Turn page of old P4 into guard page

src/memory/paging/mod.rs

@@ -196,4 +196,11 @@ pub fn remap_the_kernel<A>(allocator: &mut A, boot_info: &BootInformation)
 
     let old_table = active_table.switch(new_table);
     println!("NEW TABLE!!!");
+
+    // turn the old p4 page into a guard page
+    let old_p4_page = Page::containing_address(
+      old_table.p4_frame.start_address()
+    );
+    active_table.unmap(old_p4_page, allocator);
+    println!("guard page at {:#x}", old_p4_page.start_address());
 }