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Date: Wed, 01 May 2013 15:41:02 -0700
From: Cody P Schafer <cody@linux.vnet.ibm.com>
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To: David Rientjes <rientjes@google.com>
CC: Andrew Morton <akpm@linux-foundation.org>, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/4] memory_hotplug: use pgdat_resize_lock() when updating
 node_present_pages
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On 05/01/2013 03:30 PM, David Rientjes wrote:
> On Wed, 1 May 2013, Cody P Schafer wrote:
>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index a221fac..0bdca10 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -915,6 +915,7 @@ static void node_states_set_node(int node, struct memory_notify *arg)
>>
>>   int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_type)
>>   {
>> +	unsigned long flags;
>>   	unsigned long onlined_pages = 0;
>>   	struct zone *zone;
>>   	int need_zonelists_rebuild = 0;
>> @@ -993,7 +994,11 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
>>
>>   	zone->managed_pages += onlined_pages;
>>   	zone->present_pages += onlined_pages;
>> +
>> +	pgdat_resize_lock(zone->zone_pgdat, &flags);
>>   	zone->zone_pgdat->node_present_pages += onlined_pages;
>> +	pgdat_resize_unlock(zone->zone_pgdat, &flags);
>> +
>>   	if (onlined_pages) {
>>   		node_states_set_node(zone_to_nid(zone), &arg);
>>   		if (need_zonelists_rebuild)
>
> Why?  You can't get a partial read of a word-sized data structure.
>

Guaranteed to be stable means that if I'm a reader and 
pgdat_resize_lock(), node_present_pages had better not change at all 
until I pgdat_resize_unlock().

If nothing needs this guarantee, we should change the rules of 
pgdat_resize_lock(). I played it safe and went with following the 
existing rules.