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

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Message-ID: <1335982515.22133.610.camel@edumazet-glaptop>
Date:	Wed, 02 May 2012 20:15:15 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Alexander Duyck <alexander.duyck@...il.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 11:05 -0700, Alexander Duyck wrote:

> You're correct about the fragstolen case, I actually was thinking of the
> first patch you sent, not this second one.
> 
> However we still have a problem.  What we end up with now is a case of
> sharing in which the clone skb no longer knows that it is sharing the
> head with another skb.  The dataref will drop to 1 when we call
> __kfree_skb.  This means that any other function out there that tries to
> see if the skb is shared would return false.  This could lead to issues
> if there is anything out there that manipulates the data in head based
> on the false assumption that it is not cloned.  What we would probably
> need to do in this case is tweak the logic for skb_cloned.  If you are
> using a head_frag you should probably add a check that returns true if
> cloned is true and page_count is greater than 1.  We should be safe in
> the case of skb_header_cloned since we already dropped are dataref when
> we stole the page and freed the skb.

I really dont understand this concern.

When skb is cloned, we copy in head_frag __skb_clone()

So both skbs have the bit set, and dataref = 2.

first skb is freed, dataref becomes 1 and nothing special  happen

>>From this point, skb->head is not 'shared' anymore (taken your own
words). And we are free to do whatever we want.

second skb is freed, dataref becomes 0 and we call the right destructor.



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