This patch implements page table sharing for all shared memory regions that
span an entire page table page.  It supports sharing at multiple page
levels, depending on the architecture.

Performance testing has shown no degradation with this patch for tests with
small processes.  Preliminary tests with large benchmarks have shown as
much as 3% improvement in overall results.

For those familiar with the shared page table patch I did a couple of years
ago, this patch does not implement copy-on-write page tables for private
mappings.  Analysis showed the cost and complexity far outweighed any
potential benefit.

This version of the patch supports i386 and x86_64.  I have additional
patches to support ppc64, but they are not quite ready for public
consumption.

The patch is against 2.6.13.

Dave McCracken