NUMA topology question wrt. d4edc5b6

[David Rientjes]

On Fri, 23 May 2014, Srivatsa S. Bhat wrote:

> diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
> index c920215..58e6469 100644
> --- a/arch/powerpc/include/asm/topology.h
> +++ b/arch/powerpc/include/asm/topology.h
> @@ -18,6 +18,7 @@ struct device_node;
>   */
>  #define RECLAIM_DISTANCE 10
>  
> +#include <linux/nodemask.h>
>  #include <asm/mmzone.h>
>  
>  static inline int cpu_to_node(int cpu)
> @@ -30,7 +31,7 @@ static inline int cpu_to_node(int cpu)
>  	 * During early boot, the numa-cpu lookup table might not have been
>  	 * setup for all CPUs yet. In such cases, default to node 0.
>  	 */
> -	return (nid < 0) ? 0 : nid;
> +	return (nid < 0) ? first_online_node : nid;
>  }
>  
>  #define parent_node(node)	(node)

I wonder what would happen on ppc if we just returned NUMA_NO_NODE here 
for cpus that have not been mapped (they shouldn't even be possible).  
This would at least allow callers that do
kmalloc_node(..., cpu_to_node(cpu)) to be allocated on the local cpu 
rather than on a perhaps offline or remote node 0.

It would seem better to catch callers that do 
cpu_to_node(<not-possible-cpu>) rather than blindly return an online node.