mm: vmscan: do not allow kswapd to scan at maximum priority

Page reclaim at priority 0 will scan the entire LRU as priority 0 is
considered to be a near OOM condition.  Kswapd can reach priority 0 quite
easily if it is encountering a large number of pages it cannot reclaim
such as pages under writeback.  When this happens, kswapd reclaims very
aggressively even though there may be no real risk of allocation failure
or OOM.

This patch prevents kswapd reaching priority 0 and trying to reclaim the
world.  Direct reclaimers will still reach priority 0 in the event of an
OOM situation.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Tested-by: Zlatko Calusic <zcalusic@bitsync.net>
Cc: dormando <dormando@rydia.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cd0980393bac765218b0828b315c6ea50bfc5b64..1505c573719d9c4722eb6c52e915dfdb5e8c8353 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2929,7 +2929,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
                 */
                if (raise_priority || !sc.nr_reclaimed)
                        sc.priority--;
-       } while (sc.priority >= 0 &&
+       } while (sc.priority >= 1 &&
                 !pgdat_balanced(pgdat, order, *classzone_idx));
 
 out: