netdev - Re: [PATCH] pktgen: Clone skb to avoid corruption of skbs in ndo_start

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Date:	Thu, 21 Jul 2011 15:19:03 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	greearb@...delatech.com
Cc:	nhorman@...driver.com, eric.dumazet@...il.com, jpirko@...hat.com,
	netdev@...r.kernel.org, adobriyan@...il.com,
	robert.olsson@....uu.se
Subject: Re: [PATCH] pktgen: Clone skb to avoid corruption of skbs in
 ndo_start_xmit methods

From: Ben Greear <greearb@...delatech.com>
Date: Thu, 21 Jul 2011 15:14:32 -0700

> On 07/21/2011 03:01 PM, David Miller wrote:
>> From: Neil Horman<nhorman@...driver.com>
>> Date: Wed, 20 Jul 2011 11:18:27 -0400
>>
>>> On Wed, Jul 20, 2011 at 06:24:15AM +0200, Eric Dumazet wrote:
>>>> Le mardi 19 juillet 2011 à 22:07 -0400, Neil Horman a écrit :
>>>>>>
>>>>> I think this is a good idea.  It lets pktgen dynamically make the
>>>>> clone/share
>>>>> decision dynamically and only impacts performance for those systems.
>>>>>
>>>>
>>>> Just let pktgen refuse to use clone_skb command for these devices.
>>>>
>>> copy that, This is by no means final, but what do you think of this?
>>> If its
>>> agreeable to you, Ben, et al. I can add this to my local tree and
>>> start auditing
>>> all the drivers that may need to have the flag set.
>>
>> I think there is a much simpler solution.
>>
>> Set a flag in the SKB when pktgen does SKB sharing.
>>
>> In dev_queue_xmit() (or perhaps, dev_hard_start_xmit()), check the
>> flag
>> and if it's set then we copy the SKB.
>>
>> If this works, then we fix the crash and no driver changes are
>> necessary both now and in the future.
> 
> Doesn't that make clone-skb in pktgen much less efficient
> in all cases?

No, the copy only happens if we enter dev_queue_xmit() which pktgen
doesn't do, it calls the driver's ->ndo_start_xmit() method directly.

That's the whole idea.  Only these encapsulating software devices
will trigger the condition.
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