From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936535Ab0COOdH (ORCPT ); Mon, 15 Mar 2010 10:33:07 -0400 Received: from mail-pz0-f200.google.com ([209.85.222.200]:41980 "EHLO mail-pz0-f200.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932644Ab0COOdE convert rfc822-to-8bit (ORCPT ); Mon, 15 Mar 2010 10:33:04 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=M05BohbvxOeFy4pXvn47OTsfH7RkpxdV9EFwWm0YP8XIodP/lT1eYeQUp80XCAWJij xm2PNJScvpaxv2gwLAto3Efjnh8J2Rh6UZvTW96+wtkhnjJhOJrkg9j9KGDlftmwZKEc X8QVkiR9r3+sJ2Y7HgzrKvkNbN6fHLhRosNYI= MIME-Version: 1.0 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> Date: Mon, 15 Mar 2010 23:33:01 +0900 Message-ID: <28c262361003150733j4d0c708eyfc525e1e3c5c1f7d@mail.gmail.com> Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages From: Minchan Kim To: Mel Gorman Cc: KAMEZAWA Hiroyuki , 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 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 15, 2010 at 11:21 PM, 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 Reviewed-by: Minchan Kim -- Kind regards, Minchan Kim