linux-kernel - [59/70] mm: vmscan: do not use page

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Date:	Mon, 01 Aug 2011 16:19:14 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Sattler <tsattler@....de>, Mel Gorman <mgorman@...e.de>
Subject: [59/70] mm: vmscan: do not use page_count without a page pin

2.6.39-stable review patch.  If anyone has any objections, please let us know.

------------------


From: Andrea Arcangeli <aarcange@...hat.com>

commit: d179e84ba5da1d0024087d1759a2938817a00f3f upstream

It is unsafe to run page_count during the physical pfn scan because
compound_head could trip on a dangling pointer when reading
page->first_page if the compound page is being freed by another CPU.

[mgorman@...e.de: split out patch]
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
Signed-off-by: Mel Gorman <mgorman@...e.de>
Reviewed-by: Michal Hocko <mhocko@...e.cz>
Reviewed-by: Minchan Kim <minchan.kim@...il.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
---
 mm/vmscan.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1114,8 +1114,20 @@ static unsigned long isolate_lru_pages(u
 					nr_lumpy_dirty++;
 				scan++;
 			} else {
-				/* the page is freed already. */
-				if (!page_count(cursor_page))
+				/*
+				 * Check if the page is freed already.
+				 *
+				 * We can't use page_count() as that
+				 * requires compound_head and we don't
+				 * have a pin on the page here. If a
+				 * page is tail, we may or may not
+				 * have isolated the head, so assume
+				 * it's not free, it'd be tricky to
+				 * track the head status without a
+				 * page pin.
+				 */
+				if (!PageTail(cursor_page) &&
+				    !atomic_read(&cursor_page->_count))
 					continue;
 				break;
 			}


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