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

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Date:   Mon, 9 Nov 2020 13:28:54 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Alexander Lobakin <alobakin@...me>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO

On Mon, Nov 9, 2020 at 12:37 PM Eric Dumazet <eric.dumazet@...il.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@...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.


> ...
>