linux-kernel - Re: [PATCH net-next] ibmveth: v1 calculate correct gso_size and set gso

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Date:   Thu, 27 Oct 2016 08:26:56 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jon Maxwell <jmaxwell37@...il.com>
Cc:     tlfalcon@...ux.vnet.ibm.com, benh@...nel.crashing.org,
        paulus@...ba.org, mpe@...erman.id.au, davem@...emloft.net,
        tom@...bertland.com, jarod@...hat.com, hofrat@...dl.org,
        netdev@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, mleitner@...hat.com,
        jmaxwell@...hat.com
Subject: Re: [PATCH net-next] ibmveth: v1 calculate correct gso_size and set
 gso_type

On Wed, 2016-10-26 at 11:09 +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 <jmaxwell37@...il.com>
> ---
>  drivers/net/ethernet/ibm/ibmveth.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
> index 29c05d0..c51717e 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);
>  
>  restart_poll:
>  	while (frames_processed < budget) {
> @@ -1236,10 +1238,28 @@ 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) {
> +					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) {
> +					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++;
> +			}
> +
>  

This might break forwarding and PMTU discovery.

You force gso_size to device mtu, regardless of real MSS used by the TCP
sender.

Don't you have the MSS provided in RX descriptor, instead of guessing
the value ?