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

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Date:   Tue, 23 Mar 2021 12:54:00 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Liu Shixin <liushixin2@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>, 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 Tue, Mar 23, 2021 at 09:12:15PM +0800, 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.

This looks good to me:

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>

>  static int move_freepages(struct zone *zone,
> -			  struct page *start_page, struct page *end_page,
> +			  unsigned long start_pfn, unsigned long end_pfn,
>  			  int migratetype, int *num_movable)
>  {
>  	struct page *page;
> +	unsigned long pfn;
>  	unsigned int order;
>  	int pages_moved = 0;
>  
> -	for (page = start_page; page <= end_page;) {
> -		if (!pfn_valid_within(page_to_pfn(page))) {
> -			page++;
> +	for (pfn = start_pfn; pfn <= end_pfn;) {
> +		if (!pfn_valid_within(pfn)) {
> +			pfn++;
>  			continue;
>  		}
>  
> +		page = pfn_to_page(pfn);

I wonder if this wouldn't be even better if we did:

	struct page *start_page = pfn_to_page(start_pfn);

	for (pfn = start_pfn; pfn <= end_pfn; pfn++) {
		struct page *page = start_page + pfn - start_pfn;

		if (!pfn_valid_within(pfn))
			continue;

> -
> -			page++;
> +			pfn++;
>  			continue;

... then we can drop the increment of pfn here

>  		}
>  
> @@ -2458,7 +2459,7 @@ static int move_freepages(struct zone *zone,
>  
>  		order = buddy_order(page);
>  		move_to_free_list(page, zone, order, migratetype);
> -		page += 1 << order;
> +		pfn += 1 << order;

... and change this to pfn += (1 << order) - 1;

Do you have any numbers to quantify the benefit of this change?