my solution to lab5

lab/kern/pmap.c

@@ -84,6 +84,8 @@ static void check_page_installed_pgdir(void);
 // If we're out of memory, boot_alloc should panic.
 // This function may ONLY be used during initialization,
 // before the page_free_list list has been set up.
+// Note that when this function is called, we are still using entry_pgdir,
+// which only maps the first 4MB of physical memory.
 static void *
 boot_alloc(uint32_t n)
 {