netdev - Re: [PATCH net-next RFC 7/8] udp: gro behind static key

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Date:   Mon, 17 Sep 2018 10:10:41 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     steffen.klassert@...unet.com
Cc:     Network Development <netdev@...r.kernel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next RFC 7/8] udp: gro behind static key

On Mon, Sep 17, 2018 at 5:03 AM Steffen Klassert
<steffen.klassert@...unet.com> wrote:
>
> On Fri, Sep 14, 2018 at 01:59:40PM -0400, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > Avoid the socket lookup cost in udp_gro_receive if no socket has a
> > gro callback configured.
> >
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>
> ...
>
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index 4f6aa95a9b12..f44fe328aa0f 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -405,7 +405,7 @@ static struct sk_buff *udp4_gro_receive(struct list_head *head,
> >  {
> >       struct udphdr *uh = udp_gro_udphdr(skb);
> >
> > -     if (unlikely(!uh))
> > +     if (unlikely(!uh) || !static_branch_unlikely(&udp_encap_needed_key))
> >               goto flush;
>
> If you use udp_encap_needed_key to enalbe UDP GRO, then a UDP
> encapsulation socket will enable it too. Not sure if this is
> intentional.

Yes. That is already the case to a certain point. The function was
introduced with tunnels and is enabled by tunnels, but so far only
compiles out the encap_rcv() branch in udp_qeueue_rcv_skb.

With patch 7/8 it also toggles the GRO path. Critically, both are
enabled as soon as a tunnel is registered.

>
> That said, enabling UDP GRO on a UDP encapsulation socket
> (ESP in UPD etc.) will fail badly as then encrypted ESP
> packets might be merged together. So we somehow should
> make sure that this does not happen.

Absolutely. This initial implementation probably breaks UDP tunnels
badly. That needs to be addressed.

>
> Anyway, this reminds me that we can support GRO for
> UDP encapsulation. It just requires separate GRO
> callbacks for the different encapsulation types.