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

From: Nishanth Aravamudan <[email-masked]>
Date: Thu, 5 Mar 2015 15:15:55 -0800

Hi David,

On 05.03.2015 [13:16:35 -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.

So if we compare to x86:

arch/x86/mm/numa.c::numa_init():

        nodes_clear(numa_nodes_parsed);
        nodes_clear(node_possible_map);
        nodes_clear(node_online_map);
	...
	numa_register_memblks(...);

arch/x86/mm/numa.c::numa_register_memblks():

	node_possible_map = numa_nodes_parsed;

Basically, it looks like x86 NUMA init clears out possible map and
online map, probably for a similar reason to what I gave in the
changelog that by default, the possible map seems to be based off
MAX_NUMNODES, rather than nr_node_ids or anything dynamic.

My patch was an attempt to emulate the same thing on powerpc. You are
right that there is a window in which the node_possible_map and
node_online_map are out of sync with my patch. It seems like it
shouldn't matter given how early in boot we are, but perhaps the
following would have been clearer:

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index 0257a7d659ef..1a118b08fad2 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c
@@ -958,6 +958,13 @@ void __init initmem_init(void)
 
        memblock_dump_all();
 
+       /*
+        * Reduce the possible NUMA nodes to the online NUMA nodes,
+        * since we do not support node hotplug. This ensures that  we
+        * lower the maximum NUMA node ID to what is actually present.
+        */
+       nodes_and(node_possible_map, node_possible_map, node_online_map);
+
        for_each_online_node(nid) {
                unsigned long start_pfn, end_pfn;