netdev - Re: [RFC PATCH v2 02/10] udp: implement GRO for plain UDP sockets.

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Date:   Mon, 22 Oct 2018 11:51:00 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     steffen.klassert@...unet.com,
        Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [RFC PATCH v2 02/10] udp: implement GRO for plain UDP sockets.

> >
> > > +static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> > > +                                          struct sk_buff *skb)
> > > +{
> > > +   struct udphdr *uh = udp_hdr(skb);
> > > +   struct sk_buff *pp = NULL;
> > > +   struct udphdr *uh2;
> > > +   struct sk_buff *p;
> > > +
> > > +   /* requires non zero csum, for simmetry with GSO */
> > > +   if (!uh->check) {
> > > +           NAPI_GRO_CB(skb)->flush = 1;
> > > +           return NULL;
> > > +   }
> >
> > Why is the requirement of checksums different than in
> > udp_gro_receive? It's not that I care much about UDP
> > packets without a checksum, but you would not need
> > to implement your own loop if the requirement could
> > be the same as in udp_gro_receive.

It would be nice if we could deduplicate the loops, but even without
the checksum difference they look to me a bit too different for it to be
practical, also with the constraints on segment length and max aggregation.

> uhm....
> AFAIU, we need to generated aggregated packets that UDP GSO is able to
> process/segment. I was unable to get a nocsum packet segment (possibly
> PEBKAC) so I enforced that condition on the rx path.
>
> @Willem: did I see ghost here? is UDP_SEGMENT fine with no checksum
> segment?

udp_send_skb fails with EIO if ip_summed is anything but CHECKSUM_PARTIAL.

but that's not in the forwarding path. Still, __udp_gso_segment as is
depends on that invariant and will not handle packets with zero
checksum correctly. It unconditionally adjusts uh->check. That could
be changed, of course.