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Re: [Xen-devel] [PATCH] openvswitch: Orphan frags before sending to userspace via Netlink to avoid guest stall

On Fri, Mar 7, 2014 at 7:58 AM, Thomas Graf <tgraf@xxxxxxxxxx> wrote:
> On 03/07/2014 05:46 AM, Pravin Shelar wrote:
>> But I found bug in datapath user-space queue code. I am not sure how
>> this can work with skb fragments and MMAP-netlink socket.
>> Here is what happens, OVS allocates netlink skb and adds fragments to
>> skb using skb_zero_copy(), then calls genlmsg_unicast().
>> But if netlink sock is mmped then netlink-send queues netlink
>> allocated skb->head (linear data of skb) and ignore skb frags.
>> Currently this is not problem with OVS vswitchd since it does not use
>> netlink MMAP sockets. But if vswitchd stats using MMAP-netlink socket,
>> it can break it.
> The secret is out ;-)
> I was very surprised too when I noticed that it worked. It's not just
> OVS, it's nfqueue as well. The reason is that an netlink mmaped skb is
> setup with a giant tailroom in netlink_ring_setup_skb():
>         skb->end        = skb->tail + size;
For OVS use-case, the size is linear part of skb. so I think for
mmap-netlink socket it will fail.

> and skb_zerocopy() will consume whatever tailroom is available first:
>         /* dont bother with small payloads */
>         if (len <= skb_tailroom(to)) {
>                 skb_copy_bits(from, 0, skb_put(to, len), len);
>                 return;
>         }
> I was planning to fix this while adding GSO support to the upcall as
> that is the moment when this bug would really surface.


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