I don't completely understand this.  In the Sun 3 pmap, how does
pmap_remove_all() know which pmaps have a page mapped so that it can
be removed?

When you say the `oldest' mapping is thrown away, do you mean `oldest'
as in `first to be created' or in the sense of a LRU list?  If the
former, you almost certainly don't want the VM system to deactive the
pages prematurely, because they're probably for something like
libc.so, which you don't really want to page out in most cases.