During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.

A problem occurs if the highest zone is small.  balance_pgdat()
only considers unreclaimable zones when priority is DEF_PRIORITY
but sleeping_prematurely considers all zones. It's possible for this
sequence to occur

  1. kswapd wakes up and enters balance_pgdat()
  2. At DEF_PRIORITY, marks highest zone unreclaimable
  3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
  4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
        highest zone, clearing all_unreclaimable. Highest zone
        is still unbalanced
  5. kswapd returns and calls sleeping_prematurely
  6. sleeping_prematurely looks at *all* zones, not just the ones
     being considered by balance_pgdat. The highest small zone
     has all_unreclaimable cleared but but the zone is not
     balanced. all_zones_ok is false so kswapd stays awake

This patch corrects the behaviour of sleeping_prematurely to check
the zones balance_pgdat() checked.

Reported-and-tested-by: Pádraig Brady <P@draigBrady.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8ff834e..841e3bf 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
 		return true;
 
 	/* Check the watermark levels */
-	for (i = 0; i < pgdat->nr_zones; i++) {
+	for (i = 0; i <= classzone_idx; i++) {
 		struct zone *zone = pgdat->node_zones + i;
 
 		if (!populated_zone(zone))
-- 
1.7.3.4
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