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

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Message-Id: <51DFE0C702000078000E462A@nat28.tlf.novell.com>
Date:	Fri, 12 Jul 2013 09:56:07 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Wei Liu" <wei.liu2@...rix.com>
Cc:	"Ian Campbell" <ian.campbell@...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 12.07.13 at 10:32, Wei Liu <wei.liu2@...rix.com> wrote:
> On Mon, Jul 08, 2013 at 03:20:26PM +0100, Jan Beulich wrote:
>> >>> On 08.07.13 at 11:59, "Jan Beulich" <JBeulich@...e.com> wrote:
>> >>>> On 05.07.13 at 16:53, Wei Liu <wei.liu2@...rix.com> wrote:
>> >> 	skb->truesize += PAGE_SIZE * skb_shinfo(skb)->nr_frags;
>> >> 	skb->len += skb->data_len;
>> >>     }
>> >> 
>> >>     handle_incoming_packet();
>> >> 
>> >> You seem to be altering the behavior of the original code, because in
>> >> your patch the skb->len is incremented before use, while in the original
>> >> code (which calls skb_headlen in handle_incoming_packet) the skb->len is
>> >> correctly set.
>> > 
>> > Right. So I basically need to keep skb->len up-to-date along with
>> > ->data_len. Just handed a patch to Dion with that done; I'll defer
>> > sending a v2 for the upstream code until I know the change works
>> > for our kernel.
>> 
>> 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.
>> 
> 
> I just ported your below patch to upstream kernel and I didn't see the
> WARN_ON_ONCE. I only did iperf and netperf tests.
> 
> If the work load to trigger this bug is simple enough I can give it a
> shot...

I'm meanwhile relatively convinced that the warning isn't an effect
of the patch (final verification pending); I intend to submit v2 as
soon as 3.11-rc1 is out.

Jan

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