Re: [PATCH -next] mm, page_alloc: avoid page_to_pfn() in move_freepages()

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 23, 2021 at 09:12:15PM +0800, Liu Shixin wrote:
> From: Kefeng Wang <wangkefeng.wang@huawei.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@infradead.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?