diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 2c4743f2d50e..03d1a61b4729 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1373,8 +1373,10 @@ static void __ip6_rt_update_pmtu(struct dst_entry *dst, const struct sock *sk,
 {
 	struct rt6_info *rt6 = (struct rt6_info *)dst;
 
-	if (dst_metric_locked(dst, RTAX_MTU))
-		return;
+	/* Note: do *NOT* check dst_metric_locked(dst, RTAX_MTU)
+	 * IPv6 pmtu discovery isn't optional, so 'mtu lock' cannot disable it.
+	 * [see also comment in rt6_mtu_change_route()]
+	 */
 
 	dst_confirm(dst);
 	mtu = max_t(u32, mtu, IPV6_MIN_MTU);