We'll talk about page_alloc, page_free, alloc_kpages and free_kpages.

The first concern of OS161 virtual memory system is how to manage physical pages. Generally, we can pack a physical page's information into a structure (called struct coremap_entry) and use this struct to represent a physical page. We use an array of struct coremap_entry to keep all physical pages information. This array, aka, coremap, will be one of the most important data structure in this lab.