netdev - Re: [PATCH net v3] ipv4: allow local fragmentation in ip_finish_output

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Date:   Fri, 4 Nov 2016 11:24:09 +0200
From:   Shmulik Ladkani <shmulik.ladkani@...il.com>
To:     Lance Richardson <lrichard@...hat.com>
Cc:     netdev@...r.kernel.org, fw@...len.de, jtluka@...hat.com,
        hannes@...essinduktion.org
Subject: Re: [PATCH net v3] ipv4: allow local fragmentation in
 ip_finish_output_gso()

Hi,

On Thu, 3 Nov 2016 09:06:27 -0400 (EDT) Lance Richardson <lrichard@...hat.com> wrote:
> I'm not sure what could be added that would help, was there something
> specific you had in mind?

How about something like this (preliminary, feel free to massage):

@@ -248,10 +248,16 @@ static int ip_finish_output_gso(struct net *net, struct sock *sk,
 
 	/* Slowpath -  GSO segment length is exceeding the dst MTU.
 	 *
-	 * This can happen in two cases:
-	 * 1) TCP GRO packet, DF bit not set
-	 * 2) skb arrived via virtio-net, we thus get TSO/GSO skbs directly
-	 * from host network stack.
+	 * This can happen in several cases:
+	 *  - Forwarding of TCP GRO packet, DF bit not set
+	 *  - Forwarding of skb arrived in a virtualization environment (from
+	 *    virtio-net/vhost/tap) with TSO/GSO size set by other's network
+	 *    stack
+	 *  - Local GSO skb xmitted on an NETIF_F_TSO tunnel stacked over an
+	 *    interface with a smaller mtu
+	 *  - Arriving GRO skb (or GSO skb in a virtualized env) that gets L2
+	 *    bridged to a NETIF_F_TSO tunnel stacked over an interface with an
+	 *    insufficent mtu
 	 */
 	features = netif_skb_features(skb);
 	BUILD_BUG_ON(sizeof(*IPCB(skb)) > SKB_SGO_CB_OFFSET);