Theoretical exercises for you: :-)

- When and why would read() necessarily be any different from
  mmap()-and-access?  

- When and why would write() necessarily be any different from
  mmap()-and-access-and-munmap()/exit/otherwise lose reference
  to page(s)?

and then:

- Is it possible, feasible, and/or desirable to have the kernel
  implement read()s on page-and-block boundaries in a manner that
  makes them largely indistinguishable from mmap()-and-access?

(The 4.4BSD stdio already works fairly hard to cause reads to happen
on block boundaries contained exactly within whole pages.)

Chris