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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Xin Long <lucien.xin@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.14 038/110] ip6_tunnel: get the min mtu properly
 in ip6_tnl_xmit
Thread-Topic: [PATCH AUTOSEL for 4.14 038/110] ip6_tunnel: get the min mtu
 properly in ip6_tnl_xmit
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Date:   Sat, 3 Feb 2018 18:00:47 +0000
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From: Xin Long <lucien.xin@gmail.com>

[ Upstream commit c9fefa08190fc879fb2e681035d7774e0a8c5170 ]

Now it's using IPV6_MIN_MTU as the min mtu in ip6_tnl_xmit, but
IPV6_MIN_MTU actually only works when the inner packet is ipv6.

With IPV6_MIN_MTU for ipv4 packets, the new pmtu for inner dst
couldn't be set less than 1280. It would cause tx_err and the
packet to be dropped when the outer dst pmtu is close to 1280.

Jianlin found it by running ipv4 traffic with the topo:

  (client) gre6 <---> eth1 (route) eth2 <---> gre6 (server)

After changing eth2 mtu to 1300, the performance became very
low, or the connection was even broken. The issue also affects
ip4ip6 and ip6ip6 tunnels.

So if the inner packet is ipv4, 576 should be considered as the
min mtu.

Note that for ip4ip6 and ip6ip6 tunnels, the inner packet can
only be ipv4 or ipv6, but for gre6 tunnel, it may also be ARP.
This patch using 576 as the min mtu for non-ipv6 packet works
for all those cases.

Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 net/ipv6/ip6_tunnel.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 3f46121ad139..c3e963a2a017 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1131,8 +1131,13 @@ route_lookup:
 		max_headroom +=3D 8;
 		mtu -=3D 8;
 	}
-	if (mtu < IPV6_MIN_MTU)
-		mtu =3D IPV6_MIN_MTU;
+	if (skb->protocol =3D=3D htons(ETH_P_IPV6)) {
+		if (mtu < IPV6_MIN_MTU)
+			mtu =3D IPV6_MIN_MTU;
+	} else if (mtu < 576) {
+		mtu =3D 576;
+	}
+
 	if (skb_dst(skb) && !t->parms.collect_md)
 		skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL, skb, mtu);
 	if (skb->len - t->tun_hlen - eth_hlen > mtu && !skb_is_gso(skb)) {
--=20
2.11.0