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

[Pravin Shelar <pshelar@xxxxxxxxxx>]

On Fri, Mar 7, 2014 at 8:23 AM, Thomas Graf <tgraf@xxxxxxxxxx> wrote:
> On 02/28/2014 08:16 PM, Zoltan Kiss wrote:
>>
>> The kernel datapath now switched to zerocopy Netlink messages, but that
>> also
>> means that the pages on frags array are sent straight to userspace. If
>> those
>> pages came outside the kernel, we have to swap them out with local copies.
>>
>> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
>> ---
>>   net/openvswitch/datapath.c |    6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
>> index 36f8872..ffb563c 100644
>> --- a/net/openvswitch/datapath.c
>> +++ b/net/openvswitch/datapath.c
>> @@ -464,6 +464,12 @@ static int queue_userspace_packet(struct datapath
>> *dp, struct sk_buff *skb,
>>         }
>>         nla->nla_len = nla_attr_size(skb->len);
>>
>> +       if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC))) {
>> +               err = -ENOMEM;
>> +               skb_tx_error(skb);
>> +               goto out;
>> +       }
>> +
>>         skb_zerocopy(user_skb, skb, skb->len, hlen);
>>
>>         /* Pad OVS_PACKET_ATTR_PACKET if linear copy was performed */
>
>
> How about we move the call to skb_orphan_frags() into skb_zerocopy()
> itself and call it before we actually reference the frags?
>

Problem is mapping SKBTX_DEV_ZEROCOPY pages to userspace. skb_zerocopy
is not doing that.

Unless I missing something, Current netlink code can not handle
skb-frags with zero copy. So we have to copy skb anyways and no need
to orphan-frags here.
If you are planning on handling skb-frags without copying then
skb_orphan_frags should be done in netlink.

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