On 10/27/2016 10:26 AM, Eric Dumazet wrote:
> 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 <jmaxwel...@gmail.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 ?
>
>
>
The MSS is not always available unfortunately, so this is the best solution
there is at the moment.
