Re: [PATCH v4 2/3] mm/page_alloc: factor out fallback freepage checking

[Vlastimil Babka]

On 02/12/2015 08:15 AM, Joonsoo Kim wrote:
> This is preparation step to use page allocator's anti fragmentation logic
> in compaction. This patch just separates fallback freepage checking part
> from fallback freepage management part. Therefore, there is no functional
> change.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>   mm/page_alloc.c |  143 +++++++++++++++++++++++++++++++++++--------------------
>   1 file changed, 91 insertions(+), 52 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e64b260..64a4974 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1142,14 +1142,40 @@ static void change_pageblock_range(struct page *pageblock_page,
>    * as fragmentation caused by those allocations polluting movable pageblocks
>    * is worse than movable allocations stealing from unmovable and reclaimable
>    * pageblocks.
> - *
> - * If we claim more than half of the pageblock, change pageblock's migratetype
> - * as well.
>    */
> -static void try_to_steal_freepages(struct zone *zone, struct page *page,
> -				  int start_type, int fallback_type)
> +static bool can_steal_fallback(unsigned int order, int start_mt)
> +{
> +	/*
> +	 * Leaving this order check is intended, although there is
> +	 * relaxed order check in next check. The reason is that
> +	 * we can actually steal whole pageblock if this condition met,
> +	 * but, below check doesn't guarantee it and that is just heuristic
> +	 * so could be changed anytime.
> +	 */
> +	if (order >= pageblock_order)
> +		return true;
> +
> +	if (order >= pageblock_order / 2 ||
> +		start_mt == MIGRATE_RECLAIMABLE ||
> +		start_mt == MIGRATE_UNMOVABLE ||
> +		page_group_by_mobility_disabled)
> +		return true;
> +
> +	return false;
> +}
> +
> +/*
> + * This function implements actual steal behaviour. If order is large enough,
> + * we can steal whole pageblock. If not, we first move freepages in this
> + * pageblock and check whether half of pages are moved or not. If half of
> + * pages are moved, we can change migratetype of pageblock and permanently
> + * use it's pages as requested migratetype in the future.
> + */
> +static void steal_suitable_fallback(struct zone *zone, struct page *page,
> +							  int start_type)
>   {
>   	int current_order = page_order(page);
> +	int pages;
>
>   	/* Take ownership for orders >= pageblock_order */
>   	if (current_order >= pageblock_order) {
> @@ -1157,19 +1183,40 @@ static void try_to_steal_freepages(struct zone *zone, struct page *page,
>   		return;
>   	}
>
> -	if (current_order >= pageblock_order / 2 ||
> -	    start_type == MIGRATE_RECLAIMABLE ||
> -	    start_type == MIGRATE_UNMOVABLE ||
> -	    page_group_by_mobility_disabled) {
> -		int pages;
> +	pages = move_freepages_block(zone, page, start_type);
> +
> +	/* Claim the whole block if over half of it is free */
> +	if (pages >= (1 << (pageblock_order-1)) ||
> +			page_group_by_mobility_disabled)
> +		set_pageblock_migratetype(page, start_type);
> +}
> +
> +/* Check whether there is a suitable fallback freepage with requested order. */
> +static int find_suitable_fallback(struct free_area *area, unsigned int order,
> +					int migratetype, bool *can_steal)
> +{
> +	int i;
> +	int fallback_mt;
> +
> +	if (area->nr_free == 0)
> +		return -1;
> +
> +	*can_steal = false;
> +	for (i = 0;; i++) {
> +		fallback_mt = fallbacks[migratetype][i];
> +		if (fallback_mt == MIGRATE_RESERVE)
> +			break;
> +
> +		if (list_empty(&area->free_list[fallback_mt]))
> +			continue;
>
> -		pages = move_freepages_block(zone, page, start_type);
> +		if (can_steal_fallback(order, migratetype))
> +			*can_steal = true;
>
> -		/* Claim the whole block if over half of it is free */
> -		if (pages >= (1 << (pageblock_order-1)) ||
> -				page_group_by_mobility_disabled)
> -			set_pageblock_migratetype(page, start_type);
> +		return fallback_mt;
>   	}
> +
> +	return -1;
>   }
>
>   /* Remove an element from the buddy allocator from the fallback list */
> @@ -1179,53 +1226,45 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
>   	struct free_area *area;
>   	unsigned int current_order;
>   	struct page *page;
> +	int fallback_mt;
> +	bool can_steal;
>
>   	/* Find the largest possible block of pages in the other list */
>   	for (current_order = MAX_ORDER-1;
>   				current_order >= order && current_order <= MAX_ORDER-1;
>   				--current_order) {
> -		int i;
> -		for (i = 0;; i++) {
> -			int migratetype = fallbacks[start_migratetype][i];
> -			int buddy_type = start_migratetype;
> -
> -			/* MIGRATE_RESERVE handled later if necessary */
> -			if (migratetype == MIGRATE_RESERVE)
> -				break;
> -
> -			area = &(zone->free_area[current_order]);
> -			if (list_empty(&area->free_list[migratetype]))
> -				continue;
> -
> -			page = list_entry(area->free_list[migratetype].next,
> -					struct page, lru);
> -			area->nr_free--;
> -
> -			try_to_steal_freepages(zone, page, start_migratetype,
> -								migratetype);
> +		area = &(zone->free_area[current_order]);
> +		fallback_mt = find_suitable_fallback(area, current_order,
> +				start_migratetype, &can_steal);
> +		if (fallback_mt == -1)
> +			continue;
>
> -			/* Remove the page from the freelists */
> -			list_del(&page->lru);
> -			rmv_page_order(page);
> +		page = list_entry(area->free_list[fallback_mt].next,
> +						struct page, lru);
> +		if (can_steal)
> +			steal_suitable_fallback(zone, page, start_migratetype);
>
> -			expand(zone, page, order, current_order, area,
> -					buddy_type);
> +		/* Remove the page from the freelists */
> +		area->nr_free--;
> +		list_del(&page->lru);
> +		rmv_page_order(page);
>
> -			/*
> -			 * The freepage_migratetype may differ from pageblock's
> -			 * migratetype depending on the decisions in
> -			 * try_to_steal_freepages(). This is OK as long as it
> -			 * does not differ for MIGRATE_CMA pageblocks. For CMA
> -			 * we need to make sure unallocated pages flushed from
> -			 * pcp lists are returned to the correct freelist.
> -			 */
> -			set_freepage_migratetype(page, buddy_type);
> +		expand(zone, page, order, current_order, area,
> +					start_migratetype);
> +		/*
> +		 * The freepage_migratetype may differ from pageblock's
> +		 * migratetype depending on the decisions in
> +		 * try_to_steal_freepages(). This is OK as long as it
> +		 * does not differ for MIGRATE_CMA pageblocks. For CMA
> +		 * we need to make sure unallocated pages flushed from
> +		 * pcp lists are returned to the correct freelist.
> +		 */
> +		set_freepage_migratetype(page, start_migratetype);
>
> -			trace_mm_page_alloc_extfrag(page, order, current_order,
> -				start_migratetype, migratetype);
> +		trace_mm_page_alloc_extfrag(page, order, current_order,
> +			start_migratetype, fallback_mt);
>
> -			return page;
> -		}
> +		return page;
>   	}
>
>   	return NULL;

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