Subject: Re: [PATCH] net: take care of bonding in build_skb_flow_key (v3)

From: Wengang Wang <[email-masked]>
Date: Wed, 20 Jan 2016 15:38:35 +0800

在 2016年01月20日 14:24, zhuyj 写道:
> On 01/20/2016 01:32 PM, Wengang Wang wrote:
>> In a bonding setting, we determines fragment size according to MTU and
>> PMTU associated to the bonding master. If the slave finds the fragment
>> size is too big, it drops the fragment and calls ip_rt_update_pmtu(),
>> passing _skb_ and _pmtu_, trying to update the path MTU.
>> Problem is that the target device that function ip_rt_update_pmtu 
>> actually
>> tries to update is the slave (skb->dev), not the master. Thus since no
>> PMTU change happens on master, the fragment size for later packets 
>> doesn't
>> change so all later fragments/packets are dropped too.
>>
>> The fix is letting build_skb_flow_key() take care of the transition of
>> device index from bonding slave to the master. That makes the master 
>> become
>> the target device that ip_rt_update_pmtu tries to update PMTU to.
>>
>> Signed-off-by: Wengang Wang <[email-masked]>
>> ---
>>   net/ipv4/route.c | 13 ++++++++++++-
>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
>> index 85f184e..c59fb0d 100644
>> --- a/net/ipv4/route.c
>> +++ b/net/ipv4/route.c
>> @@ -523,10 +523,21 @@ 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;
>> +    struct net_device *master = NULL;
>>       u8 tos = RT_TOS(iph->tos);
>>       u8 prot = iph->protocol;
>>       u32 mark = skb->mark;
>> +    int oif;
>> +
>> +    if (skb->dev->flags & IFF_SLAVE) {
>> +        rtnl_lock();
>> +        master = netdev_master_upper_dev_get(skb->dev);
>> +        rtnl_unlock();
> update_pmtu is called very frequently. Is it appropriate to use 
> rtnl_lock here?
By "very frequently", how frequently it is expected? And what situation 
can cause that?
For my case, the update_pmtu is called only once.

thanks,
wengang

> That is, rtnl_lock is called frequently. Maybe other functions have 
> little chance to call rtnl_lock.
>
> Best Regards!
> Zhu Yanjun
>> +    }
>> +    if (master)
>> +        oif = master->ifindex;
>> +    else
>> +        oif = skb->dev->ifindex;
>>         __build_flow_key(fl4, sk, iph, oif, tos, prot, mark, 0);
>>   }
>