First of all we need to deal with fallout from converting the
function's previously dead checks to ASSERT(). An approach with a
weakness is proposed on patch 1; see the RFC remark there. Plus
phys_to_nid() has been somewhat inefficient with respect to all of
its present callers.

1: x86/mm: avoid phys_to_nid() calls for invalid addresses
2: NUMA: replace phys_to_nid()

Jan