How can I force a page to fault, even if it is already in the tlb?

I'm trying to write a toy working开发者_开发问答 set estimator, by keeping track of page faults over a period of time. Whenever a page is faulted in, I want to record that it was touched. The scheme breaks down when I try to keep track of accesses to already-present pages. If a page is read from or written to without triggering a fault, I have no way of tracking the access.

So then, I want to be able to cause a "lightweight" fault to occur on a page access. I've heard of some method at some point, but I didn't understand why it worked so it didn't stick in my mind. Dirty bit maybe?

You can use mprotect with PROT_NONE ("Page cannot be accessed"). Then any access to the given page will cause a fault.

The usual way to do this is to simply clear the "present" bit for the page, while leaving the page in memory and the necessary kernel data structures in place so that the kernel knows this.

However, depending on the architecture in question you may have better options - for example, on x86 there is an "Accessed" flag (bit 5 in the PTE) that is set whenever the PTE is used in a linear address translation. You can simply clear this bit whenever you like, and the hardware will set it to record that the page was touched.

Using either of these methods you will need to clear the cached translation for that page out of the TLB - on x86 you can use the INVLPG instruction.