netdev - Re: [PATCH] net: take care of bonding in build_skb_flow

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Date:	Thu, 21 Jan 2016 13:15:43 +0800
From:	Wengang Wang <wen.gang.wang@...cle.com>
To:	Sabrina Dubroca <sd@...asysnail.net>
Cc:	netdev@...r.kernel.org, jay.vosburgh@...onical.com
Subject: Re: [PATCH] net: take care of bonding in build_skb_flow_key (v3)



在 2016年01月20日 23:18, Sabrina Dubroca 写道:
> 2016-01-20, 13:32:13 +0800, 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 <wen.gang.wang@...cle.com>
>> ---
>>   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) {
> Maybe use netif_is_bond_slave here instead, since you have this
> problem with bonding slaves?
>
>
>> +		rtnl_lock();
>> +		master = netdev_master_upper_dev_get(skb->dev);
>> +		rtnl_unlock();
>> +	}
> As zhuyj said, this is called from dev_queue_xmit, so you cannot take
> rtnl_lock here.
>
>> +	if (master)
>> +		oif = master->ifindex;
> You cannot dereference master after you release the rtnl lock.
>
> So it would probably be best to use netdev_master_upper_dev_get_rcu,
> as zhuyj suggested earlier, and make sure that you only use the result
> between rcu_read_lock()/rcu_read_unlock():
>
>      rcu_read_lock();
>      master = netdev_master_upper_dev_get_rcu(skb->dev);
>      if (master)
>          oif = master->ifindex;
>      rcu_read_unlock();
>
OK, thanks for advising.

thanks,
wengang

> Thanks,
>