From: Xin Long <lucien....@gmail.com>
Date: Wed, 20 Dec 2017 01:05:32 +0800
> On Wed, Dec 20, 2017 at 12:12 AM, David Miller <da...@davemloft.net> wrote:
>> You're going to have to find a way to fix this without
>> invoking ->update_pmtu() on every single transmit. That's
>> really excessive, especially for an operation which is
>> going to be a NOP %99.9999 of the time.
> understand, I couldn't find a better way, and all iptunnels are
> doing it in this way.
>
> Or is it possible to go with an unlikely here ?
>
> if (unlikely(skb_dst(skb) && mtu < dst_mtu(skb_dst(skb))))
> skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL,
> skb, mtu);
>
>
...
> how about doing it in vxlan_get_route():
> @@ -1896,6 +1896,13 @@ static struct rtable *vxlan_get_route(struct
> vxlan_dev *vxlan, struct net_device
> *saddr = fl4.saddr;
> if (use_cache)
> dst_cache_set_ip4(dst_cache, &rt->dst, fl4.saddr);
> +
> + if (skb_dst(skb)) {
> + int mtu = dst_mtu(ndst) - VXLAN_HEADROOM;
> +
> + skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL,
> + skb, mtu);
> + }
>
>
> This would do it only when no dst_cache and it has to do real route lookup.
>
> Note that even when update_pmtu is hit, mostly it will do nothing and
> just return
> as usually new mtu >= skb_dst(skb)'s pmtu.
Ok, yeah, this is really difficult.
I'll apply your patch for now, but generally speaking we have to handle this
issue better.
