Notes:
A new address space (and a new mm_struct) could be created by opening /proc/mm. A copy of the current address space could be created by opening either /proc/self/mm/clone or /proc/self/clone_mm, depending on whether you like treating /proc/self/mm both as a file and a directory.
The newly created mm_struct would have a reference count of 1. Opening /proc/self/mm would increment its reference count.