Also consider that file I/O also goes through the virtual memory system in normal circumstances. Memory serves as a disk cache - and will be released first if the system needs memory. (Data written typically gets flushed to disk but is kept in this in-memory cache).
This is why simply using “free memory” numbers is misleading. If free memory is low it does not mean that your system is out of memory.
As is stated, the degree of paging activity is a better indication of whether you have over subscribed memory.