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

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
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.


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