[PATCH] mm: page_alloc: Avoid marking zones full prematurely afterzone_reclaim()

From: Mel Gorman
Date: Wed Mar 20 2013 - 14:20:10 EST

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
cd38b11 (mm: page allocator: initialise ZLC for first zone eligible for
zone_reclaim) and commit 76d3fbf (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

Reported-and-tested-by: Hedi Berriche <hedi@xxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
mm/page_alloc.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8fcced7..adce823 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1940,9 +1940,24 @@ zonelist_scan:
/* 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_MIN) ||
goto this_zone_full;
+ continue;

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