linux-kernel - Re: [RFC][PATCH 1/2] memcg: avoiding unnecessary get_page at move

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Date:	Sat, 16 Oct 2010 07:36:05 +0900
From:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	Greg Thelen <gthelen@...gle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC][PATCH 1/2] memcg: avoiding unnecessary get_page at move_charge

2010/10/16 Christoph Lameter <cl@...ux.com>:
> On Fri, 15 Oct 2010, KAMEZAWA Hiroyuki wrote:
>
>> But above is called all under pte_offset_map_lock().
>> get_page_unless_zero() #1 is not necessary because we do all under a
>> pte_offset_map_lock().
>
> The two (ptl and refcount) are entirely different. The ptl is for
> protecting the page table. The refcount handles only the page.
>
> However, if the entry in the page table is pointing to the page then there
> must have been a refcount taken on the page. So if you know that the page
> is in the page table and you took the ptl then you can be sure that the
> page refcount will not become zero. Therefore get_page_unless_zero() will
> never fail and there is no need to take additional refcounts as long as
> the page table lock is held and the page is not removed from the page
> table.
>

Ok, thank you for explanation. I can make this function faster.

>> Index: mmotm-1013/mm/vmscan.c
>> ===================================================================
>> --- mmotm-1013.orig/mm/vmscan.c
>> +++ mmotm-1013/mm/vmscan.c
>> @@ -1166,7 +1166,8 @@ static unsigned long clear_active_flags(
>>   * found will be decremented.
>>   *
>>   * Restrictions:
>> - * (1) Must be called with an elevated refcount on the page. This is a
>> + * (1) Must be called with an elevated refcount on the page, IOW, the
>> + *     caller must guarantee that there is a stable reference. This is a
>>   *     fundamentnal difference from isolate_lru_pages (which is called
>>   *     without a stable reference).
>>   * (2) the lru_lock must not be held.
>
> There is no need for this change since you have an elevated refcount.
> IMH The words "stable reference" may be confusing since the refcount may
> change. The elevated refcount protects against the freeing of the page.
>

Sure, drop change this in v2. I misunderstand "elevated refcount"
means "extra get_page()".

Thanks,
-Kame
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