linux-kernel - Re: [PATCH 1/4] mm/compaction: fix wrong order check in compact

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Date:	Mon, 08 Dec 2014 10:06:37 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Mel Gorman <mgorman@...e.de>, David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/4] mm/compaction: fix wrong order check in compact_finished()

On 12/08/2014 08:16 AM, Joonsoo Kim wrote:
> What we want to check here is whether there is highorder freepage
> in buddy list of other migratetype in order to steal it without
> fragmentation. But, current code just checks cc->order which means
> allocation request order. So, this is wrong.
>
> Without this fix, non-movable synchronous compaction below pageblock order
> would not stopped until compaction is complete, because migratetype of most
> pageblocks are movable and high order freepage made by compaction is usually
> on movable type buddy list.
>
> There is some report related to this bug. See below link.
>
> http://www.spinics.net/lists/linux-mm/msg81666.html
>
> Although the issued system still has load spike comes from compaction,
> this makes that system completely stable and responsive according to
> his report.
>
> stress-highalloc test in mmtests with non movable order 7 allocation doesn't
> show any notable difference in allocation success rate, but, it shows more
> compaction success rate and reduced elapsed time.
>
> Compaction success rate (Compaction success * 100 / Compaction stalls, %)
> 18.47 : 28.94
>
> Elapsed time (sec)
> 1429 : 1411
>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>   mm/compaction.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c9ee464..1a5f465 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1105,7 +1105,7 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
>   			return COMPACT_PARTIAL;
>
>   		/* Job done if allocation would set block type */
> -		if (cc->order >= pageblock_order && area->nr_free)
> +		if (order >= pageblock_order && area->nr_free)
>   			return COMPACT_PARTIAL;
>   	}
>
>

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