mmap() tricks you into thinking you can directly access the disk. What really happens when you access the mapped data is that the CPU generates a page fault, the OS takes control, copies the data from disk to physical memory and maps the page in your address space. So essentially it is a convient way to do read().

The mmap() model is good. The current page fault mechanism, not so much.

The difference is that you only take the page fault once per page (or less than once if you do some prefetching in the kernel), while you have to do a system call for every read().

It costs a fortune to map a file on every OS I've tried, it makes little sense unless you're consuming a lot of data from few files rather than little data from many files