From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759610Ab0COG2V (ORCPT ); Mon, 15 Mar 2010 02:28:21 -0400 Received: from mail-pz0-f200.google.com ([209.85.222.200]:48434 "EHLO mail-pz0-f200.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752565Ab0COG2Q convert rfc822-to-8bit (ORCPT ); Mon, 15 Mar 2010 02:28:16 -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=bTa66dPtSF7Sdy4gU2/s8EwOH6rh6ysgc4H/cxRanq/HdqcvJsPuv0Z9f5GoUfbGzs DTsykWY0hCjMIBpxoR0eWtJqal4Xxitt4KndzQljsJJALldlZ/KvK5X+RCn5qIsAIG9o XLvxLCaNEmJbqUG67Fd9ziAXTQmz9uAbFZbdc= MIME-Version: 1.0 In-Reply-To: <20100315143420.6ec3bdf9.kamezawa.hiroyu@jp.fujitsu.com> 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> Date: Mon, 15 Mar 2010 15:28:15 +0900 Message-ID: <28c262361003142328w610f0478sbc17880ffa454fe8@mail.gmail.com> Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages From: Minchan Kim To: KAMEZAWA Hiroyuki Cc: Mel Gorman , 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 2:34 PM, 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. > >> 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) > > 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. >  - 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). 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. > > Thanks, > -Kame > Thanks for detail explanation, Kame. But it can't understand me enough, Sorry. Mel said he met "use-after-free errors in anon_vma". So added the check in unmap_and_move. if (PageAnon(page)) { .... if (!page_mapcount(page)) goto uncharge; rcu_read_lock(); My concern what protects racy mapcount of the page? For example, CPU A CPU B unmap_and_move page_mapcount check pass zap_pte_range <-- some stall --> pte_lock <-- some stall --> page_remove_rmap(map_count is zero!) <-- some stall --> pte_unlock <-- some stall --> anon_vma_unlink <-- some stall --> anon_vma free !!!! rcu_read_lock anon_vma has gone!! I think above scenario make error "use-after-free", again. What prevent above scenario? -- Kind regards, Minchan Kim