FAQ

Do we need a working Project 2 to implement Project 3?

Yes.

How do we resume a process after we have handled a page fault?

Returning from page_fault() resumes the current user process. It will then retry the instruction to which the instruction pointer points. Why do user processes sometimes fault above the stack pointer? You might notice that, in the stack growth tests, the user program faults on an address that is above the user program’s current stack pointer.

Does the virtual memory system need to support data segment growth?

No. The size of the data segment is determined by the linker. We still have no dynamic allocation in Pintos (although it is possible to “fake” it at the user level by using memory-mapped files). Supporting data segment growth should add little additional complexity to a well-designed system.

Why should I use PAL_USER for allocating page frames?

Passing PAL_USER to palloc_get_page() causes it to allocate memory from the user pool, instead of the main kernel pool. Running out of pages in the user pool just causes user programs to page, but running out of pages in the kernel pool will cause many failures because so many kernel functions need to obtain memory. You can layer some other allocator on top of palloc_get_page() if you like, but it should be the underlying mechanism. Also, you can use the -ul kernel command-line option to limit the size of the user pool, which makes it easy to test your VM implementation with various user memory sizes.