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

[Wengang Wang]

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.w...@oracle.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,