- virtual address KPAGE to the page table. Fails if UPAGE is
- already mapped or if memory allocation fails. */
+ virtual address KPAGE to the page table.
+ UPAGE must not already be mapped.
+ KPAGE should probably be a page obtained from the user pool
+ with palloc_get_page().
+ Returns true on success, false if UPAGE is already mapped or
+ if memory allocation fails. */