This patch is somewhat of a companion to the "no buddy bitmap" patches.
It messes with core allocator functions, so it probably deserves a long
turn in -mm.

Appended is a patch which stops using the zone->zone_mem_map to
calculate the buddy and combined page pointers.  It uses the fact
that the mem_map array is guaranteed to be contigious for the
surrounding (1 << MAX_ORDER) pages.  The relative positions of
the pages in the physical address space to provide the alignment;
which conicidentally fixes the issue where zones are not aligned
at MAX_ORDER.  There is a very comprehensive comment in the new
code explaining the mathematical relationship between a page and
its buddy so I won't reproduce it here.

This kind of approach is required for CONFIG_NONLINEAR systems
where the mem_map is not contiguous within a zone, and the
zone->zone_mem_map is not used at all.

This patch has been boot-tested on a large variety of systems and
architectures: my P4 laptop, 16-way NUMAQ, 16-way Summit, 4-way
x86 SMP, ppc64 LPAR, x86_64, and several ia64 configurations.

It has been performance-tested on a 16-way NUMAQ. SDET shows a
very slight (within margin of error) performance gain.  Kernbench
shows an approximately ~1% decrease in system time with this
patch applied.  So, it has a likely small positive performance impact.

However, the patch has the potential to have a negative performance
impact on systems with an expensive page_to_pfn() implementation.
But, I think the NUMAQ has one of the more expensive ones around,
and it doesn't seem mind too much.

Applies against 2.6.11-rc3.  

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>

-- Dave