linux-kernel - Re: [PATCH] mm: page_alloc: simpify page del and expand

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Message-ID: <e3276bb1-6d70-41c8-9d04-c7eade3dae95@suse.cz>
Date: Thu, 29 Aug 2024 17:12:54 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Huan Yang <link@...o.com>, Andrew Morton <akpm@...ux-foundation.org>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: opensource.kernel@...o.com
Subject: Re: [PATCH] mm: page_alloc: simpify page del and expand

On 8/26/24 08:40, Huan Yang wrote:
> When page del from buddy and need expand, it will account free_pages
> in zone's migratetype.
> 
> The current way is to subtract the page number of the current order
> when deleting, and then add it back when expanding.
> 
> This is unnecessary, as when migrating the same type, we can directly
> record the difference between the high-order pages and the expand
> added, and then subtract it directly.
> 
> This patch merge that, only when del and expand done, then account
> free_pages.
> 
> Signed-off-by: Huan Yang <link@...o.com>

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

can't hurt to reduce the number of calls to __mod_zone_page_state() (via
account_freepages()) in the allocator

> ---
>  mm/page_alloc.c | 35 +++++++++++++++++++++++++----------
>  1 file changed, 25 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 023132f66d29..900b0947c6e1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1363,11 +1363,11 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
>   *
>   * -- nyc
>   */
> -static inline void expand(struct zone *zone, struct page *page,
> -	int low, int high, int migratetype)
> +static inline unsigned int expand(struct zone *zone, struct page *page, int low,
> +				  int high, int migratetype)
>  {
> -	unsigned long size = 1 << high;
> -	unsigned long nr_added = 0;
> +	unsigned int size = 1 << high;
> +	unsigned int nr_added = 0;
>  
>  	while (high > low) {
>  		high--;
> @@ -1387,7 +1387,19 @@ static inline void expand(struct zone *zone, struct page *page,
>  		set_buddy_order(&page[size], high);
>  		nr_added += size;
>  	}
> -	account_freepages(zone, nr_added, migratetype);
> +
> +	return nr_added;
> +}
> +
> +static __always_inline void page_del_and_expand(struct zone *zone,
> +						struct page *page, int low,
> +						int high, int migratetype)
> +{
> +	int nr_pages = 1 << high;
> +
> +	__del_page_from_free_list(page, zone, high, migratetype);
> +	nr_pages -= expand(zone, page, low, high, migratetype);
> +	account_freepages(zone, -nr_pages, migratetype);
>  }
>  
>  static void check_new_page_bad(struct page *page)
> @@ -1557,8 +1569,9 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>  		page = get_page_from_free_area(area, migratetype);
>  		if (!page)
>  			continue;
> -		del_page_from_free_list(page, zone, current_order, migratetype);
> -		expand(zone, page, order, current_order, migratetype);
> +
> +		page_del_and_expand(zone, page, order, current_order,
> +				    migratetype);
>  		trace_mm_page_alloc_zone_locked(page, order, migratetype,
>  				pcp_allowed_order(order) &&
>  				migratetype < MIGRATE_PCPTYPES);
> @@ -1888,9 +1901,12 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
>  
>  	/* Take ownership for orders >= pageblock_order */
>  	if (current_order >= pageblock_order) {
> +		unsigned int nr_added;
> +
>  		del_page_from_free_list(page, zone, current_order, block_type);
>  		change_pageblock_range(page, current_order, start_type);
> -		expand(zone, page, order, current_order, start_type);
> +		nr_added = expand(zone, page, order, current_order, start_type);
> +		account_freepages(zone, nr_added, start_type);
>  		return page;
>  	}
>  
> @@ -1943,8 +1959,7 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
>  	}
>  
>  single_page:
> -	del_page_from_free_list(page, zone, current_order, block_type);
> -	expand(zone, page, order, current_order, block_type);
> +	page_del_and_expand(zone, page, order, current_order, block_type);
>  	return page;
>  }
>