netdev - Re: [PATCH net-next] net: take care of cloned skbs in tcp_try

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Message-ID: <1336014869.22133.821.camel@edumazet-glaptop>
Date:	Thu, 03 May 2012 05:14:29 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Michael Chan <mchan@...adcom.com>,
	Matt Carlson <mcarlson@...adcom.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH net-next] net: take care of cloned skbs in
 tcp_try_coalesce()

On Wed, 2012-05-02 at 20:00 -0700, Alexander Duyck wrote:

> This is exactly my point.  The way your current code works the check in 
> pskb_expand_head will not detect the header is cloned.
> 
> So for example lets say you have one of your skbs that goes through and 
> is merged.
> 
> 1.  You start with a cloned skb.  dataref = 2, head_frag page count = 1;
> 2.  You go through tcp_try_coalesce.  dataref = 2, head_frag page count = 2;
> 3.  You call __kfree_skb on the skb.  dataref = 1, head_frag page count = 2;


If page count was incremented in 2., this is because we stole the head.

But we could not stole the head since dataref = 2

So try to find a real example, based on current state, not on the first
patch I sent, since we made progress since this time.

Thanks



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