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Date: Fri, 14 Sep 2012 13:14:28 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: qiuxishi <qiuxishi@gmail.com>, <mgorman@suse.de>, <tony.luck@intel.com>,
        Jiang Liu <jiang.liu@huawei.com>, <qiuxishi@huawei.com>,
        <bessel.wang@huawei.com>, <wujianguo@huawei.com>,
        <paul.gortmaker@windriver.com>, <kamezawa.hiroyu@jp.fujitsu.com>,
        <kosaki.motohiro@jp.fujitsu.com>, <rientjes@google.com>,
        Minchan Kim <minchan@kernel.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, Wen Congyang <wency@cn.fujitsu.com>
Subject: Re: [PATCH RESEND] memory hotplug: fix a double register section
 info bug
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On Fri, 14 Sep 2012 20:00:09 +0900
Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com> wrote:

> > @@ -187,9 +184,10 @@ void register_page_bootmem_info_node(struct pglist_data *pgdat)
> >   	end_pfn = pfn + pgdat->node_spanned_pages;
> >
> >   	/* register_section info */
> > -	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION)
> > -		register_page_bootmem_info_section(pfn);
> > -
> > +	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
> > +		if (pfn_valid(pfn) && (pfn_to_nid(pfn) == node))
> 
> I cannot judge whether your configuration is correct or not.
> Thus if it is correct, I want a comment of why the node check is
> needed. In usual configuration, a node does not span the other one.
> So it is natural that "pfn_to_nid(pfn) is same as "pgdat->node_id".
> Thus we may remove the node check in the future.

yup.  How does this look?

--- a/mm/memory_hotplug.c~memory-hotplug-fix-a-double-register-section-info-bug-fix
+++ a/mm/memory_hotplug.c
@@ -185,6 +185,12 @@ void register_page_bootmem_info_node(str
 
 	/* register_section info */
 	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+		/*
+		 * Some platforms can assign the same pfn to multiple nodes - on
+		 * node0 as well as nodeN.  To avoid registering a pfn against
+		 * multiple nodes we check that this pfn does not already
+		 * reside in some other node.
+		 */
 		if (pfn_valid(pfn) && (pfn_to_nid(pfn) == node))
 			register_page_bootmem_info_section(pfn);
 	}
_