netdev - Re: [PATCH] act_mirred: don't clone skb when skb isn't shared

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Date:	Sat, 5 Jun 2010 21:07:55 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	hadi@...erus.ca
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] act_mirred: don't clone skb when skb isn't shared

On Sat, Jun 5, 2010 at 8:53 PM, jamal <hadi@...erus.ca> wrote:
> On Fri, 2010-06-04 at 21:43 +0800, Changli Gao wrote:
>> don't clone skb when skb isn't shared
>>
>> When the tcf_action is TC_ACT_STOLEN, and the skb isn't shared, we don't need
>> to clone a new skb. As the skb will be freed after this function returns, we
>> can use it freely once we get a reference to it.
>
> It looks like a good optimization - but i am not a big fan of one-offs
> [because usability goes down and I am forced to explain it longer in the
> rules (refer to: Documentation/networking/tc-actions-env-rules.txt)]

Thanks. BTW: act_nat.c doesn't obey the following rule, and you plan
to remove TC_MUNGED and TC_OK2MUNGE?

2) If you munge any packet thou shalt call pskb_expand_head in the case
someone else is referencing the skb. After that you "own" the skb.
You must also tell us if it is ok to munge the packet (TC_OK2MUNGE),
this way any action downstream can stomp on the packet.

>
> How about you update skb_act_clone to take take the action code as well
> and do the check the if stolen/queued it does a skb_get otherwise it
> calls skb_clone?
>

Good idea. Thanks.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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