linux-kernel - Re: [PATCH v3 3/4] memory_hotplug: use pgdat_resize_lock() in online

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Message-Id: <20130515162054.1c76200ee9514ca8a2054628@linux-foundation.org>
Date:	Wed, 15 May 2013 16:20:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cody P Schafer <cody@...ux.vnet.ibm.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/4] memory_hotplug: use pgdat_resize_lock() in
 online_pages()

On Mon, 13 May 2013 16:13:06 -0700 Cody P Schafer <cody@...ux.vnet.ibm.com> wrote:

> mmzone.h documents node_size_lock (which pgdat_resize_lock() locks) as
> follows:
> 
>         * Must be held any time you expect node_start_pfn, node_present_pages
>         * or node_spanned_pages stay constant.  [...]

Yeah, I suppose so.  Although no present code sites actually do that.

> So actually hold it when we update node_present_pages in online_pages().
> 
> ...
>
> --- 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)

afaict the only benefits of making this change are

a) so that code which does

	a = p->node_present_pages;
	...
	b = p->node_present_pages;

   can ensure that `a' and `b' are equal, by taking pgdat_resize_lock().

   Which is somewhat odd, and

b) to make the comment truthful ;)
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