Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO

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

On Mon, Nov 9, 2020 at 11:57 AM Alexander Lobakin <alobakin@pm.me> wrote:
>
> While testing UDP GSO fraglists forwarding through driver that uses
> Fast GRO (via napi_gro_frags()), I was observing lots of out-of-order
> iperf packets:
>
> [ ID] Interval           Transfer     Bitrate         Jitter
> [SUM]  0.0-40.0 sec  12106 datagrams received out-of-order
>
> Simple switch to napi_gro_receive() any other method without frag0
> shortcut completely resolved them.
>
> I've found that UDP GRO uses udp_hdr(skb) in its .gro_receive()
> callback. While it's probably OK for non-frag0 paths (when all
> headers or even the entire frame are already in skb->data), this
> inline points to junk when using Fast GRO (napi_gro_frags() or
> napi_gro_receive() with only Ethernet header in skb->data and all
> the rest in shinfo->frags) and breaks GRO packet compilation and
> the packet flow itself.
> To support both modes, skb_gro_header_fast() + skb_gro_header_slow()
> are typically used. UDP even has an inline helper that makes use of
> them, udp_gro_udphdr(). Use that instead of troublemaking udp_hdr()
> to get rid of the out-of-order delivers.
>
> Present since the introduction of plain UDP GRO in 5.0-rc1.
>
> Since v1 [1]:
>  - added a NULL pointer check for "uh" as suggested by Willem.
>
> [1] https://lore.kernel.org/netdev/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@cp4-web-034.plabs.ch
>
> Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>

Acked-by: Willem de Bruijn <willemb@google.com>

Thanks