swapouts and pageouts aren't identical in Linux, and are instrumented separately (pswpout and pgpgout, respectively; see /proc/vmstat). mmap() and other page-ins won't be counted under the swap statistics.

A pageout might suggest memory pressure, but not nearly as much as a swapout does. (pgmajfault is a better indicator.) Writing dirty pages is just something the kernel does even when there's no memory pressure at all. Also, unfortunately you can't use pgpgout for anything useful as ordinary file writes are counted there.