Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756887Ab0LAUFn (ORCPT ); Wed, 1 Dec 2010 15:05:43 -0500 Received: from mx2.netapp.com ([216.240.18.37]:45404 "EHLO mx2.netapp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755947Ab0LAUFl convert rfc822-to-8bit (ORCPT ); Wed, 1 Dec 2010 15:05:41 -0500 X-IronPort-AV: E=Sophos;i="4.59,285,1288594800"; d="scan'208";a="489661250" Subject: Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir From: Trond Myklebust To: Hugh Dickins Cc: Linus Torvalds , Nick Bowler , Linux Kernel Mailing List , linux-nfs@vger.kernel.org, Andrew Morton , Rik van Riel , Christoph Hellwig , Al Viro , Nick Piggin 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> <1291229669.6609.24.camel@heimdal.trondhjem.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Organization: NetApp Inc Date: Wed, 01 Dec 2010 15:05:38 -0500 Message-ID: <1291233938.6609.37.camel@heimdal.trondhjem.org> Mime-Version: 1.0 X-Mailer: Evolution 2.32.1 (2.32.1-1.fc14) X-OriginalArrivalTime: 01 Dec 2010 20:05:40.0537 (UTC) FILETIME=[20B87290:01CB9193] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3905 Lines: 95 On Wed, 2010-12-01 at 11:23 -0800, Hugh Dickins wrote: > On Wed, 1 Dec 2010, Trond Myklebust wrote: > > On Wed, 2010-12-01 at 08:17 -0800, Linus Torvalds wrote: > > > 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: > > Yes, I was wondering quite how we would define this ->freepage thing, > if it gets called from one place that removes from page cache and not > from others. > > Another minor problem with it: it would probably need to take the > struct address_space *mapping as arg as well as struct page *page: > because by this time page->mapping has been reset to NULL. > > But I'm not at all keen on adding a calllback in this very special > frozen-to-0-references place: please let's not do it without an okay > from Nick Piggin (now Cc'ed). > > I agree completely with what Linus said originally about how the > page cannot be freed while there's a reference to it, and it should > be possible to work this without your additional page locks. > > Your ->releasepage should be able to judge whether the page is likely > (not certain) to be freed - page_count 3? 1 for the page cache, 1 for > the page_private reference, 1 for vmscan's reference, I think. Then > it can mark !PageUptodate and proceed with freeing the stuff you had > allocated, undo page_has_private and its reference, and return 1 (or > return 0 if it decides to hold on to the page). That is very brittle. I'd prefer not to have to scan linux-mm every week in order to find out if someone changed the page_count. However, while reading Documentation/filesystems/vfs.txt (in order to add documentation for freepage) I was surprised to read that the ->releasepage() is itself supposed to be allowed to actually remove the page from the address space if it so desires. Looking at the actual code in shrink_page_list() and friends I can't see how that can possibly fail to break things, but if it were true, then that might enable us to call remove_mapping() in order to safely free the page before it gets cleared. Cheers Trond -- Trond Myklebust Linux NFS client maintainer NetApp Trond.Myklebust@netapp.com www.netapp.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/