Re: [PATCH v2] mm/compaction : check the watermark when cc->order is -1

[Minchan Kim <minchan@kernel.org>]

On Fri, Jan 06, 2012 at 10:50:01AM +0800, Huang Shijie wrote:
> We get cc->order is -1 when user echos to /proc/sys/vm/compact_memory.
> In this case, we should check that if we have enough pages for
> the compaction in the zone.
> 
> If we do not check this, in our MX6Q board(arm), i ever observed
> COMPACT_CLUSTER_MAX pages were compaction failed in per migrate_pages().
> Thats mean we can not alloc any pages by the free scanner in the zone.
> 

It seems this patch is useful but I can't parse your this sentense.
Could you elaborate on it?

We should know about exactly why order == -1 is COMPACT_CLUSTER_MAX * 2 and other case
2UL << order. If you write down description more clear, it will help.

Thanks.

> This patch checks the watermark to avoid this problem.
> Tested this patch in the MX6Q board.
> 
> Signed-off-by: Huang Shijie <b32955@freescale.com>
> ---
>  mm/compaction.c |   18 +++++++++---------
>  1 files changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 899d956..bf8e8b2 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -479,21 +479,21 @@ unsigned long compaction_suitable(struct zone *zone, int order)
>  	unsigned long watermark;
>  
>  	/*
> +	 * Watermarks for order-0 must be met for compaction.
> +	 * During the migration, copies of pages need to be
> +	 * allocated and for a short time, so the footprint is higher.
>  	 * order == -1 is expected when compacting via
> -	 * /proc/sys/vm/compact_memory
> +	 * /proc/sys/vm/compact_memory.
>  	 */
> -	if (order == -1)
> -		return COMPACT_CONTINUE;
> +	watermark = low_wmark_pages(zone) +
> +		((order == -1) ? (COMPACT_CLUSTER_MAX * 2) : (2UL << order));
>  
> -	/*
> -	 * Watermarks for order-0 must be met for compaction. Note the 2UL.
> -	 * This is because during migration, copies of pages need to be
> -	 * allocated and for a short time, the footprint is higher
> -	 */
> -	watermark = low_wmark_pages(zone) + (2UL << order);
>  	if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
>  		return COMPACT_SKIPPED;
>  
> +	if (order == -1)
> +		return COMPACT_CONTINUE;
> +
>  	/*
>  	 * fragmentation index determines if allocation failures are due to
>  	 * low memory or external fragmentation
> -- 
> 1.7.3.2
> 
> 
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