Here's a new version of my shared page tables patch.

The primary purpose of sharing page tables is improved performance for
large applications that share big memory areas between multiple processes.
It eliminates the redundant page tables and significantly reduces the
number of minor page faults.  Tests show significant performance
improvement for large database applications, including those using large
pages.  There is no measurable performance degradation for small processes.

This version of the patch uses Hugh's new locking mechanism, extending it
up the page table tree as far as necessary for proper concurrency control.

The patch also includes the proper locking for following the vma chains.

Hugh, I believe I have all the lock points nailed down.  I'd appreciate
your input on any I might have missed.

The architectures supported are i386 and x86_64.  I'm working on 64 bit
ppc, but there are still some issues around proper segment handling that
need more testing.  This will be available in a separate patch once it's
solid.

Dave McCracken