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

[Michal Hocko]

On Wed 20-03-13 18:19:57, 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@xxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

Reviewed-by: Michal Hocko <mhocko@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:
>  				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;
>  			}
>  		}
>  

-- 
Michal Hocko
SUSE Labs
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