If we can't isolate pages from LRU list, we don't have to account page

movement, either.  Already, in commit 5343daceec, KOSAKI did it about
shrink_inactive_list.

This patch removes unnecessary overhead of page accounting and locking in
shrink_active_list as follow-up work of commit 5343daceec.
Signed-off-by: Vincent Li <macli@brc.ubc.ca>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -1333,9 +1333,12 @@ static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
 	if (scanning_global_lru(sc)) {
 		zone->pages_scanned += pgscanned;
 	}
-	reclaim_stat->recent_scanned[file] += nr_taken;
-
 	__count_zone_vm_events(PGREFILL, zone, pgscanned);
+
+	if (nr_taken == 0)
+		goto done;
+
+	reclaim_stat->recent_scanned[file] += nr_taken;
 	if (file)
 		__mod_zone_page_state(zone, NR_ACTIVE_FILE, -nr_taken);
 	else
@@ -1393,6 +1396,7 @@ static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
 	move_active_pages_to_lru(zone, &l_inactive,
 						LRU_BASE   + file * LRU_FILE);
 	__mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
+done:
 	spin_unlock_irq(&zone->lru_lock);
 }