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 Mon, Apr 15, 2024 at 8:00 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > 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.
> 
> I was referring more to the fact that this code is one of two
> branches. So there is this path, and then the is_flist branch that
> comes before this. I would think this logic would apply to both
> wouldn't it? I am not familiar with the code so I cannot say for
> certain if it does or doesn't. If it doesn't then yes. I suppose it
> doesn't matter.

With if_flist, all original segments are preserved in the frag_list,
so can be sent out as is.

Good point that that is no excuse for combining three or more
segments where some have a fixed id and others an incrementing id.