windows - 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 address space, but not loaded in physical memory. (emphasis mine)

Okay, that makes sense.

But if that's the case, why is it that whenever the process information in Process Hacker is refreshed, I see about 15 page faults?

Or in other words, why is any memory getting paged out? (I have no idea if it's user or kernel memory.) I have no page file, and the RAM usage is about 1.2 GB out of 4 GB, which is after a clean reboot. There's no shortage of any resource; why would anything get paged out?

Answer 1

(I'm the author of Process Hacker.)

Firstly:

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 address space, but not loaded in physical memory.

That's not entirely correct, as explained later in the same article (Minor page fault). There are soft page faults, where all the kernel needs to do is add a page to the working set of the process. Here's a table from the Windows Internals book (I've excluded the ones that result in an access violation):

Reason for Fault	Result
Accessing a page that isn’t resident in memory but is on disk in a page file or a mapped file	Allocate a physical page, and read the desired page from disk and into the relevant working set
Accessing a page that is on the standby or modified list	Transition the page to the relevant process, session, or system working set
Accessing a demand-zero page	Add a zero-filled page to the relevant working set
Writing to a copy-on-write page	Make process-private (or session-private) copy of page, and replace original in process or system working set