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

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Date:	Wed, 20 Jan 2016 16:18:20 +0100
From:	Sabrina Dubroca <sd@...asysnail.net>
To:	Wengang Wang <wen.gang.wang@...cle.com>
Cc:	netdev@...r.kernel.org, jay.vosburgh@...onical.com
Subject: Re: [PATCH] net: take care of bonding in build_skb_flow_key (v3)

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();


Thanks,

-- 
Sabrina