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

[David Rientjes]

On Thu, 5 Mar 2015, Nishanth Aravamudan wrote:

> 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);

If you don't support node hotplug, then a node should always be possible 
if it's online unless there are other tricks powerpc plays with 
node_possible_map.  Shouldn't this just be 
node_possible_map = node_online_map?

> +
>         for_each_online_node(nid) {
>                 unsigned long start_pfn, end_pfn;
>  
> 
>