There is a quite nasty condition that can happen particularly if some program misbehaves and eats all the kva space, leaving affected system live-locked and admin with no breadcrumbs to follow. At least this one-liner would be a good start. In general, whichever pid is eating it out should be killed with extreme prejudice, since right now it's basically a system-wide deadlock implementation, as it blocks any fork+exec from completing. :(
As documented: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=272332