --git a/include/linux/fs.h b/include/linux/fs.h > index c9e06cc..090f0ea 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -602,6 +602,7 @@ struct address_space_operations { > sector_t (*bmap)(struct address_space *, sector_t); > void (*invalidatepage) (struct page *, unsigned long); > int (*releasepage) (struct page *, gfp_t); > + void (*freepage)(struct page *); > ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec > *iov, > loff_t offset, unsigned long nr_segs); > int (*get_xip_mem)(struct address_space *, pgoff_t, int, > diff --git a/mm/vmscan.c b/mm/vmscan.c > index d31d7ce..1accb01 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -499,6 +499,9 @@ static int __remove_mapping(struct address_space > *mapping, struct page *page) > mem_cgroup_uncharge_cache_page(page); > } > > + if (mapping->a_ops->freepage) > + mapping->a_ops->freepage(page); Hmm... Looking again at the problem, it appears that the same callback needs to be added to truncate_complete_page() and invalidate_complete_page2(). Otherwise we end up in a situation where the page can sometimes be removed from the page cache without calling freepage(). > + > return 1; > > cannot_free: Cheers Trond -- Trond Myklebust Linux NFS client maintainer NetApp Trond.Myklebust@netapp.com www.netapp.com