I think it's harder for you to write "correct code" here because the crate is hiding most of the actual detail from you. I put that in quotes because there's absolutely nothing incorrect about the code, it's really just suboptimal, and most probably because it can't even use the full syscall interface.

Seriously, I hate to be a curmudgeon, but that crate looks like a particularly bad and naive wrapper around mmap. It works very hard to provide you things you don't need when the basic interface is much more flexible. Aside from having to put `unsafe` around the call and re-import the kernel header constants, there's almost no reason to even have this in a crate.