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 SMTP id BF1426B01A0 for ; Mon, 15 Mar 2010 01:38:04 -0400 (EDT) Received: from m4.gw.fujitsu.co.jp ([10.0.50.74]) by fgwmail7.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id o2F5c2uk013280 for (envelope-from kamezawa.hiroyu@jp.fujitsu.com); Mon, 15 Mar 2010 14:38:02 +0900 Received: from smail (m4 [127.0.0.1]) by outgoing.m4.gw.fujitsu.co.jp (Postfix) with ESMTP id DEE9945DE4D for ; Mon, 15 Mar 2010 14:38:01 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (s4.gw.fujitsu.co.jp [10.0.50.94]) by m4.gw.fujitsu.co.jp (Postfix) with ESMTP id A3A9345DE79 for ; Mon, 15 Mar 2010 14:38:01 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 6DB43E18003 for ; Mon, 15 Mar 2010 14:38:01 +0900 (JST) Received: from ml13.s.css.fujitsu.com (ml13.s.css.fujitsu.com [10.249.87.103]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 088AB1DB8037 for ; Mon, 15 Mar 2010 14:38:01 +0900 (JST) Date: Mon, 15 Mar 2010 14:34:20 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages Message-Id: <20100315143420.6ec3bdf9.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <28c262361003141728g4aa40901hb040144c5a4aeeed@mail.gmail.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> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org To: Minchan Kim 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 List-ID: 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. > 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 -- 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