The Darwin man page for munmap has more detail: https://developer.apple.com/library/mac/documentation/Darwin... See DIRTY PAGE HANDLING.

  If the mapping maps data from a file (MAP_SHARED), then the memory will eventually be written back to disk if it's dirty.  This will happen automatically at some point in the future (implementation dependent).

The XNU source may be useful: http://fxr.watson.org/fxr/ident?v=xnu-2050.18.24&im=excerpts...