[PATCH 2/4] mm: vmscan: Do not use page_count without a page pin

[Mel Gorman <mgorman@suse.de>]

From: Andrea Arcangeli <aarcange@redhat.com>

From: Andrea Arcangeli <aarcange@redhat.com>

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@suse.de: Split out patch]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |   16 ++++++++++++++--
 1 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index faa0a08..001a504 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1124,8 +1124,20 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 					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;
 			}
-- 
1.7.3.4

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