Re: [Xen-devel] [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Paul Durrant
> Sent: 07 October 2013 11:24
> To: David Vrabel
> Cc: Wei Liu; Ian Campbell; netdev@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxx;
> Annie Li; Matt Wilson; Xi Xiong
> Subject: Re: [Xen-devel] [PATCH net-next] xen-netback: fix
> xenvif_count_skb_slots()
> 
> > -----Original Message-----
> > From: David Vrabel
> > Sent: 07 October 2013 10:50
> > To: Paul Durrant
> > Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; Ian
> Campbell;
> > Annie Li; Matt Wilson; Xi Xiong
> > Subject: Re: [Xen-devel] [PATCH net-next] xen-netback: fix
> > xenvif_count_skb_slots()
> >
> > On 04/10/13 17:26, 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 4f0581d2582 is causing the skb->data to be fully packed into a
> > minimal number of slots then the simple
> > DIV_ROUND_UP(skb_headlen(skb))
> > is correct.
> >
> > I think this change will miscount in the number of slots,
> > over-estimating the count which I think will eventually cause netback to
> > think the ring has no space when it has some.
> >
> > Is the problem here not the miscounting of slots but running out of
> > space in the grant table op array because we know use more copy ops?
> >
> 
> Essentially yes. Netback is built on the assumption of no more than two grant
> copies per ring slot.
> 

To be clear; I believe that, with the packing change, a third grant copy may be 
used for the initial slot and that is why we blow the array.

  Paul

> > I didn't think there was any real merit in the problematic commit (or at
> > least there was no evidence that it was better) so I would suggest just
> > reverting it instead of trying to fix it up.
> >
> 
> I'd be happy with a reversion.
> 
> > If we do want to change how netback fills the ring then netback needs
> > some redesign (i.e., change it so it doesn't have to this counting in
> > advance) to make it much less fragile to changes in this area.
> >
> 
> Yes, that would be much better.
> 
>   Paul
> 
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