You can tell that you understand how modern OS memory management works when you realize that the OS "automatically read[ing] the pages...from disk" and "flush[ing them] to disk" on memory pressure is paging whether those pages are anonymous pages or mmaped file pages. :-)

[Edit: flushing dirty file-paged pages is analogous to swapping anonymous memory to the swapfile. Discarding clean file-backed pages is a bit like discarding anonymous pages that have been made unused through munmap, process death, etc.]

But to the GP's point: you don't need (except to conserve address space) to limit file mapping size. I think he really wants something like MADV_FREE. But it's complicated.