netdev - Re: [PATCH] net: ip_finish_output_gso: Allow fragmenting segments of tunneled skbs if their DF is unset

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Message-Id: <1471859303.3970877.702207417.066809DC@webmail.messagingengine.com>
Date:   Mon, 22 Aug 2016 11:48:23 +0200
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Shmulik Ladkani <shmulik.ladkani@...il.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     pravin shelar <pshelar@....org>, wenxu <wenxu@...oud.cn>
Subject: Re: [PATCH] net: ip_finish_output_gso: Allow fragmenting segments of
 tunneled skbs if their DF is unset

Hi,

On Sun, Aug 21, 2016, at 10:22, Shmulik Ladkani wrote:
> In b8247f095e,
> 
>    "net: ip_finish_output_gso: If skb_gso_network_seglen exceeds MTU,
>    allow segmentation for local udp tunneled skbs"
> 
> gso skbs arriving from an ingress interface that go through UDP
> tunneling, are allowed to be fragmented if the resulting encapulated
> segments exceed the dst mtu of the egress interface.
> 
> This aligned the behavior of gso skbs to non-gso skbs going through udp
> encapsulation path.
> 
> However the non-gso vs gso anomaly is present also in the following
> cases of a GRE tunnel:
>  - ip_gre in collect_md mode, where TUNNEL_DONT_FRAGMENT is not set
>    (e.g. OvS vport-gre with df_default=false)
>  - ip_gre in nopmtudisc mode, where IFLA_GRE_IGNORE_DF is set
> 
> In both of the above cases, the non-gso skbs get fragmented, whereas the
> gso skbs (having skb_gso_network_seglen that exceeds dst mtu) get
> dropped,
> as they don't go through the segment+fragment code path.
> 
> Fix: Setting IPSKB_FRAG_SEGS if the tunnel specified IP_DF bit is NOT
> set.
> 
> Tunnels that do set IP_DF, will not go to fragmentation of segments.
> This preserves behavior of ip_gre in (the default) pmtudisc mode.
> 
> Fixes: b8247f095e ("net: ip_finish_output_gso: If skb_gso_network_seglen
> exceeds MTU, allow segmentation for local udp tunneled skbs")
> Reported-by: wenxu <wenxu@...oud.cn>
> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Signed-off-by: Shmulik Ladkani <shmulik.ladkani@...il.com>

Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

Your dissecting of the current state of fragmentation handling also
looked fine. I wonder if it would now make sense to add a sysctl to add
back the dropping of packets in case they can't be fragmented by a
bridge, as we made sure that the defaults don't break for anyone.

Thanks,
Hannes