What causes page faults?

Question

According to Wikipedia:

A page fault is a trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual add

Answer 1

A slow but steady source of page faults is the OS probing for infrequently accessed pages. In this case, the operating system marks some pages not present, but leaves them in memory as-is. If an application accesses the page, then the #PF trap occurs and the OS simply marks the page present again without further ado. If a "long time" passes and a page never trips a fault, then the OS knows the page is a good candidate for swapping should the need arise. This mechanism can run proactively even in times of no resource pressure.