netdev - Re: [Xen-devel] [PATCH] xen-netfront: pull on receive skb may need to happen earlier

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Date:	Tue, 09 Jul 2013 08:47:20 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
Cc:	"Ian Campbell" <ian.campbell@...rix.com>,
	"Wei Liu" <wei.liu2@...rix.com>, <davem@...emloft.net>,
	"Dion Kant" <g.w.kant@...enet.nl>, <xen-devel@...ts.xen.org>,
	<netdev@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen-netfront: pull on receive skb may
 need to happen earlier

>>> On 08.07.13 at 17:22, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2013-07-08 at 15:20 +0100, Jan Beulich wrote:
> 
>> Okay, so with that done (see below) Dion is now seeing the
>> WARN_ON_ONCE(delta < len) in skb_try_coalesce() triggering. Of
>> course, with it having crashed before, it's hard to tell whether the
>> triggering now is an effect of the patch, or just got unmasked by it.
> 
> Please note this warning is not the sign of an error.

Thanks for pointing this out.

> It can be triggered because network stack (IP + TCP) had to reallocate
> skb->head to hold all the headers.
> 
> pskb_may_pull() doesn't change skb->truesize, for various reasons, so we
> can have a situation where the warning triggers.
> 
> For example, it can happen when drivers use a really small skb->head,
> like 64 bytes, as its not big enough to hold ethernet+IP+TCP headers.

We know from the crashes prior to the patch here that this is
exactly what is happening.

Jan


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