netdev - Re: [PATCH net] ip_tunnel: fix over-mtu packet send fail without TUNNEL_DONT

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Message-ID: <20201023141254.7102795d@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net>
Date:   Fri, 23 Oct 2020 14:12:54 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     wenxu@...oud.cn
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net] ip_tunnel: fix over-mtu packet send fail without
 TUNNEL_DONT_FRAGMENT flags

On Wed, 21 Oct 2020 17:21:55 +0800 wenxu@...oud.cn wrote:
> From: wenxu <wenxu@...oud.cn>
> 
> The TUNNEL_DONT_FRAGMENT flags specific the tunnel outer ip can do
> fragment or not in the md mode. Without the TUNNEL_DONT_FRAGMENT
> should always do fragment. So it should not care the frag_off in
> inner ip.

Can you describe the use case better? My understanding is that we
should propagate DF in normally functioning networks, and let PMTU 
do its job.

> Fixes: cfc7381b3002 ("ip_tunnel: add collect_md mode to IPIP tunnel")
> Signed-off-by: wenxu <wenxu@...oud.cn>
> ---
>  net/ipv4/ip_tunnel.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> index 8b04d1d..ee65c92 100644
> --- a/net/ipv4/ip_tunnel.c
> +++ b/net/ipv4/ip_tunnel.c
> @@ -608,9 +608,6 @@ void ip_md_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
>  			ttl = ip4_dst_hoplimit(&rt->dst);
>  	}
>  
> -	if (!df && skb->protocol == htons(ETH_P_IP))
> -		df = inner_iph->frag_off & htons(IP_DF);
> -
>  	headroom += LL_RESERVED_SPACE(rt->dst.dev) + rt->dst.header_len;
>  	if (headroom > dev->needed_headroom)
>  		dev->needed_headroom = headroom;