linux-kernel - [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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Date:	Fri, 12 Mar 2010 16:41:18 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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 <mel@....ul.ie>
Acked-by: Rik van Riel <riel@...hat.com>
---
 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))
+			goto uncharge;
+
 		rcu_read_lock();
 		rcu_locked = 1;
 		anon_vma = page_anon_vma(page);
-- 
1.6.5

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