mm: shrink_inactive_list() nr_scan accounting fix fix

If sc->isolate_pages() return 0, we don't need to call shrink_page_list().
In past days, shrink_inactive_list() handled it properly.

But commit fb8d14e1 (three years ago commit!) breaked it.  current
shrink_inactive_list() always call shrink_page_list() although
isolate_pages() return 0.

This patch restore proper return value check.

Requirements:
  o "nr_taken == 0" condition should stay before calling shrink_page_list().
  o "nr_taken == 0" condition should stay after nr_scan related statistics
     modification.

Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1076,6 +1076,20 @@ static unsigned long shrink_inactive_list(unsigned long max_scan,
 		nr_taken = sc->isolate_pages(sc->swap_cluster_max,
 			     &page_list, &nr_scan, sc->order, mode,
 				zone, sc->mem_cgroup, 0, file);
+
+		if (scanning_global_lru(sc)) {
+			zone->pages_scanned += nr_scan;
+			if (current_is_kswapd())
+				__count_zone_vm_events(PGSCAN_KSWAPD, zone,
+						       nr_scan);
+			else
+				__count_zone_vm_events(PGSCAN_DIRECT, zone,
+						       nr_scan);
+		}
+
+		if (nr_taken == 0)
+			goto done;
+
 		nr_active = clear_active_flags(&page_list, count);
 		__count_vm_events(PGDEACTIVATE, nr_active);
 
@@ -1088,8 +1102,6 @@ static unsigned long shrink_inactive_list(unsigned long max_scan,
 		__mod_zone_page_state(zone, NR_INACTIVE_ANON,
 						-count[LRU_INACTIVE_ANON]);
 
-		if (scanning_global_lru(sc))
-			zone->pages_scanned += nr_scan;
 
 		reclaim_stat->recent_scanned[0] += count[LRU_INACTIVE_ANON];
 		reclaim_stat->recent_scanned[0] += count[LRU_ACTIVE_ANON];
@@ -1123,18 +1135,12 @@ static unsigned long shrink_inactive_list(unsigned long max_scan,
 		}
 
 		nr_reclaimed += nr_freed;
+
 		local_irq_disable();
-		if (current_is_kswapd()) {
-			__count_zone_vm_events(PGSCAN_KSWAPD, zone, nr_scan);
+		if (current_is_kswapd())
 			__count_vm_events(KSWAPD_STEAL, nr_freed);
-		} else if (scanning_global_lru(sc))
-			__count_zone_vm_events(PGSCAN_DIRECT, zone, nr_scan);
-
 		__count_zone_vm_events(PGSTEAL, zone, nr_freed);
 
-		if (nr_taken == 0)
-			goto done;
-
 		spin_lock(&zone->lru_lock);
 		/*
 		 * Put back any unfreeable pages.
@@ -1164,9 +1170,9 @@ static unsigned long shrink_inactive_list(unsigned long max_scan,
 			}
 		}
   	} while (nr_scanned < max_scan);
-	spin_unlock(&zone->lru_lock);
+
 done:
-	local_irq_enable();
+	spin_unlock_irq(&zone->lru_lock);
 	pagevec_release(&pvec);
 	return nr_reclaimed;
 }