Re: [PATCH] mm: page_alloc: Avoid marking zones full prematurely after zone_reclaim()

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Wed, Mar 20, 2013 at 06:19:57PM +0000, Mel Gorman wrote:
>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
>progress.
>
>Reported-and-tested-by: Hedi Berriche <hedi@sgi.com>
>Signed-off-by: Mel Gorman <mgorman@suse.de>

Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>

>---
> 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:
> 				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_MIN) ||
>+				    ret == ZONE_RECLAIM_SOME)
> 					goto this_zone_full;
>+
>+				continue;
> 			}
> 		}
>
>
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