There is one extra copy that is not shown in this diagram (you don't write directly into my I/O buffer, you write into something that I copy into my system I/O buffer when you ask me to do a write()).

Not on win--I can't speak to *nix. There is no "I/O buffer" involved. And no "copying".

The point is: The file is already on disk! And it doesn't get read in to a physical pages of RAM until you attempt to access it. And it doesn't get written (anywhere) unless you write to it. All that has happened up to the point where you attempt to read or write to the mapped address space is that a few virtual-to-physical mapping tables have been set up.