mm: page_alloc: avoid marking zones full prematurely after zone_reclaim()

The following problem was reported against a distribution kernel when
zone_reclaim was enabled but the same problem applies to the mainline
kernel.  The reproduction case was as follows

1. Run numactl -m +0 dd if=largefile of=/dev/null
   This allocates a large number of clean pages in node 0

2. numactl -N +0 memhog 0.5*Mg
   This start a memory-using application in node 0.

The expected behaviour is that the clean pages get reclaimed and the
application uses node 0 for its memory.  The observed behaviour was that
the memory for the memhog application was allocated off-node since
commits cd38b115d5 ("mm: page allocator: initialise ZLC for first zone
eligible for zone_reclaim") and commit 76d3fbf8fb ("mm: page
allocator: reconsider zones for allocation after direct reclaim").

The assumption of those patches was that it was always preferable to
allocate quickly than stall for long periods of time and they were meant
to take care that the zone was only marked full when necessary but an
important case was missed.

In the allocator fast path, only the low watermarks are checked.  If the
zones free pages are between the low and min watermark then allocations
from the allocators slow path will succeed.  However, zone_reclaim will
only reclaim SWAP_CLUSTER_MAX or 1<<order pages.  There is no guarantee
that this will meet the low watermark causing the zone to be marked full
prematurely.

This patch will only mark the zone full after zone_reclaim if it the min
watermarks are checked or if page reclaim failed to make sufficient
progress.

[mhocko@suse.cz: fix alloc_flags test]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Reported-by: Hedi Berriche <hedi@sgi.com>
Tested-by: Hedi Berriche <hedi@sgi.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -1942,9 +1942,24 @@ zonelist_scan:
 				continue;
 			default:
 				/* did we reclaim enough */
-				if (!zone_watermark_ok(zone, order, mark,
+				if (zone_watermark_ok(zone, order, mark,
 						classzone_idx, alloc_flags))
+					goto try_this_zone;
+
+				/*
+				 * Failed to reclaim enough to meet watermark.
+				 * Only mark the zone full if checking the min
+				 * watermark or if we failed to reclaim just
+				 * 1<<order pages or else the page allocator
+				 * fastpath will prematurely mark zones full
+				 * when the watermark is between the low and
+				 * min watermarks.
+				 */
+				if (((alloc_flags & ALLOC_WMARK_MASK) == ALLOC_WMARK_MIN) ||
+				    ret == ZONE_RECLAIM_SOME)
 					goto this_zone_full;
+
+				continue;
 			}
 		}