From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
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@xxxxxxx: Split out patch]
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
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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