Re: [RFC PATCH] powerpc/numa: reset node_possible_map to only node_online_map

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

On 06.03.2015 [08:48:52 +1100], Michael Ellerman wrote:
> On Thu, 2015-03-05 at 13:16 -0800, David Rientjes wrote:
> > On Thu, 5 Mar 2015, Nishanth Aravamudan wrote:
> > 
> > > diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> > > index 0257a7d659ef..24de29b3651b 100644
> > > --- a/arch/powerpc/mm/numa.c
> > > +++ b/arch/powerpc/mm/numa.c
> > > @@ -958,9 +958,17 @@ void __init initmem_init(void)
> > >  
> > >  	memblock_dump_all();
> > >  
> > > +	/*
> > > +	 * zero out the possible nodes after we parse the device-tree,
> > > +	 * so that we lower the maximum NUMA node ID to what is actually
> > > +	 * present.
> > > +	 */
> > > +	nodes_clear(node_possible_map);
> > > +
> > >  	for_each_online_node(nid) {
> > >  		unsigned long start_pfn, end_pfn;
> > >  
> > > +		node_set(nid, node_possible_map);
> > >  		get_pfn_range_for_nid(nid, &start_pfn, &end_pfn);
> > >  		setup_node_data(nid, start_pfn, end_pfn);
> > >  		sparse_memory_present_with_active_regions(nid);
> > 
> > This seems a bit strange, node_possible_map is supposed to be a superset 
> > of node_online_map and this loop is iterating over node_online_map to set 
> > nodes in node_possible_map.
>  
> Yeah. Though at this point in boot I don't think it matters that the two maps
> are out-of-sync temporarily.
> 
> But it would simpler to just set the possible map to be the online
> map. That would also maintain the invariant that the possible map is
> always a superset of the online map.

Yes, we could do that (see my reply to David just now). I didn't
consider just setting the map directly, that would be clearer. I didn't
want to post my nodes_and() version, because the cost of nodes_and
seemed higher than nodes_clear & node_set appropriately.

-Nish