Welcome to... ... the GNU Hurd!
Used in glibc for implementing mmap, brk and sbrk, and attempt to map page zero redzoned ([glibc]/hurd/hurdstartup.c:_hurd_startup).
[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.
[glibc]/sysdeps/mach/hurd/dl-sysdep.c:fmh