On x86, prior to ("mm: handle uninitialized numa nodes gracecully"), NUMA
nodes could be allocated at three different places.

 - numa_register_memblks
 - init_cpu_to_node
 - init_gi_nodes

All these calls happen at setup_arch, and have the following order:

setup_arch
  ...
  x86_numa_init
   numa_init
    numa_register_memblks
  ...
  init_cpu_to_node
   init_memory_less_node
    alloc_node_data
    free_area_init_memoryless_node
  init_gi_nodes
   init_memory_less_node
    alloc_node_data
    free_area_init_memoryless_node

numa_register_memblks() is only interested in those nodes which have
memory, so it skips over any memoryless node it founds.  Later on, when
we have read ACPI's SRAT table, we call init_cpu_to_node() and
init_gi_nodes(), which initialize any memoryless node we might have that
have either CPU or Initiator affinity, meaning we allocate pg_data_t
struct for them and we mark them as ONLINE.

So far so good, but the thing is that after ("mm...