Message-ID: <5035020E.5040806@lwfinger.net>
Date: Wed, 22 Aug 2012 11:00:14 -0500
From: Larry Finger <Larry.Finger@lwfinger.net>
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To: Eric Dumazet <eric.dumazet@gmail.com>
CC: Neal Cardwell <ncardwell@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        John W Linville <linville@tuxdriver.com>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: Regression associated with commit c8628155ece3 - "tcp: reduce
 out_of_order memory use"
References: <50345B12.1050600@lwfinger.net> <1345612503.5158.566.camel@edumazet-glaptop>
In-Reply-To: <1345612503.5158.566.camel@edumazet-glaptop>
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On 08/22/2012 12:15 AM, Eric Dumazet wrote:
>
> This particular commit is the start of a patches batch that ended in the
> generic TCP coalescing mechanism.
>
> It is known to have problem on drivers doing skb_clone() in their rx
> path.
>
> Current kernels should be ok, because coalescing doesnt happen if the
> destination skb is cloned (skb_cloned(to) in skb_try_coalesce())
>
> For 3.4 kernel, I guess I need to backport this skb_cloned(to) check fo
> stable 3.4 kernel
>
> But these skb_clone() in various USB drivers should be killed for good,
> they really can kill the box because of skb->truesize lies.

Applying your patch to 3.4 does not fix the problem. In addition, all of the 
problems I noted happen with 3.6-rc2 - current kernels are not OK.

Of the drivers I reported, only r8712u in staging/rtl8712/rtl8712_recv.c has a 
direct call to skb_clone(). As its symptoms are different, the cloning is likely 
the source of the problems there. In addition, the problem is intermittent. The 
driver normally allocates a new buffer and copies to it, and the clone only 
occurs when that skb allocation fails. I will try to fix that. My skb skills are 
minimal and I will have to do some studying.

Thanks,

Larry


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