Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755489Ab0DTVDw (ORCPT ); Tue, 20 Apr 2010 17:03:52 -0400 Received: from gir.skynet.ie ([193.1.99.77]:47004 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755252Ab0DTVBM (ORCPT ); Tue, 20 Apr 2010 17:01:12 -0400 From: Mel Gorman To: Andrew Morton Cc: Andrea Arcangeli , Christoph Lameter , Adam Litke , Avi Kivity , David Rientjes , Minchan Kim , KAMEZAWA Hiroyuki , KOSAKI Motohiro , Rik van Riel , Mel Gorman , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH 03/14] mm,migration: Do not try to migrate unmapped anonymous pages Date: Tue, 20 Apr 2010 22:01:05 +0100 Message-Id: <1271797276-31358-4-git-send-email-mel@csn.ul.ie> X-Mailer: git-send-email 1.6.5 In-Reply-To: <1271797276-31358-1-git-send-email-mel@csn.ul.ie> References: <1271797276-31358-1-git-send-email-mel@csn.ul.ie> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2104 Lines: 56 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 Reviewed-by: KAMEZAWA Hiroyuki --- mm/migrate.c | 11 +++++++++++ 1 files changed, 11 insertions(+), 0 deletions(-) diff --git a/mm/migrate.c b/mm/migrate.c index 42a3d24..b114635 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -600,6 +600,17 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, if (PageAnon(page)) { rcu_read_lock(); rcu_locked = 1; + + /* + * 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_mapped(page)) + goto rcu_unlock; + anon_vma = page_anon_vma(page); atomic_inc(&anon_vma->external_refcount); } -- 1.6.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/