linux-kernel - Re: [PATCH 03/11] mm: mmzone: introduce zone_pfn_same

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Date:	Mon, 12 Dec 2011 15:51:55 +0100
From:	"Michal Nazarewicz" <mina86@...a86.com>
To:	"Mel Gorman" <mel@....ul.ie>,
	"Dave Hansen" <dave@...ux.vnet.ibm.com>
Cc:	"Marek Szyprowski" <m.szyprowski@...sung.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	"Kyungmin Park" <kyungmin.park@...sung.com>,
	"Russell King" <linux@....linux.org.uk>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Ankita Garg" <ankita@...ibm.com>,
	"Daniel Walker" <dwalker@...eaurora.org>,
	"Arnd Bergmann" <arnd@...db.de>,
	"Jesse Barker" <jesse.barker@...aro.org>,
	"Jonathan Corbet" <corbet@....net>,
	"Shariq Hasnain" <shariq.hasnain@...aro.org>,
	"Chunsang Jeong" <chunsang.jeong@...aro.org>
Subject: Re: [PATCH 03/11] mm: mmzone: introduce zone_pfn_same_memmap()

> On Fri, Nov 18, 2011 at 05:43:10PM +0100, Marek Szyprowski wrote:
>> From: Michal Nazarewicz <mina86@...a86.com>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 6afae0e..09c9702 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -111,7 +111,10 @@ skip:
>>
>>  next:
>>  		pfn += isolated;
>> -		page += isolated;
>> +		if (zone_pfn_same_memmap(pfn - isolated, pfn))
>> +			page += isolated;
>> +		else
>> +			page = pfn_to_page(pfn);
>>  	}

On Mon, 12 Dec 2011 15:19:53 +0100, Mel Gorman <mel@....ul.ie> wrote:
> Is this necessary?
>
> We are isolating pages, the largest of which is a MAX_ORDER_NR_PAGES
> page.  [...]

On Mon, 12 Dec 2011 15:40:30 +0100, Mel Gorman <mel@....ul.ie> wrote:
> To be clear, I'm referring to a single page being isolated here. It may
> or may not be a high-order page but it's still going to be less then
> MAX_ORDER_NR_PAGES so you should be able check when a new block is
> entered and pfn_to_page is necessary.

Do you mean something like:

if (same pageblock)
	just do arithmetic;
else
	use pfn_to_page;

?

I've discussed it with Dave and he suggested that approach as an
optimisation since in some configurations zone_pfn_same_memmap()
is always true thus compiler will strip the else part, whereas
same pageblock test will be false on occasions regardless of kernel
configuration.

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +----<email/xmpp: mpn@...gle.com>--------------ooO--(_)--Ooo--
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