Re: [Xen-devel] [PATCH net v4] xen-netback: Fix grant ref resolution in RX path

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 15/05/14 11:37, Ian Campbell wrote:
> On Thu, 2014-05-15 at 11:08 +0100, Zoltan Kiss wrote:
> > The original series for reintroducing grant mapping for netback had a patch [1]
> > to handle receiving of packets from an another VIF. Grant copy on the receiving
> > side needs the grant ref of the page to set up the op.
> > The original patch assumed (wrongly) that the frags array haven't changed. In
> > the case reported by Sander, the sending guest sent a packet where the linear
> > buffer and the first frag were under PKT_PROT_LEN (=128) bytes.
> > xenvif_tx_submit() then pulled up the linear area to 128 bytes, and ditched the
> > first frag. The receiving side had an off-by-one problem when gathered the grant
> > refs.
> > This patch fixes that by checking whether the actual frag's page pointer is the
> > same as the page in the original frag list. It can handle any kind of changes on
> > the original frags array, like:
> > - removing granted frags from the array at any point
> > - adding local pages to the frags list anywhere
> > - reordering the frags
> > It's optimized to the most common case, when there is 1:1 relation between the
> > frags and the list, plus works optimal when frags are removed from the end or
> > the beginning.
> >
> > [1]: 3e2234: xen-netback: Handle foreign mapped pages on the guest RX path
> >
> > Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> > Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
>
> (Remind me, do we need both this and Eric's fix, or is it either/or?)
When you say Eric's fix, which one do you talk about? The one for 
skb_try_coalesce? That's not too burning, at the moment it is only used 
from the IP layer and above, as far as I can tell, and zerocopy skbs get 
copied to local pages in deliver_skb.
Zoli

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