linux-kernel - Re: [RFC v8 PATCH 13/20] memory-hotplug: check page type in get_page

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Date:	Tue, 4 Sep 2012 18:54:30 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Wen Congyang <wency@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>, <x86@...nel.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-acpi@...r.kernel.org>,
	<linux-s390@...r.kernel.org>, <linux-sh@...r.kernel.org>,
	<linux-ia64@...r.kernel.org>, <cmetcalf@...era.com>,
	<sparclinux@...r.kernel.org>, <rientjes@...gle.com>,
	<liuj97@...il.com>, <len.brown@...el.com>,
	<benh@...nel.crashing.org>, <paulus@...ba.org>, <cl@...ux.com>,
	<minchan.kim@...il.com>, <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC v8 PATCH 13/20] memory-hotplug: check page type in get_page_bootmem

Hi Wen,

2012/09/04 12:46, Wen Congyang wrote:
> Hi, isimatu-san
>
> At 09/01/2012 05:30 AM, Andrew Morton Wrote:
>> On Tue, 28 Aug 2012 18:00:20 +0800
>> wency@...fujitsu.com wrote:
>>
>>> From: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
>>>
>>> There is a possibility that get_page_bootmem() is called to the same page many
>>> times. So when get_page_bootmem is called to the same page, the function only
>>> increments page->_count.
>>
>> I really don't understand this explanation, even after having looked at
>> the code.  Can you please have another attempt at the changelog?
>
> What is the problem that you want to fix? The function get_page_bootmem()
> may be called to the same page more than once, but I don't find any problem
> about current implementation.

The patch is just optimization. The patch does not fix a problems.
As you know, the function may be called many times for the same page.
I think if a page is sets to page_type and Page Private flag and page->private,
the page need not be set the same things again. So we check page_type when
get_page_bootmem() is called. And if the page has been set to them, the page
is only incremented page->_count.

Thanks,
Yasuaki Ishimatsu

>
> Thanks
> Wen Congyang
>
>>
>>> --- a/mm/memory_hotplug.c
>>> +++ b/mm/memory_hotplug.c
>>> @@ -95,10 +95,17 @@ static void release_memory_resource(struct resource *res)
>>>   static void get_page_bootmem(unsigned long info,  struct page *page,
>>>   			     unsigned long type)
>>>   {
>>> -	page->lru.next = (struct list_head *) type;
>>> -	SetPagePrivate(page);
>>> -	set_page_private(page, info);
>>> -	atomic_inc(&page->_count);
>>> +	unsigned long page_type;
>>> +
>>> +	page_type = (unsigned long) page->lru.next;
>>> +	if (page_type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||
>>> +	    page_type > MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE){
>>> +		page->lru.next = (struct list_head *) type;
>>> +		SetPagePrivate(page);
>>> +		set_page_private(page, info);
>>> +		atomic_inc(&page->_count);
>>> +	} else
>>> +		atomic_inc(&page->_count);
>>>   }
>>
>> And a code comment which explains what is going on would be good.  As
>> is always the case ;)
>>
>>
>


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