The current implementation of the Mmap wrappers for the Unix-derived OSes does not dispatch directly to the relevant syscall but relies on the mmapper layer. The mmapper adds address range validation and returns EINVAL for the ranges not matching the content of the mapping registry. While it might be arguably correct for most of the use cases, it also prevents partial Munmaps and similar valid parts of the pretty rich mmap semantics. I suggest that the existing implementation should be left as-is, but undisturbed access to the underlying OS primitives should also be provided. The scope of the extension would be minimal, as the low-level part already is there -- the package should just export more entry points. The naming details are irrelevant to me, so I leave them unspecified: I will be equally happy with MmapUnsafe as with MmapRaw.

The only suggestion I'd make is the return type: the functions should operate with uintptr, as their WinAPI VirtualAlloc() counterparts do. Wrapping the uintptr in a slice just to unwrap it in the follow-up step to recover the uintptr adds no value. I'll make the slice when I'm done.