When a superpage promotion occurs, the page table page that the superpage
mapping replaces is added to an ordered collection of page table pages.
Rather than preserving the code that implements the splay tree of pages
in the pmap for just this one purpose, use the new MI radix tree. The
extra overhead of using a radix tree for this purpose is small enough,
about 4% added run-time to pmap_promote_pde(), that I don't see the point
of preserving the splay tree code.
Sponsored by: EMC / Isilon Storage Division
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