Subject: Re: [PATCH net-next] ibmveth: calculate correct gso_size and set gso_type

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Tue, 25 Oct 2016 08:31:26 -0200

On Tue, Oct 25, 2016 at 04:13:41PM +1100, Jon Maxwell wrote:
> We recently encountered a bug where a few customers using ibmveth on the 
> same LPAR hit an issue where a TCP session hung when large receive was
> enabled. Closer analysis revealed that the session was stuck because the 
> one side was advertising a zero window repeatedly.
> 
> We narrowed this down to the fact the ibmveth driver did not set gso_size 
> which is translated by TCP into the MSS later up the stack. The MSS is 
> used to calculate the TCP window size and as that was abnormally large, 
> it was calculating a zero window, even although the sockets receive buffer 
> was completely empty. 
> 
> We were able to reproduce this and worked with IBM to fix this. Thanks Tom 
> and Marcelo for all your help and review on this.
> 
> The patch fixes both our internal reproduction tests and our customers tests.
> 
> Signed-off-by: Jon Maxwell <[email-masked]>
> ---
>  drivers/net/ethernet/ibm/ibmveth.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
> index 29c05d0..3028c33 100644
> --- a/drivers/net/ethernet/ibm/ibmveth.c
> +++ b/drivers/net/ethernet/ibm/ibmveth.c
> @@ -1182,6 +1182,8 @@ static int ibmveth_poll(struct napi_struct *napi, int budget)
>  	int frames_processed = 0;
>  	unsigned long lpar_rc;
>  	struct iphdr *iph;
> +	bool large_packet = 0;
> +	u16 hdr_len = ETH_HLEN + sizeof(struct tcphdr);

Compiler may optmize this, but maybe move hdr_len to [*] ?

>  
>  restart_poll:
>  	while (frames_processed < budget) {
> @@ -1236,10 +1238,27 @@ static int ibmveth_poll(struct napi_struct *napi, int budget)
>  						iph->check = 0;
>  						iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
>  						adapter->rx_large_packets++;
> +						large_packet = 1;
>  					}
>  				}
>  			}
>  
> +			if (skb->len > netdev->mtu) {

[*]

> +				iph = (struct iphdr *)skb->data;
> +				if (be16_to_cpu(skb->protocol) == ETH_P_IP && iph->protocol == IPPROTO_TCP) {

The if line above is too long, should be broken in two.

> +					hdr_len += sizeof(struct iphdr);
> +					skb_shinfo(skb)->gso_type = SKB_GSO_TCPV4;
> +					skb_shinfo(skb)->gso_size = netdev->mtu - hdr_len;
> +				} else if (be16_to_cpu(skb->protocol) == ETH_P_IPV6 &&
> +					iph->protocol == IPPROTO_TCP) {
                                        ^
And this one should start 3 spaces later, right below be16_....

  Marcelo

> +					hdr_len += sizeof(struct ipv6hdr);
> +					skb_shinfo(skb)->gso_type = SKB_GSO_TCPV6;
> +					skb_shinfo(skb)->gso_size = netdev->mtu - hdr_len;
> +				}
> +				if (!large_packet)
> +					adapter->rx_large_packets++;
> +			}
> +
>  			napi_gro_receive(napi, skb);	/* send it up */
>  
>  			netdev->stats.rx_packets++;
> -- 
> 1.8.3.1
>