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

[Mel Gorman <mgorman@suse.de>]

On Sat, Dec 31, 2011 at 02:18:43PM +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().
> That's mean we can not alloc any pages by the free scanner in the zone.
> 
> This patch checks the watermark to avoid this problem.
> 
> Signed-off-by: Huang Shijie <b32955@freescale.com>
> ---
>  mm/compaction.c |   14 ++++++++++++--
>  1 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 899d956..0f12cc9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -442,8 +442,13 @@ static int compact_finished(struct zone *zone,
>  	 * order == -1 is expected when compacting via
>  	 * /proc/sys/vm/compact_memory
>  	 */
> -	if (cc->order == -1)
> +	if (cc->order == -1) {
> +		/* Check if we have enough pages now. */
> +		watermark = low_wmark_pages(zone) + COMPACT_CLUSTER_MAX * 2;
> +		if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
> +			return COMPACT_SKIPPED;
>  		return COMPACT_CONTINUE;
> +	}
>  

We already do the watermark check in compact_finished. Would moving
the cc->order == -1 check below it not be functionally equivalent? This
would be preferable to duplicating the code for the watermark check.

Same for the second check in your patch.

-- 
Mel Gorman
SUSE Labs

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