From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964869Ab0COL2w (ORCPT ); Mon, 15 Mar 2010 07:28:52 -0400 Received: from gir.skynet.ie ([193.1.99.77]:46705 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936099Ab0COL2s (ORCPT ); Mon, 15 Mar 2010 07:28:48 -0400 Date: Mon, 15 Mar 2010 11:28:29 +0000 From: Mel Gorman To: KAMEZAWA Hiroyuki 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 Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-ID: <20100315112829.GI18274@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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20100315143420.6ec3bdf9.kamezawa.hiroyu@jp.fujitsu.com> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 15, 2010 at 02:34:20PM +0900, KAMEZAWA Hiroyuki wrote: > On Mon, 15 Mar 2010 09:28:08 +0900 > Minchan Kim wrote: > > > Hi, Mel. > > On Sat, Mar 13, 2010 at 1:41 AM, Mel Gorman wrote: > > > rmap_walk_anon() was triggering errors in memory compaction that looks like > > > use-after-free errors in anon_vma. The problem appears to be 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 was freed. This patch > > > skips the migration of anon pages that are not mapped by anyone. > > > > > > Signed-off-by: Mel Gorman > > > Acked-by: Rik van Riel > > > --- > > >  mm/migrate.c |   10 ++++++++++ > > >  1 files changed, 10 insertions(+), 0 deletions(-) > > > > > > diff --git a/mm/migrate.c b/mm/migrate.c > > > index 98eaaf2..3c491e3 100644 > > > --- a/mm/migrate.c > > > +++ b/mm/migrate.c > > > @@ -602,6 +602,16 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, > > >         * just care Anon page here. > > >         */ > > >        if (PageAnon(page)) { > > > +               /* > > > +                * 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)) > > > > As looking code about mapcount of page, I got confused. > > I think mapcount of page is protected by pte lock. > > But I can't find pte lock in unmap_and_move. > > There is no pte_lock. > Indeed. It is manipulated while some other lock is held but it can be read without locks held. For example, when mapping a page either tha anon_vma lock or i_mmap_lock is held but it is read without special locking in places like page_referenced_ksm(). > > If I am right, what protects race between this condition check and > > rcu_read_lock? > > This patch makes race window very small but It can't remove race totally. > > > > I think I am missing something. > > Pz, point me out. :) > > > > Hmm. This is my understanding of old story. > > At migration. > 1. we increase page_count(). > 2. isolate it from LRU. > 3. call try_to_unmap() under rcu_read_lock(). Then, > 4. replace pte with swp_entry_t made by PFN. under pte_lock. > 5. do migarate > 6. remap new pages. under pte_lock()> > 7. release rcu_read_lock(). > > Here, we don't care whether page->mapping holds valid anon_vma or not. > > Assume a racy threads which calls zap_pte_range() (or some other) > I believe the race being hit is related to processes existing. A racy thread calling zap_pte_range() while pages within were being migrated does appear to be the problem. > a) When the thread finds valid pte under pte_lock and successfully call > page_remove_rmap(). > In this case, migration thread finds try_to_unmap doesn't unmap any pte. > Then, at 6, remap pte will not work. > b) When the thread finds migrateion PTE(as swap entry) in zap_page_range(). > In this case, migration doesn't find migrateion PTE and remap fails. > > Why rcu_read_lock() is necessary.. > - When page_mapcount() goes to 0, we shouldn't trust page->mapping is valid. I also believe this to be true. > - Possible cases are > i) anon_vma (= page->mapping) is freed and used for other object. > ii) anon_vma (= page->mapping) is freed > iii) anon_vma (= page->mapping) is freed and used as anon_vma again. > > Here, anon_vma_cachep is created by SLAB_DESTROY_BY_RCU. Then, possible cases > are only ii) and iii). I believe it's (ii) that was being hit. > While anon_vma is anon_vma, try_to_unmap and remap_page > can work well because of the list of vmas and address check. IOW, remap routine > just do nothing if anon_vma is freed. > > I'm not sure by what logic "use-after-free anon_vma" is caught. But yes, > there will be case, "anon_vma is touched after freed.", I think. > 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. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id 58A426B01C3 for ; Mon, 15 Mar 2010 07:28:52 -0400 (EDT) Date: Mon, 15 Mar 2010 11:28:29 +0000 From: Mel Gorman Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-ID: <20100315112829.GI18274@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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20100315143420.6ec3bdf9.kamezawa.hiroyu@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki 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, Mar 15, 2010 at 02:34:20PM +0900, KAMEZAWA Hiroyuki wrote: > On Mon, 15 Mar 2010 09:28:08 +0900 > Minchan Kim wrote: > > > Hi, Mel. > > On Sat, Mar 13, 2010 at 1:41 AM, Mel Gorman wrote: > > > rmap_walk_anon() was triggering errors in memory compaction that looks like > > > use-after-free errors in anon_vma. The problem appears to be 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 was freed. This patch > > > skips the migration of anon pages that are not mapped by anyone. > > > > > > Signed-off-by: Mel Gorman > > > Acked-by: Rik van Riel > > > --- > > > A mm/migrate.c | A 10 ++++++++++ > > > A 1 files changed, 10 insertions(+), 0 deletions(-) > > > > > > diff --git a/mm/migrate.c b/mm/migrate.c > > > index 98eaaf2..3c491e3 100644 > > > --- a/mm/migrate.c > > > +++ b/mm/migrate.c > > > @@ -602,6 +602,16 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, > > > A A A A * just care Anon page here. > > > A A A A */ > > > A A A A if (PageAnon(page)) { > > > + A A A A A A A /* > > > + A A A A A A A A * If the page has no mappings any more, just bail. An > > > + A A A A A A A A * unmapped anon page is likely to be freed soon but worse, > > > + A A A A A A A A * it's possible its anon_vma disappeared between when > > > + A A A A A A A A * the page was isolated and when we reached here while > > > + A A A A A A A A * the RCU lock was not held > > > + A A A A A A A A */ > > > + A A A A A A A if (!page_mapcount(page)) > > > > As looking code about mapcount of page, I got confused. > > I think mapcount of page is protected by pte lock. > > But I can't find pte lock in unmap_and_move. > > There is no pte_lock. > Indeed. It is manipulated while some other lock is held but it can be read without locks held. For example, when mapping a page either tha anon_vma lock or i_mmap_lock is held but it is read without special locking in places like page_referenced_ksm(). > > If I am right, what protects race between this condition check and > > rcu_read_lock? > > This patch makes race window very small but It can't remove race totally. > > > > I think I am missing something. > > Pz, point me out. :) > > > > Hmm. This is my understanding of old story. > > At migration. > 1. we increase page_count(). > 2. isolate it from LRU. > 3. call try_to_unmap() under rcu_read_lock(). Then, > 4. replace pte with swp_entry_t made by PFN. under pte_lock. > 5. do migarate > 6. remap new pages. under pte_lock()> > 7. release rcu_read_lock(). > > Here, we don't care whether page->mapping holds valid anon_vma or not. > > Assume a racy threads which calls zap_pte_range() (or some other) > I believe the race being hit is related to processes existing. A racy thread calling zap_pte_range() while pages within were being migrated does appear to be the problem. > a) When the thread finds valid pte under pte_lock and successfully call > page_remove_rmap(). > In this case, migration thread finds try_to_unmap doesn't unmap any pte. > Then, at 6, remap pte will not work. > b) When the thread finds migrateion PTE(as swap entry) in zap_page_range(). > In this case, migration doesn't find migrateion PTE and remap fails. > > Why rcu_read_lock() is necessary.. > - When page_mapcount() goes to 0, we shouldn't trust page->mapping is valid. I also believe this to be true. > - Possible cases are > i) anon_vma (= page->mapping) is freed and used for other object. > ii) anon_vma (= page->mapping) is freed > iii) anon_vma (= page->mapping) is freed and used as anon_vma again. > > Here, anon_vma_cachep is created by SLAB_DESTROY_BY_RCU. Then, possible cases > are only ii) and iii). I believe it's (ii) that was being hit. > While anon_vma is anon_vma, try_to_unmap and remap_page > can work well because of the list of vmas and address check. IOW, remap routine > just do nothing if anon_vma is freed. > > I'm not sure by what logic "use-after-free anon_vma" is caught. But yes, > there will be case, "anon_vma is touched after freed.", I think. > 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. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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