Did you see a compiler warning? Indeed, this function (mmap.move) decodes its arguments with:
   PyArg_ParseTuple(args, "kkk:move", &dest, &src, &cnt)
This looks wrong to me: these three numbers should be Py_ssize_t, and decoded with "nnn".

An example of wrong behavior (on win32):
>>> m = mmap.mmap(-1, 500)
>>> m.move(2**32, 10, 4)   # Should throw a ValueError