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Subject: Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO
To:     Alexander Lobakin <alobakin@pm.me>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
Cc:     Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Willem de Bruijn <willemb@google.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <0eaG8xtbtKY1dEKCTKUBubGiC9QawGgB3tVZtNqVdY@cp4-web-030.plabs.ch>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <d9d09931-8cd3-1eb6-673c-3ae5ebc3ee57@gmail.com>
Date:   Mon, 9 Nov 2020 18:37:36 +0100
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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.


> +		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);

...