diff --git a/hfo2/src/mpool.rs b/hfo2/src/mpool.rs
index 6339627b5..f174a2e31 100644
--- a/hfo2/src/mpool.rs
+++ b/hfo2/src/mpool.rs
@@ -179,6 +179,9 @@ impl Pool {
     }
 }
 
+/// Memory pool equipped with spinlock and fallback pool.
+/// TODO(HfO2): Make a trait, which generalizes Pool (linked list) and a pair of
+/// (pool, fallback) (#35.)
 #[repr(C)]
 pub struct MPool {
     pool: SpinLock<Pool>,