Re: [PATCH v2 1/4] mm/compaction: fix wrong order check in compact_finished()

[Vlastimil Babka]

On 01/30/2015 01:34 PM, 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.

The bug has been introduced by 1fb3f8ca0e92 ("mm: compaction: capture a suitable
high-order page immediately when it is made available") and survived the later
partial revert 8fb74b9fb2b1.

> 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.
> 
> Compaction success rate (Compaction success * 100 / Compaction stalls, %)
> 18.47 : 28.94
> 
> Cc: <stable@xxxxxxxxxxxxxxx>

# v3.7+
Fixes: 1fb3f8ca0e9222535a39b884cb67a34628411b9f

> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Thanks.

> ---
>  mm/compaction.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index b68736c..4954e19 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1173,7 +1173,7 @@ 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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