netdev - Re: [PATCH net-next 3/8] udp: properly complete L4 GRO over UDP tunnel packet

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Date:   Tue, 23 Mar 2021 22:13:10 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alexander Lobakin <alobakin@...me>
Subject: Re: [PATCH net-next 3/8] udp: properly complete L4 GRO over UDP
 tunnel packet

On Mon, Mar 22, 2021 at 1:00 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Mon, 2021-03-22 at 09:30 -0400, Willem de Bruijn wrote:
> > On Sun, Mar 21, 2021 at 1:01 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > > After the previous patch the stack can do L4 UDP aggregation
> > > on top of an UDP tunnel.
> > >
> > > The current GRO complete code tries frag based aggregation first;
> > > in the above scenario will generate corrupted frames.
> > >
> > > We need to try first UDP tunnel based aggregation, if the GRO
> > > packet requires that. We can use time GRO 'encap_mark' field
> > > to track the need GRO complete action. If encap_mark is set,
> > > skip the frag_list aggregation.
> > >
> > > On tunnel encap GRO complete clear such field, so that an inner
> > > frag_list GRO complete could take action.
> > >
> > > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > > ---
> > >  net/ipv4/udp_offload.c | 8 +++++++-
> > >  net/ipv6/udp_offload.c | 3 ++-
> > >  2 files changed, 9 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > > index 25134a3548e99..54e06b88af69a 100644
> > > --- a/net/ipv4/udp_offload.c
> > > +++ b/net/ipv4/udp_offload.c
> > > @@ -642,6 +642,11 @@ int udp_gro_complete(struct sk_buff *skb, int nhoff,
> > >                 skb_shinfo(skb)->gso_type = uh->check ? SKB_GSO_UDP_TUNNEL_CSUM
> > >                                         : SKB_GSO_UDP_TUNNEL;
> > >
> > > +               /* clear the encap mark, so that inner frag_list gro_complete
> > > +                * can take place
> > > +                */
> > > +               NAPI_GRO_CB(skb)->encap_mark = 0;
> > > +
> > >                 /* Set encapsulation before calling into inner gro_complete()
> > >                  * functions to make them set up the inner offsets.
> > >                  */
> > > @@ -665,7 +670,8 @@ INDIRECT_CALLABLE_SCOPE int udp4_gro_complete(struct sk_buff *skb, int nhoff)
> > >         const struct iphdr *iph = ip_hdr(skb);
> > >         struct udphdr *uh = (struct udphdr *)(skb->data + nhoff);
> > >
> > > -       if (NAPI_GRO_CB(skb)->is_flist) {
> > > +       /* do fraglist only if there is no outer UDP encap (or we already processed it) */
> > > +       if (NAPI_GRO_CB(skb)->is_flist && !NAPI_GRO_CB(skb)->encap_mark) {
> >
> > Sorry, I don't follow. I thought the point was to avoid fraglist if an
> > outer udp tunnel header is present. But the above code clears the mark
> > and allows entering the fraglist branch exactly when such a header is
> > encountered?
>
> The relevant UDP packet has gone through:
>
> [l2/l3 GRO] -> udp_gro_receive  -> udp_sk(sk)->gro_receive -> [some
> more GRO layers] -> udp_gro_receive (again)
>
> The first udp_gro_receive set NAPI_GRO_CB(skb)->encap_mark, the
> latter udp_gro_receive set NAPI_GRO_CB(skb)->is_flist.
>
> Then, at GRO complete time:
>
> [l2/l3 GRO] -> udp{4,6}_gro_complete -> udp_sk(sk)->gro_complete ->
> [more GRO layers] -> udp{4,6}_gro_complete (again).
>
> In the first udp{4,6}_gro_complete invocation 'encap_mark' is 1, so
> with this patch we do the 'udp_sk(sk)->gro_complete' path. In the
> second udp{4,6}_gro_complete invocation 'encap_mark' has been cleared
> (by udp_gro_complete), so we do the SKB_GSO_FRAGLIST completion.
>
> In case SKB_GSO_FRAGLIST with no UDP tunnel, 'encap_mark' is 0 and we
> do the SKB_GSO_FRAGLIST completion.
>
> Another alternative, possibly more readable, would be avoid clearing
> 'encap_mark' in udp_gro_complete() and replacing the above check with:
>
>         if (NAPI_GRO_CB(skb)->is_flist &&
>             (!NAPI_GRO_CB(skb)->encap_mark ||
>              (NAPI_GRO_CB(skb)->encap_mark && skb->encapsulation))) {
>
> I opted otherwise to simplify the conditional expression.
>
> Please let me know if the above is somewhat more clear and if you have
> preferecens between the two options.

Got it now, thanks. Yes, this patch makes sense.

When the GRO layer has built a GSO packet with both inner UDP
(GSO_UDP_L4) and UDP tunnel header (GSO_UDP_TUNNEL), then on
completion udp{4,6}_gro_complete will be called twice. This function
will enter its is_flist branch immediately, even though that is only
correct on the second call, as GSO_FRAGLIST is only relevant for the
inner packet. Skip this while encap_mark == 1, identifying processing
of the outer tunnel header. Clear the field to enter the path on the next
round, for the inner header.

This is subject to only supporting a single layer of encap, but that
is indeed the current state afaik.