Re: [PATCH net v1 1/2] net: gro: add flush check in udp_gro_receive_segment

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Alexander Duyck wrote:
> On Sat, Apr 13, 2024 at 11:38 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > Richard Gobert wrote:
> > > GRO-GSO path is supposed to be transparent and as such L3 flush checks are
> > > relevant to all flows which call skb_gro_receive. This patch uses the same
> > > logic and code from tcp_gro_receive but in the relevant flow path in
> > > udp_gro_receive_segment.
> > >
> > > Fixes: 36707061d6ba ("udp: allow forwarding of plain (non-fraglisted) UDP GRO packets")
> > > Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
> >
> > Reviewed-by: Willem de Bruijn <willemb@google.com>
> >
> > > ---
> > >  net/ipv4/udp_offload.c | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > > index 3498dd1d0694..1f4e08f43c4b 100644
> > > --- a/net/ipv4/udp_offload.c
> > > +++ b/net/ipv4/udp_offload.c
> > > @@ -471,6 +471,7 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> > >       struct sk_buff *p;
> > >       unsigned int ulen;
> > >       int ret = 0;
> > > +     int flush;
> > >
> > >       /* requires non zero csum, for symmetry with GSO */
> > >       if (!uh->check) {
> > > @@ -528,7 +529,17 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> > >                               skb_gro_postpull_rcsum(skb, uh,
> > >                                                      sizeof(struct udphdr));
> > >
> > > -                             ret = skb_gro_receive(p, skb);
> > > +                             flush = NAPI_GRO_CB(p)->flush;
> > > +
> > > +                             if (NAPI_GRO_CB(p)->flush_id != 1 ||
> > > +                                 NAPI_GRO_CB(p)->count != 1 ||
> > > +                                 !NAPI_GRO_CB(p)->is_atomic)
> > > +                                     flush |= NAPI_GRO_CB(p)->flush_id;
> > > +                             else
> > > +                                     NAPI_GRO_CB(p)->is_atomic = false;
> > > +
> > > +                             if (flush || skb_gro_receive(p, skb))
> > > +                                     ret = 1;
> >
> > UDP_L4 does not have the SKB_GSO_TCP_FIXEDID that uses is_atomic as
> > input.
> >
> > And I still don't fully internalize the flush_id logic after staring
> > at it for more than one coffee.
> 
> The flush_id field is there to indicate the difference between the
> current IPv4 ID of the previous IP header. It is meant to be used in
> conjunction with the is_atomic for the frame coalescing. Basically
> after the second frame we can decide the pattern either incrementing
> IPv4 ID or fixed, so on frames 3 or later we can decide to drop the
> frame if it doesn't follow that pattern.
> 
> > But even ignoring those, the flush signal of NAPI_GRO_CB(p)->flush
> > set the network layer must be followed, so ACK. Thanks for the fix.
> 
> I'm not sure about the placement of this code though. That is the one
> thing that seems off to me. Specifically this seems like it should be
> done before we start the postpull, not after. It should be something
> that can terminate the flow before we attempt to aggregate the UDP
> headers.

In principle agreed that we should conclude the flush checks before
doing prep for coalescing.

In practice it does not matter? NAPI_GRO_CB(skb)->csum will be ignored
if the packet gets flushed.