So, this patch started out with me trying to keep from passing contiguous,
node-specific mem_map into free_area_init_node() and cousins.  Instead, I
relied on some calls to pfn_to_page().

This works fine and dandy when all you need is the pgdat->node_mem_map to
do pfn_to_page().  However, the non-NUMA/DISCONTIG architectures use the
real, global mem_map[] instead of a node_mem_map in the pfn_to_page()
calculation.  So, I ended up effectively trying to initialize mem_map from
itself, when it was NULL.  That was bad, and caused some very pretty colors
on someone's screen when he tested it.

So, I had to make sure to initialize the global mem_map[] before calling
into free_area_init_node().  Then, I realized how many architectures do
this on their own, and have comments like this:

        /* XXX: MRB-remove - this doesn't seem sane, should this be done somewhere else ?*/
        mem_map = NODE_DATA(0)->node_mem_map;

The following patch does what my first one did (don't pass mem_map into the
init functions), incorporates Jesse Barnes' ia64 fixes on top of that, and
gets rid of all but one of the global mem_map initializations (parisc is
weird).  It also magically removes more code than it adds.  It could be
smaller, but I shamelessly added some comments. 

Boot-tested on ppc64, i386 (NUMAQ, plain SMP, laptop), UML (i386).
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>