-- 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. */