mm: do not use unnecessary atomic operations when adding pages to the LRU

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@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/swap.c b/mm/swap.c
index 30b6a37c74af538538a1893d9e45469fa171f56c..1fb25f8bb1553663cfbec690732265b3b6ea99e7 100644 (file)
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -631,13 +631,15 @@ static void __lru_cache_add(struct page *page)
  */
 void lru_cache_add_anon(struct page *page)
 {
-       ClearPageActive(page);
+       if (PageActive(page))
+               ClearPageActive(page);
        __lru_cache_add(page);
 }
 
 void lru_cache_add_file(struct page *page)
 {
-       ClearPageActive(page);
+       if (PageActive(page))
+               ClearPageActive(page);
        __lru_cache_add(page);
 }
 EXPORT_SYMBOL(lru_cache_add_file);