netdev - RE: [PATCH net-next] xen-netback: fix xenvif_count_skb

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Date:	Mon, 7 Oct 2013 10:12:15 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Xi Xiong <xixiong@...zon.com>, Matt Wilson <msw@...zon.com>,
	Annie Li <annie.li@...cle.com>, Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
Subject: RE: [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@...rix.com]
> Sent: 07 October 2013 11:02
> To: Paul Durrant
> Cc: xen-devel@...ts.xen.org; netdev@...r.kernel.org; Xi Xiong; Matt Wilson;
> Annie Li; Wei Liu; Ian Campbell
> Subject: Re: [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()
> 
> On Fri, Oct 04, 2013 at 05:26:23PM +0100, Paul Durrant wrote:
> > Commit 4f0581d25827d5e864bcf07b05d73d0d12a20a5c introduced an error
> into
> > xenvif_count_skb_slots() for skbs with a linear area spanning a page
> > boundary. The alignment of skb->data needs to be taken into account, not
> > just the head length. This patch fixes the issue by dry-running the code
> > from xenvif_gop_skb() (and adjusting the comment above the function to
> note
> > that).
> >
> 
> If I'm not mistaken the change in commit 4f0581d2 is correct because we
> changed the way that the ring is packed. Now you seem to fall back to
> the original scheme (or something in between without reverting later
> other changes in that commit).
> 

It's not possible to use a single grant copy to copy to even a 2-byte linear area that spans a page boundary so you have to take into account the alignment of skb->data. How the ring is packed is not relevant.

> Do you have instruction to reproduce the bug? Can you paste some
> detailed oops message?
> 

I don't have the message to hand, but it's this BUG_ON that I hit:

	BUG_ON(npo.copy_prod > ARRAY_SIZE(vif->grant_copy_op));

I.e. we blow the grant copy op array.

  Paul
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