netdev - Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu discovery on encap sockets

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Date:   Fri, 17 Jul 2020 20:43:05 +0200
From:   Florian Westphal <fw@...len.de>
To:     David Ahern <dsahern@...il.com>
Cc:     Stefano Brivio <sbrivio@...hat.com>,
        Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
        aconole@...hat.com
Subject: Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu
 discovery on encap sockets

David Ahern <dsahern@...il.com> wrote:
> On 7/17/20 6:27 AM, Stefano Brivio wrote:
> >> Every type of bridge port that needs to add additional header on egress
> >> has this problem in the bridge scenario once the peer of the IP tunnel
> >> signals a PMTU event.
> > 
> > Yes :(
> > 
> 
> The vxlan/tunnel device knows it is a bridge port, and it knows it is
> going to push a udp and ip{v6} header. So why not use that information
> in setting / updating the MTU? That's what I was getting at on Monday
> with my comment about lwtunnel_headroom equivalent.

What action should be taken in the vxlan driver?  Say, here:

static inline void skb_dst_update_pmtu_no_confirm(struct sk_buff *skb,
	u32 mtu)
{
 struct dst_entry *dst = skb_dst(skb);

 if (dst && dst->ops->update_pmtu)
    dst->ops->update_pmtu(dst, NULL, skb, mtu, false);
 else
    /* ??? HERE */
 }

We hit the (non-existent) else branch as skb has no dst entry.