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

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Date:   Mon, 9 Nov 2020 14:06:07 -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>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO

On Mon, Nov 9, 2020 at 1:54 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 11/9/20 7:28 PM, Willem de Bruijn wrote:
> > 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.
>
> My feeling was that tcp_gro_receive() is terminal in the GRO stack.
>
> While UDP could be encapsulated in UDP :)
>
> I guess we do not support this yet.
>
> Years ago we made sure to propagate the current header offset into GRO stack
> (when we added SIT/IPIP/GRE support to GRO)
> 299603e8370a93dd5d8e8d800f0dff1ce2c53d36 ("net-gro: Prepare GRO stack for the upcoming tunneling support")

On which note, and Alexander's mention of udp4_lib_lookup_skb(), that
performs a standard ip_hdr on the possibly frag0 optimized incoming
packet:

struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
                                 __be16 sport, __be16 dport)
{
        const struct iphdr *iph = ip_hdr(skb);

        return __udp4_lib_lookup(dev_net(skb->dev), iph->saddr, sport,
                                 iph->daddr, dport, inet_iif(skb),
                                 inet_sdif(skb), &udp_table, NULL);
}

This should use skb_gro_header_.. too, then?