Re: Node 0 not necessary for powerpc?

On Wed, 21 May 2014, Nishanth Aravamudan wrote:

For context: I was looking at why N_ONLINE was statically setting Node 0
to be online, whether or not the topology is that way -- I've been
getting several bugs lately where Node 0 is online, but has no CPUs and
no memory on it, on powerpc. 

On powerpc, setup_per_cpu_areas calls into ___alloc_bootmem_node using
NODE_DATA(cpu_to_node(cpu)).

Currently, cpu_to_node() in arch/powerpc/include/asm/topology.h does:

        /*
         * 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;

And so early at boot, if node 0 is not present, we end up accessing an
unitialized NODE_DATA(). So this seems buggy (I'll contact the powerpc
deveopers separately on that).

I think what this really wants to do is NODE_DATA(cpu_to_mem(cpu)) and I 
thought ppc had the cpu-to-local-memory-node mappings correct?