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

From: Zoltan Kiss <[email-masked]>
Date: Thu, 15 May 2014 11:53:10 +0100

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 <[email-masked]>
>> Signed-off-by: Zoltan Kiss <[email-masked]>
>
> Acked-by: Ian Campbell <[email-masked]>
>
> (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