Devices have been doing that since forever, so Plan9 has an mmap equivalent with segment/segattach, where you use file I/O (only) to define a memory mapping and

Devices have been doing that since forever, so Plan9 has an mmap equivalent with segment/segattach, where you use file I/O (only) to define a memory mapping and attach it to the current process. Everything from that point on is regular memory I/O.