I don't disagree. Are you interested in trying to write it up? It sounds
like a potentially good idea, with few downsides (but I can imagine some:
it does bad things to threads that just happen to share the same 4M area
for other stuff, and that start getting spurious page faults on another
CPU because _their_ area temporarily went away from under them).
I also suspect that it might simplify the TLB shootdown enough that we
wouldn't _have_ to split out the exit case and could use the shared
zapping. But I'm kind of worried about the potential threading issues.
(Rule of thumb: it's always a bad idea to cut down on parallelism, and
we'll _really_ be up shit creek if some threaded app comes along later
where munmap() ends up serializing threads too much).
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