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Date: Sat, 27 Mar 2021 11:34:05 +0800 From: Liu Shixin <liushixin2@...wei.com> To: Matthew Wilcox <willy@...radead.org> 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() Sorry to reply to you after a so long time and thanks for your advice. It does seem that your proposed change will make the code cleaner and more efficient. I repeated move_freepages_block() 2000000 times on the VM and counted jiffies. The average value before and after the change was both about 12,000. I think it's probably because I'm using the Sparse Memory Model, so pfn_to_page() is not time-consuming. On 2021/3/23 20:54, Matthew Wilcox wrote: > 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? > . >
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