Re: [PATCH v3, part4 38/39] mm/hotplug: prepare for removing num_physpages

From: Sam Ravnborg <sam@ravnborg.org>
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>
Subject: Re: [PATCH v3, part4 38/39] mm/hotplug: prepare for removing num_physpages
Date: Fri, 29 Mar 2013 17:40:14 +0100	[thread overview]
Message-ID: <20130329164014.GA6266@merkur.ravnborg.org> (raw)
In-Reply-To: <5155C196.5020909@gmail.com>

On Sat, Mar 30, 2013 at 12:30:14AM +0800, Jiang Liu wrote:
> 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

I assumed that num_physpages was used run-time to check for the
available number of pages. And __online_page_set_limits() were used
to reduced the number of online pages.

But a quick grep did not reveal any such use.

So it looks OK - thanks for the clarification.

	Sam