Why *not* rmap, anyway?

[Joseph A Knapka <jknapka@earthlink.net>]

Hi folks,

I was just reading Bill's reply regaring rmap, and it
seems to me that rmap is the most obvious and clean
way to handle unmapping pages. So now I wonder why
it wasn't done that way from the beginning?

It took me a while to figure out all the complicated
interactions between virtual and physical scanning
in the Linux mm system. If I were writing a VM system
and I got to the point where I wanted to be able to
unmap a possibly-shared page, I would say to myself,
"Hmm, this will require a map of physical pages
to all their virtual addresses. Ick. But on the
other hand, the alternatives are probably a lot more
complicated," and I would just go ahead and implement
physical-to-virtual mappings. So why did Linus and/or
the MM hackers of ages past implement the parallel
virtual-and-physical-scanning thing? What are the
advantages, besides less data overhead? It seems
to me that the old method really complicates the
code a lot, and gives the CPU more work to do to
boot.

Thanks,

-- Joe
  Using open-source software: free.
  Pissing Bill Gates off: priceless.
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