> every access in the mapped region would be really slow.
Would certainly be slower. The compiler would need to be aware we want this behaviour and split the access in two parts, one to trigger the page read and yield to the app’s async loop, and another to resolve the read when the page has loaded. This would only need to happen for explicitly marked asynchronous memory reads (doing that without hardware support for all memory reads would be painful).
Normal syscalls also "leverage hardware support for trapping into the kernel". Mmap is usually used because it is a simple way to do 0-copy disk I/O.
i think adding software indirection to every access in the mapped region would be really slow.
i think a better answer would be to impose more structure on the planned memory access, then maybe given some constraints (like say, "this loop is embarrassingly parallel") the system could be smarter about working on the stuff in ram first while the rest is loaded in.