vmscan: check high watermark after shrink zone

Kswapd checks that zone has sufficient pages free via zone_watermark_ok().

If any zone doesn't have enough pages, we set all_zones_ok to zero.
!all_zone_ok makes kswapd retry rather than sleeping.

I think the watermark check before shrink_zone() is pointless.  Only after
kswapd has tried to shrink the zone is the check meaningful.

Move the check to after the call to shrink_zone().

[akpm@linux-foundation.org: fix comment, layout]
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2060,9 +2060,6 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
 					priority != DEF_PRIORITY)
 				continue;
 
-			if (!zone_watermark_ok(zone, order,
-					high_wmark_pages(zone), end_zone, 0))
-				all_zones_ok = 0;
 			temp_priority[i] = priority;
 			sc.nr_scanned = 0;
 			note_zone_scanning_priority(zone, priority);
@@ -2102,13 +2099,18 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
 			    total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
 				sc.may_writepage = 1;
 
+			if (!zone_watermark_ok(zone, order,
+					high_wmark_pages(zone), end_zone, 0)) {
+				all_zones_ok = 0;
 				/*
-			 * We are still under min water mark. it mean we have
-			 * GFP_ATOMIC allocation failure risk. Hurry up!
+				 * We are still under min water mark.  This
+				 * means that we have a GFP_ATOMIC allocation
+				 * failure risk. Hurry up!
 				 */
-			if (!zone_watermark_ok(zone, order, min_wmark_pages(zone),
-					      end_zone, 0))
+				if (!zone_watermark_ok(zone, order,
+					    min_wmark_pages(zone), end_zone, 0))
 					has_under_min_watermark_zone = 1;
+			}
 
 		}
 		if (all_zones_ok)