[PATCH 16/19] mm: Do not use unnecessary atomic operations when adding pages to the LRU

[Mel Gorman]

When adding pages to the LRU we clear the active bit unconditionally. As the
page could be reachable from other paths we cannot use unlocked operations
without risk of corruption such as a parallel mark_page_accessed. This
patch tests if is necessary to clear the active flag before using an atomic
operation. This potentially opens a tiny race when PageActive is checked
as mark_page_accessed could be called after PageActive was checked. The
race already exists but this patch changes it slightly. The consequence
is that that the page may be promoted to the active list that might have
been left on the inactive list before the patch. It's too tiny a race and
too marginal a consequence to always use atomic operations for.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 include/linux/swap.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index da8a250..395dcab 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -329,13 +329,15 @@ extern void add_page_to_unevictable_list(struct page *page);
  */
 static inline void lru_cache_add_anon(struct page *page)
 {
-	ClearPageActive(page);
+	if (PageActive(page))
+		ClearPageActive(page);
 	__lru_cache_add(page);
 }
 
 static inline void lru_cache_add_file(struct page *page)
 {
-	ClearPageActive(page);
+	if (PageActive(page))
+		ClearPageActive(page);
 	__lru_cache_add(page);
 }
 
-- 
1.8.4.5

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