Subject: Re: [RFC PATCH v2 1/7] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.

From: Eric Ren <[email-masked]>
Date: Fri, 10 Dec 2021 15:43:56 +0800

Hi,

On 2021/12/10 07:04, Zi Yan wrote:
> From: Zi Yan <[email-masked]>
>
> This is done in addition to MIGRATE_ISOLATE pageblock merge avoidance.
> It prepares for the upcoming removal of the MAX_ORDER-1 alignment
> requirement for CMA and alloc_contig_range().
>
> MIGRARTE_HIGHATOMIC should not merge with other migratetypes like
> MIGRATE_ISOLATE and MIGRARTE_CMA[1], so this commit prevents that too.
> Also add MIGRARTE_HIGHATOMIC to fallbacks array for completeness.
>
> [1] https://lore.kernel.org/linux-mm/[email-masked]/
>
> Signed-off-by: Zi Yan <[email-masked]>
> ---
>   include/linux/mmzone.h |  6 ++++++
>   mm/page_alloc.c        | 28 ++++++++++++++++++----------
>   2 files changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 58e744b78c2c..b925431b0123 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -83,6 +83,12 @@ static inline bool is_migrate_movable(int mt)
>   	return is_migrate_cma(mt) || mt == MIGRATE_MOVABLE;
>   }
>   
> +/* See fallbacks[MIGRATE_TYPES][3] in page_alloc.c */
> +static inline bool migratetype_has_fallback(int mt)
> +{
> +	return mt < MIGRATE_PCPTYPES;
> +}
> +

I would suggest spliting the patch into 2 parts.  The first part: no 
functioning change, just introduce migratetype_has_fallback()
and replace where it applys to.

>   #define for_each_migratetype_order(order, type) \
>   	for (order = 0; order < MAX_ORDER; order++) \
>   		for (type = 0; type < MIGRATE_TYPES; type++)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index edfd6c81af82..107a5f186d3b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1041,6 +1041,12 @@ buddy_merge_likely(unsigned long pfn, unsigned long buddy_pfn,
>   	return page_is_buddy(higher_page, higher_buddy, order + 1);
>   }
>   
> +static inline bool has_non_fallback_pageblock(struct zone *zone)
> +{
> +	return has_isolate_pageblock(zone) || zone_cma_pages(zone) != 0 ||
> +		zone->nr_reserved_highatomic != 0;

Make zone->nr_reserved_highatomic != 0 a helper as zone_cma_pages()?
> +}
> +
>   /*
>    * Freeing function for a buddy system allocator.
>    *
> @@ -1116,14 +1122,15 @@ static inline void __free_one_page(struct page *page,
>   	}
>   	if (order < MAX_ORDER - 1) {
>   		/* If we are here, it means order is >= pageblock_order.
> -		 * We want to prevent merge between freepages on isolate
> -		 * pageblock and normal pageblock. Without this, pageblock
> -		 * isolation could cause incorrect freepage or CMA accounting.
> +		 * We want to prevent merge between freepages on pageblock
> +		 * without fallbacks and normal pageblock. Without this,
> +		 * pageblock isolation could cause incorrect freepage or CMA
> +		 * accounting or HIGHATOMIC accounting.
>   		 *
>   		 * We don't want to hit this code for the more frequent
>   		 * low-order merging.
>   		 */
> -		if (unlikely(has_isolate_pageblock(zone))) {
> +		if (unlikely(has_non_fallback_pageblock(zone))) {
I'm not familiar with the code details, just wondering if this change 
would has side effects on cma
pageblock merging as it the condition stronger?

Thanks,
Eric
>   			int buddy_mt;
>   
>   			buddy_pfn = __find_buddy_pfn(pfn, order);
> @@ -1131,8 +1138,8 @@ static inline void __free_one_page(struct page *page,
>   			buddy_mt = get_pageblock_migratetype(buddy);
>   
>   			if (migratetype != buddy_mt
> -					&& (is_migrate_isolate(migratetype) ||
> -						is_migrate_isolate(buddy_mt)))
> +					&& (!migratetype_has_fallback(migratetype) ||
> +						!migratetype_has_fallback(buddy_mt)))
>   				goto done_merging;
>   		}
>   		max_order = order + 1;
> @@ -2483,6 +2490,7 @@ static int fallbacks[MIGRATE_TYPES][3] = {
>   	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },
>   	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
>   	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
> +	[MIGRATE_HIGHATOMIC] = { MIGRATE_TYPES }, /* Never used */
>   #ifdef CONFIG_CMA
>   	[MIGRATE_CMA]         = { MIGRATE_TYPES }, /* Never used */
>   #endif
> @@ -2794,8 +2802,8 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
>   
>   	/* Yoink! */
>   	mt = get_pageblock_migratetype(page);
> -	if (!is_migrate_highatomic(mt) && !is_migrate_isolate(mt)
> -	    && !is_migrate_cma(mt)) {
> +	/* Only reserve normal pageblock */
> +	if (migratetype_has_fallback(mt)) {
>   		zone->nr_reserved_highatomic += pageblock_nr_pages;
>   		set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
>   		move_freepages_block(zone, page, MIGRATE_HIGHATOMIC, NULL);
> @@ -3544,8 +3552,8 @@ int __isolate_free_page(struct page *page, unsigned int order)
>   		struct page *endpage = page + (1 << order) - 1;
>   		for (; page < endpage; page += pageblock_nr_pages) {
>   			int mt = get_pageblock_migratetype(page);
> -			if (!is_migrate_isolate(mt) && !is_migrate_cma(mt)
> -			    && !is_migrate_highatomic(mt))
> +			/* Only change normal pageblock */
> +			if (migratetype_has_fallback(mt))
>   				set_pageblock_migratetype(page,
>   							  MIGRATE_MOVABLE);
>   		}