Implement add_free_region

src/allocator/linked_list.rs

@@ -1,3 +1,6 @@
+use super::align_up;
+use core::mem;
+
 struct ListNode {
     size: usize,
     next: Option<&'static mut ListNode>,
@@ -43,6 +46,15 @@ impl LinkedListAllocator {
 
     /// Adds the given memory region to the front of the list.
     unsafe fn add_free_region(&mut self, addr: usize, size: usize) {
-        todo!();
+        // ensure that the freed region is capable of holding ListNode
+        assert!(align_up(addr, mem::align_of::<ListNode>()) == addr);
+        assert!(size >= mem::size_of::<ListNode>());
+
+        // create a new list node and append it at the start of the list
+        let mut node = ListNode::new(size);
+        node.next = self.head.next.take();
+        let node_ptr = addr as *mut ListNode;
+        node_ptr.write(node);
+        self.head.next = Some(&mut *node_ptr)
     }
 }