From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with SMTP id C82E66B01F8 for ; Mon, 15 Mar 2010 19:53:29 -0400 (EDT) Received: from m3.gw.fujitsu.co.jp ([10.0.50.73]) by fgwmail7.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id o2FNrQNU000394 for (envelope-from kamezawa.hiroyu@jp.fujitsu.com); Tue, 16 Mar 2010 08:53:27 +0900 Received: from smail (m3 [127.0.0.1]) by outgoing.m3.gw.fujitsu.co.jp (Postfix) with ESMTP id A144F45DE50 for ; Tue, 16 Mar 2010 08:53:26 +0900 (JST) Received: from s3.gw.fujitsu.co.jp (s3.gw.fujitsu.co.jp [10.0.50.93]) by m3.gw.fujitsu.co.jp (Postfix) with ESMTP id 82FF245DE4E for ; Tue, 16 Mar 2010 08:53:26 +0900 (JST) Received: from s3.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s3.gw.fujitsu.co.jp (Postfix) with ESMTP id 6A0A21DB803C for ; Tue, 16 Mar 2010 08:53:26 +0900 (JST) Received: from m105.s.css.fujitsu.com (m105.s.css.fujitsu.com [10.249.87.105]) by s3.gw.fujitsu.co.jp (Postfix) with ESMTP id 1C7541DB803E for ; Tue, 16 Mar 2010 08:53:26 +0900 (JST) Date: Tue, 16 Mar 2010 08:49:34 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-Id: <20100316084934.3798576c.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20100315142124.GL18274@csn.ul.ie> References: <1268412087-13536-1-git-send-email-mel@csn.ul.ie> <1268412087-13536-3-git-send-email-mel@csn.ul.ie> <28c262361003141728g4aa40901hb040144c5a4aeeed@mail.gmail.com> <20100315143420.6ec3bdf9.kamezawa.hiroyu@jp.fujitsu.com> <20100315112829.GI18274@csn.ul.ie> <1268657329.1889.4.camel@barrios-desktop> <20100315142124.GL18274@csn.ul.ie> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Mel Gorman Cc: Minchan Kim , Andrew Morton , Andrea Arcangeli , Christoph Lameter , Adam Litke , Avi Kivity , David Rientjes , KOSAKI Motohiro , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: On Mon, 15 Mar 2010 14:21:24 +0000 Mel Gorman wrote: > On Mon, Mar 15, 2010 at 09:48:49PM +0900, Minchan Kim wrote: > > On Mon, 2010-03-15 at 11:28 +0000, Mel Gorman wrote: > > > The use after free looks like > > > > > > 1. page_mapcount(page) was zero so anon_vma was no longer reliable > > > 2. rcu lock taken but the anon_vma at this point can already be garbage because the > > > process exited > > > 3. call try_to_unmap, looks up tha anon_vma and locks it. This causes problems > > > > > > I thought the race would be closed but there is still a very tiny window there all > > > right. The following alternative should close it. What do you think? > > > > > > if (PageAnon(page)) { > > > rcu_read_lock(); > > > > > > /* > > > * If the page has no mappings any more, just bail. An > > > * unmapped anon page is likely to be freed soon but worse, > > > * it's possible its anon_vma disappeared between when > > > * the page was isolated and when we reached here while > > > * the RCU lock was not held > > > */ > > > if (!page_mapcount(page)) { > > > rcu_read_unlock(); > > > goto uncharge; > > > } > > > > > > rcu_locked = 1; > > > anon_vma = page_anon_vma(page); > > > atomic_inc(&anon_vma->external_refcount); > > > } > > > > > > The rcu_unlock label is not used here because the reference counts were not taken in > > > the case where page_mapcount == 0. > > > > > > > Please, repost above code with your use-after-free scenario comment. > > > > This will be the replacement patch so. > > ==== CUT HERE ==== > mm,migration: Do not try to migrate unmapped anonymous pages > > rmap_walk_anon() was triggering errors in memory compaction that look like > use-after-free errors. The problem is that between the page being isolated > from the LRU and rcu_read_lock() being taken, the mapcount of the page > dropped to 0 and the anon_vma gets freed. This can happen during memory > compaction if pages being migrated belong to a process that exits before > migration completes. Hence, the use-after-free race looks like > > 1. Page isolated for migration > 2. Process exits > 3. page_mapcount(page) drops to zero so anon_vma was no longer reliable > 4. unmap_and_move() takes the rcu_lock but the anon_vma is already garbage > 4. call try_to_unmap, looks up tha anon_vma and "locks" it but the lock > is garbage. > > This patch checks the mapcount after the rcu lock is taken. If the > mapcount is zero, the anon_vma is assumed to be freed and no further > action is taken. > > Signed-off-by: Mel Gorman > Acked-by: Rik van Riel Reviewd-by: KAMEZAWA Hiroyuki > --- > mm/migrate.c | 13 +++++++++++++ > 1 files changed, 13 insertions(+), 0 deletions(-) > > diff --git a/mm/migrate.c b/mm/migrate.c > index 98eaaf2..6eb1efe 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, > */ > if (PageAnon(page)) { > rcu_read_lock(); > + > + /* > + * If the page has no mappings any more, just bail. An > + * unmapped anon page is likely to be freed soon but worse, > + * it's possible its anon_vma disappeared between when > + * the page was isolated and when we reached here while > + * the RCU lock was not held > + */ > + if (!page_mapcount(page)) { > + rcu_read_unlock(); > + goto uncharge; > + } > + > rcu_locked = 1; > anon_vma = page_anon_vma(page); > atomic_inc(&anon_vma->migrate_refcount); > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org