linux-kernel - Re: [PATCH v2 1/2] mm: page_alloc: simplify pageblock migratetype check in __free_one

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Date:   Sat, 2 Apr 2022 00:14:25 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Zi Yan <ziy@...dia.com>, linux-mm@...ck.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: page_alloc: simplify pageblock migratetype
 check in __free_one_page().

On 4/1/22 20:11, Zi Yan wrote:
> From: Zi Yan <ziy@...dia.com>
> 
> Move pageblock migratetype check code in the while loop to simplify the
> logic. It also saves redundant buddy page checking code.
> 
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Link: https://lore.kernel.org/linux-mm/27ff69f9-60c5-9e59-feb2-295250077551@suse.cz/
> Signed-off-by: Zi Yan <ziy@...dia.com>

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

> ---
>  mm/page_alloc.c | 46 +++++++++++++++++-----------------------------
>  1 file changed, 17 insertions(+), 29 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 856473e54155..2ea106146686 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1054,7 +1054,6 @@ static inline void __free_one_page(struct page *page,
>  		int migratetype, fpi_t fpi_flags)
>  {
>  	struct capture_control *capc = task_capc(zone);
> -	unsigned int max_order = pageblock_order;
>  	unsigned long buddy_pfn;
>  	unsigned long combined_pfn;
>  	struct page *buddy;
> @@ -1070,8 +1069,7 @@ static inline void __free_one_page(struct page *page,
>  	VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
>  	VM_BUG_ON_PAGE(bad_range(zone, page), page);
>  
> -continue_merging:
> -	while (order < max_order) {
> +	while (order < MAX_ORDER - 1) {
>  		if (compaction_capture(capc, page, order, migratetype)) {
>  			__mod_zone_freepage_state(zone, -(1 << order),
>  								migratetype);
> @@ -1082,6 +1080,22 @@ static inline void __free_one_page(struct page *page,
>  
>  		if (!page_is_buddy(page, buddy, order))
>  			goto done_merging;
> +
> +		if (unlikely(order >= pageblock_order)) {
> +			/*
> +			 * 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.
> +			 */
> +			int buddy_mt = get_pageblock_migratetype(buddy);
> +
> +			if (migratetype != buddy_mt
> +					&& (!migratetype_is_mergeable(migratetype) ||
> +						!migratetype_is_mergeable(buddy_mt)))
> +				goto done_merging;
> +		}
> +
>  		/*
>  		 * Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
>  		 * merge with it and move up one order.
> @@ -1095,32 +1109,6 @@ static inline void __free_one_page(struct page *page,
>  		pfn = combined_pfn;
>  		order++;
>  	}
> -	if (order < MAX_ORDER - 1) {
> -		/* If we are here, it means order is >= pageblock_order.
> -		 * 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.
> -		 */
> -		int buddy_mt;
> -
> -		buddy_pfn = __find_buddy_pfn(pfn, order);
> -		buddy = page + (buddy_pfn - pfn);
> -
> -		if (!page_is_buddy(page, buddy, order))
> -			goto done_merging;
> -		buddy_mt = get_pageblock_migratetype(buddy);
> -
> -		if (migratetype != buddy_mt
> -				&& (!migratetype_is_mergeable(migratetype) ||
> -					!migratetype_is_mergeable(buddy_mt)))
> -			goto done_merging;
> -		max_order = order + 1;
> -		goto continue_merging;
> -	}
>  
>  done_merging:
>  	set_buddy_order(page, order);