Subject: Re: [PATCH] net: take care of bonding in build_skb_flow_key

From: Wengang Wang <[email-masked]>
Date: Wed, 6 Jan 2016 15:06:18 +0800

Hi Yanjun,

Thanks for your review.
Master MTU is same as that for slaves.
Maybe fixing in bonding driver is a good idea, but I don't find a good 
place to do that.
Let's go through the simplified follow:

...
1) Fragmentation.
    --This is is done is against the bonding master device(device MTU 
and path MTU)
2) bond_start_xmit
3) ipoib_start_xmit(slaves are IPoIB interfaces)

For the first send
1) fragment size is 7000(in my case)
2) bond_start_xmit its self is fine
3) ipoib_start_xmit sees the packet size 7000 is larger than the 
internal limit 2044, drops the packet and try to update PMTU.
     without the patch, it tried update PMTU on slave device(no changes 
to master).

the seconds send comes, since no changes happen on bonding master(PMTU), 
the fragment size is still 7000 and the behavior is just the same as the 
first send.

With the patch, the bonding master PMTU is changed to 2044 after the 
first send(hopefully), for the seconds send the fragment size is set to 
2044.

To fix in bonding code, I don't find where we can.

thanks,
wengang

在 2016年01月06日 14:19, zhuyj 写道:
>
> IMHO, this should fix in bonding driver because the active slave mtu 
> should be the same with the master.
> bonding master's mtu is changed to path MTU, then slave dev's MTU 
> should be changed, too.
>
> Zhu Yanjun
> On 01/06/2016 01:49 PM, Wengang Wang wrote:
>> A problem is found that we are looking for route basing a bonding 
>> device and
>> deal with path MTU there: The path MTU is set to the active slave 
>> device, not
>> the bonding master.
>>
>> The patch tries to fix the issue by letting build_skb_flow_key() take 
>> care
>> of the transition of device index from bonding slave to the master.
>>
>> Signed-off-by: Wengang Wang <[email-masked]>
>> ---
>>   net/ipv4/route.c | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
>> index 85f184e..3053f10 100644
>> --- a/net/ipv4/route.c
>> +++ b/net/ipv4/route.c
>> @@ -523,11 +523,20 @@ static void build_skb_flow_key(struct flowi4 
>> *fl4, const struct sk_buff *skb,
>>                      const struct sock *sk)
>>   {
>>       const struct iphdr *iph = ip_hdr(skb);
>> -    int oif = skb->dev->ifindex;
>> +    int oif;
>> +    struct net_device *master = NULL;
>> +
>>       u8 tos = RT_TOS(iph->tos);
>>       u8 prot = iph->protocol;
>>       u32 mark = skb->mark;
>>   +    if (skb->dev->flags & IFF_SLAVE)
>> +        master = netdev_master_upper_dev_get(skb->dev);
>> +    if (master)
>> +        oif = master->ifindex;
>> +    else
>> +        oif = skb->dev->ifindex;
>> +
>>       __build_flow_key(fl4, sk, iph, oif, tos, prot, mark, 0);
>>   }
>