Re: [PATCH v3 8/8] mm, compaction: finish whole pageblock to reduce fragmentation

[Joonsoo Kim]

On Tue, Mar 07, 2017 at 02:15:45PM +0100, Vlastimil Babka wrote:
> The main goal of direct compaction is to form a high-order page for allocation,
> but it should also help against long-term fragmentation when possible. Most
> lower-than-pageblock-order compactions are for non-movable allocations, which
> means that if we compact in a movable pageblock and terminate as soon as we
> create the high-order page, it's unlikely that the fallback heuristics will
> claim the whole block. Instead there might be a single unmovable page in a
> pageblock full of movable pages, and the next unmovable allocation might pick
> another pageblock and increase long-term fragmentation.
> 
> To help against such scenarios, this patch changes the termination criteria for
> compaction so that the current pageblock is finished even though the high-order
> page already exists. Note that it might be possible that the high-order page
> formed elsewhere in the zone due to parallel activity, but this patch doesn't
> try to detect that.
> 
> This is only done with sync compaction, because async compaction is limited to
> pageblock of the same migratetype, where it cannot result in a migratetype
> fallback. (Async compaction also eagerly skips order-aligned blocks where
> isolation fails, which is against the goal of migrating away as much of the
> pageblock as possible.)
> 
> As a result of this patch, long-term memory fragmentation should be reduced.
> 
> In testing based on 4.9 kernel with stress-highalloc from mmtests configured
> for order-4 GFP_KERNEL allocations, this patch has reduced the number of
> unmovable allocations falling back to movable pageblocks by 20%. The number
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  mm/compaction.c | 36 ++++++++++++++++++++++++++++++++++--
>  mm/internal.h   |  1 +
>  2 files changed, 35 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 2c288e75840d..bc7903130501 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1318,6 +1318,17 @@ static enum compact_result __compact_finished(struct zone *zone,
>  	if (is_via_compact_memory(cc->order))
>  		return COMPACT_CONTINUE;
>  
> +	if (cc->finishing_block) {
> +		/*
> +		 * We have finished the pageblock, but better check again that
> +		 * we really succeeded.
> +		 */
> +		if (IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
> +			cc->finishing_block = false;
> +		else
> +			return COMPACT_CONTINUE;
> +	}
> +
>  	/* Direct compactor: Is a suitable page free? */
>  	for (order = cc->order; order < MAX_ORDER; order++) {
>  		struct free_area *area = &zone->free_area[order];
> @@ -1338,8 +1349,29 @@ static enum compact_result __compact_finished(struct zone *zone,
>  		 * other migratetype buddy lists.
>  		 */
>  		if (find_suitable_fallback(area, order, migratetype,
> -						true, &can_steal) != -1)
> -			return COMPACT_SUCCESS;
> +						true, &can_steal) != -1) {
> +
> +			/* movable pages are OK in any pageblock */
> +			if (migratetype == MIGRATE_MOVABLE)
> +				return COMPACT_SUCCESS;
> +
> +			/*
> +			 * We are stealing for a non-movable allocation. Make
> +			 * sure we finish compacting the current pageblock
> +			 * first so it is as free as possible and we won't
> +			 * have to steal another one soon. This only applies
> +			 * to sync compaction, as async compaction operates
> +			 * on pageblocks of the same migratetype.
> +			 */
> +			if (cc->mode == MIGRATE_ASYNC ||
> +					IS_ALIGNED(cc->migrate_pfn,
> +							pageblock_nr_pages)) {
> +				return COMPACT_SUCCESS;
> +			}

If cc->migratetype and cc->migrate_pfn's migratetype is the same, stopping
the compaction here doesn't cause any fragmentation. Do we need to
compact full pageblock in this case?

Thanks.