Used in glibc for implementing mmap, brk and sbrk, and attempt
to map page zero redzoned ([glibc]/hurd/hurdstartup.c:_hurd_startup).
It has an awkward mention in
[glibc]/sysdeps/mach/hurd/dl-sysdep.c:fmh: XXX loser kludge for vm_map
kernel bug.
GNU Hurd/
microkernel/
mach/
interface/
vm map