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

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Date:	Thu, 15 Aug 2013 10:32:50 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>
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 2:00, Mel Gorman wrote:

>>> Even if the page is still page buddy, there is no guarantee that it's
>>> the same page order as the first read. It could have be currently
>>> merging with adjacent buddies for example. There is also a really
>>> small race that a page was freed, allocated with some number stuffed
>>> into page->private and freed again before the second PageBuddy check.
>>> It's a bit of a hand grenade. How much of a performance benefit is there
>>
>> 1. Just worst case is skipping pageblock_nr_pages
> 
> No, the worst case is that page_order returns a number that is
> completely garbage and low_pfn goes off the end of the zone
> 
>> 2. Race is really small
>> 3. Higher order page allocation customer always have graceful fallback.
>>

Hi Minchan, 
I think in this case, we may get the wrong value from page_order(page).

1. page is in page buddy

> if (PageBuddy(page)) {

2. someone allocated the page, and set page->private to another value

> 	int nr_pages = (1 << page_order(page)) - 1;

3. someone freed the page

> 	if (PageBuddy(page)) {

4. we will skip wrong pages

> 		nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1);
> 		low_pfn += nr_pages;
> 		continue;
> 	}
> }
> 
> It's still race-prone meaning that it really should be backed by some
> performance data justifying it.
> 



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