There are three places we define pfn_to_nid().  Two in linux/mmzone.h and one
in asm/mmzone.h.  These in essence represent the three memory models.  The
definition in linux/mmzone.h under !NEED_MULTIPLE_NODES is both the FLATMEM
definition and the optimisation for single NUMA nodes; the one under SPARSEMEM
is the NUMA sparsemem one; the one in asm/mmzone.h under DISCONTIGMEM is the
discontigmem one.  This is not in the least bit obvious, particularly the
connection between the non-NUMA optimisations and the memory models.

Two patches:

flatmem-split-out-memory-model: simplifies the selection of pfn_to_nid()
implementations.  The selection is based primarily off the memory model
selected.  Optimisations for non-NUMA are applied where needed.

sparse-provide-pfn_to_nid: implement pfn_to_nid() for SPARSEMEM

This patch:

pfn_to_nid is memory model specific

The pfn_to_nid() call is memory model specific.  It represents the locality
identifier for the memory passed.  Classically this would be a NUMA node,
but not a chunk of memory under DISCONTIGMEM.

The SPARSEMEM and FLATMEM memory model non-NUMA versions of pfn_to_nid()
are folded together under NEED_MULTIPLE_NODES, while DISCONTIGMEM has its
own optimisation.  This is all very confusing.

This patch splits out each implementation of pfn_to_nid() so that we can
see them and the optimisations to each.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>