netdev - Re: [net-next 2/2] macvlan: Enable qdisc backoff logic.

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Date:	Thu, 26 Aug 2010 10:45:58 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	netdev@...r.kernel.org
Subject: Re: [net-next 2/2] macvlan:  Enable qdisc backoff logic.

On 08/26/2010 06:55 AM, Arnd Bergmann wrote:
> On Wednesday 25 August 2010, Ben Greear wrote:
>> On 08/25/2010 12:59 PM, Arnd Bergmann wrote:
>>> On Wednesday 25 August 2010 21:27:43 Ben Greear wrote:
>>>>> I suppose we need to do something in macvtap to handle this as
>>>>> well, right? A guest trying to send a frame through qemu
>>>>> or vhost net into macvtap needs to be prevented from sending
>>>>> more when we get into this path. Right now, we just ignore
>>>>> the return value of macvlan_start_xmit.
>>>>
>>>> I have a similar, though slightly more complex, patch for 802.1q
>>>> vlans, but I haven't looked at macvtap at all.
>>>>
>>>> If these two patches are accepted, I'll post the .1q patch as well.
>>>
>>> I think one of us needs to fix macvtap in order for your patch to
>>> go in, because otherwise there is a memory leak or worse when
>>> macvtap fails to retransmit the frame.
>>
>> With no change, the try_ logic will not be called, so it should
>> be fully backwards compatible.
>
> How? The macvlan driver is used as the back-end for macvtap,
> so it calls all the same functions:
>
> macvtap_write
> ->  macvtap_get_user
> ->  macvlan_start_xmit
> ->  macvlan->queue_xmit
> ->  try_dev_queue_xmit

I think this will keep today's functionality.  Someone that knows and uses this
code might can figure out how to properly do backpressure to calling code
and re-queue the skb instead of just deleting it when the underlying device
complains of being busy.

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 4256727..5abf0c0 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -571,9 +571,15 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q,

         rcu_read_lock_bh();
         vlan = rcu_dereference(q->vlan);
-       if (vlan)
-               macvlan_start_xmit(skb, vlan->dev);
-       else
+       if (vlan) {
+               /* TODO:  Deal with BUSY properly by somehow re-queuing
+                * skb for later transmit and let calling logic know it
+                * needs to back off for a short time.
+                */
+               if (macvlan_start_xmit(skb, vlan->dev) == NETDEV_TX_BUSY)
+                       goto free_skb;
+       } else
+free_skb:
                 kfree_skb(skb);
         rcu_read_unlock_bh();


If this looks good, I'll do up an official patch set containing this and the other
two patches I sent previously.

Thanks,
Ben

>
> 	Arnd
> --
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-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com

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