linux-kernel - Re: [PATCH] mm: skip the page buddy block instead of one page

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Date:	Thu, 15 Aug 2013 15:45:11 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>, <riel@...hat.com>,
	<aquini@...hat.com>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Xishi Qiu <qiuxishi@...wei.com>
Subject: Re: [PATCH] mm: skip the page buddy block instead of one page

On 2013/8/15 12:24, Minchan Kim wrote:

>> Please read full thread in detail.
>>
>> Mel suggested following as
>>
>> if (PageBuddy(page)) {
>>         int nr_pages = (1 << page_order(page)) - 1;
>>         if (PageBuddy(page)) {
>>                 nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1);
>>                 low_pfn += nr_pages;
>>                 continue;
>>         }
>> }
>>
>> min(nr_pages, xxx) removes your concern but I think Mel's version
>> isn't right. It should be aligned with pageblock boundary so I 
>> suggested following.
>>
>> if (PageBuddy(page)) {
>> #ifdef CONFIG_MEMORY_ISOLATION
>> 	unsigned long order = page_order(page);
>> 	if (PageBuddy(page)) {
>> 		low_pfn += (1 << order) - 1;
>> 		low_pfn = min(low_pfn, end_pfn);
>> 	}
>> #endif
>> 	continue;
>> }
>>

Hi Minchan,

I understand now, but why use "end_pfn" here? 
Maybe like this:

if (PageBuddy(page)) {
	/*
	 * page_order is racy without zone->lock but worst case
	 * by the racing is just skipping pageblock_nr_pages.
	 */
	unsigned long nr_pages = 1 << page_order(page);
	if (likely(PageBuddy(page))) {
		nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES);

		/* Align with pageblock boundary */
		if ((low_pfn & (pageblock_nr_pages - 1)) + nr_pages >
		    pageblock_nr_pages)
			low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
		else
			low_pfn += nr_pages - 1;
	}
	continue;
}

Thanks,
Xishi Qiu

>> so worst case is (pageblock_nr_pages - 1).
>> but we don't need to add CONFIG_MEMORY_ISOLATION so my suggestion
>> is following as.
>>
>> if (PageBuddy(page)) {
>> 	unsigned long order = page_order(page);
>> 	if (PageBuddy(page)) {
>> 		low_pfn += (1 << order) - 1;
>> 		low_pfn = min(low_pfn, end_pfn);
> 
> Maybe it should be low_pfn = min(low_pfn, end_pfn - 1).
> 
> 
>> 	}
>> 	continue;
>> }
>>
>>
> 



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