diff --git a/mm/thrash.c b/mm/thrash.c
index 42ffb01..e53f7d0 100644
--- a/mm/thrash.c
+++ b/mm/thrash.c
@@ -67,6 +67,17 @@
 	if (!swap_token_mm)
 		goto replace_token;
 
+	/*
+	 * Usually, we don't need priority aging because long interval faults
+	 * makes priority decrease quickly. But there is one exception. If the
+	 * token owner task is sleeping, it never make long interval faults.
+	 * Thus, we need a priority aging mechanism instead. The requirements
+	 * of priority aging are
+	 *  1) An aging interval is reasonable enough long. Too short aging
+	 *     interval makes quick swap token lost and decrease performance.
+	 *  2) The swap token owner task have to get priority aging even if
+	 *     it's under sleep.
+	 */
 	if ((global_faults - last_aging) > TOKEN_AGING_INTERVAL) {
 		swap_token_mm->token_priority /= 2;
 		last_aging = global_faults;