Here's my initial coding of shared page tables.  It sets the pmd read-only,
so it forks really fast, then unshares it as necessary.  I've tried to keep
the sharing semantics clean so if/when we add pte sharing for shared files
the existing code should handle it just fine.

The few feeble attempts I've made at putting in locks are clearly wrong, so
it only works on UP.

I don't see any reason why swap won't work, but I haven't tested it.

This is also against 2.5.29.  I'm gonna work to merge it forward, but there
are significant changes since then so I figured I'd toss this out for
people to get an early look at it.

Dave McCracken

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Dave McCracken          IBM Linux Base Kernel Team      1-512-838-3059
dmccr@us.ibm.com                                        T/L   678-3059