linux-kernel - Re: [PATCH v3, part4 38/39] mm/hotplug: prepare for removing num

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Message-ID: <5155C196.5020909@gmail.com>
Date:	Sat, 30 Mar 2013 00:30:14 +0800
From:	Jiang Liu <liuj97@...il.com>
To:	Sam Ravnborg <sam@...nborg.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	David Howells <dhowells@...hat.com>,
	Mark Salter <msalter@...hat.com>,
	Jianguo Wu <wujianguo@...wei.com>, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Tang Chen <tangchen@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Subject: Re: [PATCH v3, part4 38/39] mm/hotplug: prepare for removing num_physpages

On 03/30/2013 12:17 AM, Sam Ravnborg wrote:
>>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>>> index 97454b3..9b1b494 100644
>>>> --- a/mm/memory_hotplug.c
>>>> +++ b/mm/memory_hotplug.c
>>>> @@ -751,10 +751,6 @@ EXPORT_SYMBOL_GPL(restore_online_page_callback);
>>>>  
>>>>  void __online_page_set_limits(struct page *page)
>>>>  {
>>>> -	unsigned long pfn = page_to_pfn(page);
>>>> -
>>>> -	if (pfn >= num_physpages)
>>>> -		num_physpages = pfn + 1;
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(__online_page_set_limits);
>>>
>>> How can this be correct?
>>> With this change __online_page_set_limits() is now a nop.
>> Hi Sam,
>> 	We will eventually remove the global variable num_physpages in the last patch.
>> I kept the nop __online_page_set_limits() because I have a plan to use it to fix other
>> bugs in memory hotplug, otherwise it may be killed too.
> 
> The xen ballon driver uses __online_page_set_limits for memory
> hotplug - so this will break this driver afaics.
Hi Sam,
	I haven't gotten your point yet here. 
	Function __online_page_set_limits() was only used to update the global variable
num_physpages, and one of the goals of this patch set is to get rid of num_physpages.
So I think it won't break Xen balloon driver.
	Please refer to the patch here, which eventually kills num_physpages.
http://marc.info/?l=linux-mm&m=136431387813309&w=2

	Regards!
	Gerry

> 
> 	Sam
> 

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