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Date: Fri, 29 Mar 2013 17:17:00 +0100
From: Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH v3, part4 38/39] mm/hotplug: prepare for removing
	num_physpages
Message-ID: <20130329161700.GA6201@merkur.ravnborg.org>
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>
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To: Jiang Liu <liuj97@gmail.com>
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>

> >> 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.

	Sam

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