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

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