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Used in [[glibc]] for implementing [[glibc/mmap]], brk and sbrk, and *attempt
to map page zero redzoned* (`[glibc]/hurd/hurdstartup.c:_hurd_startup`).

[[!tag open_issue_glibc open_issue_gnumach]]It has an awkward mention in
`[glibc]/sysdeps/mach/hurd/dl-sysdep.c:fmh`: *XXX loser kludge for vm\_map
kernel bug*.