Re: [PATCH 4/4] memory_hotplug: use pgdat_resize_lock() when updatingnode_present_pages

[Cody P Schafer]

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.

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