linux-kernel - Re: [PATCH -next] mm, page_alloc: avoid page_to_pfn() in move

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Date:   Mon, 29 Mar 2021 17:31:09 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Liu Shixin <liushixin2@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH -next] mm, page_alloc: avoid page_to_pfn() in
 move_freepages()

On 3/23/21 2:12 PM, Liu Shixin wrote:
> From: Kefeng Wang <wangkefeng.wang@...wei.com>
> 
> The start_pfn and end_pfn are already available in move_freepages_block(),
> there is no need to go back and forth between page and pfn in move_freepages
> and move_freepages_block, and pfn_valid_within() should validate pfn first
> before touching the page.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> Signed-off-by: Liu Shixin <liushixin2@...wei.com>

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

Agreed with Matthew's suggestion, also:

> @@ -2468,25 +2469,22 @@ static int move_freepages(struct zone *zone,
>  int move_freepages_block(struct zone *zone, struct page *page,
>  				int migratetype, int *num_movable)
>  {
> -	unsigned long start_pfn, end_pfn;
> -	struct page *start_page, *end_page;
> +	unsigned long start_pfn, end_pfn, pfn;
>  
>  	if (num_movable)
>  		*num_movable = 0;
>  
> -	start_pfn = page_to_pfn(page);
> -	start_pfn = start_pfn & ~(pageblock_nr_pages-1);
> -	start_page = pfn_to_page(start_pfn);
> -	end_page = start_page + pageblock_nr_pages - 1;
> +	pfn = page_to_pfn(page);
> +	start_pfn = pfn & ~(pageblock_nr_pages - 1);

Since you touch this already, consider pageblock_start_pfn()

>  	end_pfn = start_pfn + pageblock_nr_pages - 1;

I'd also drop the "- 1" here and instead adjust the for loop's ending condition
from "pfn <= end_pfn" to "pfn < end_pfn" as that's more common way of doing it.

Thanks.

>  
>  	/* Do not cross zone boundaries */
>  	if (!zone_spans_pfn(zone, start_pfn))
> -		start_page = page;
> +		start_pfn = pfn;
>  	if (!zone_spans_pfn(zone, end_pfn))
>  		return 0;
>  
> -	return move_freepages(zone, start_page, end_page, migratetype,
> +	return move_freepages(zone, start_pfn, end_pfn, migratetype,
>  								num_movable);
>  }
>  
>