From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jiang Liu <liuj97@gmail.com>
Subject: Re: [PATCH v3, part4 38/39] mm/hotplug: prepare for removing num_physpages
Date: Sat, 30 Mar 2013 00:30:14 +0800
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References: <1364313298-17336-1-git-send-email-jiang.liu@huawei.com> <1364313298-17336-39-git-send-email-jiang.liu@huawei.com> <20130329111856.GA3824@merkur.ravnborg.org> <5155B517.3040501@gmail.com> <20130329161700.GA6201@merkur.ravnborg.org>
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To: Sam Ravnborg <sam@ravnborg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, David Rientjes <rientjes@google.com>, Jiang Liu <jiang.liu@huawei.com>, Wen Congyang <wency@cn.fujitsu.com>, Mel Gorman <mgorman@suse.de>, Minchan Kim <minchan@kernel.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Michal Hocko <mhocko@suse.cz>, James Bottomley <James.Bottomley@HansenPartnership.com>, Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>, David Howells <dhowells@redhat.com>, Mark Salter <msalter@redhat.com>, Jianguo Wu <wujianguo@huawei.com>, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Tang Chen <tangchen@cn.fujitsu.com>, Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

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
>