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    SubjectRe: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO
    On Mon, Nov 9, 2020 at 12:37 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
    >
    >
    >
    > On 11/9/20 5:56 PM, Alexander Lobakin 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>
    > > ---
    > > net/ipv4/udp_offload.c | 7 ++++++-
    > > 1 file changed, 6 insertions(+), 1 deletion(-)
    > >
    > > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
    > > index e67a66fbf27b..7f6bd221880a 100644
    > > --- a/net/ipv4/udp_offload.c
    > > +++ b/net/ipv4/udp_offload.c
    > > @@ -366,13 +366,18 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
    > > static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
    > > struct sk_buff *skb)
    > > {
    > > - struct udphdr *uh = udp_hdr(skb);
    > > + struct udphdr *uh = udp_gro_udphdr(skb);
    > > struct sk_buff *pp = NULL;
    > > struct udphdr *uh2;
    > > struct sk_buff *p;
    > > unsigned int ulen;
    > > int ret = 0;
    > >
    > > + if (unlikely(!uh)) {
    >
    > How uh could be NULL here ?
    >
    > My understanding is that udp_gro_receive() is called
    > only after udp4_gro_receive() or udp6_gro_receive()
    > validated that udp_gro_udphdr(skb) was not NULL.

    Oh indeed. This has already been checked before.

    > > + NAPI_GRO_CB(skb)->flush = 1;
    > > + return NULL;
    > > + }
    > > +
    > > /* requires non zero csum, for symmetry with GSO */
    > > if (!uh->check) {
    > > NAPI_GRO_CB(skb)->flush = 1;
    > >
    >
    > Why uh2 is left unchanged ?
    >
    > uh2 = udp_hdr(p);

    Isn't that the same as th2 = tcp_hdr(p) in tcp_gro_receive? no frag0
    optimization to worry about for packets on the list.


    > ...
    >

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