netdev - Re: [PATCH net] tuntap: raise EPOLLOUT on device up

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Date:   Sat, 19 May 2018 09:09:11 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] tuntap: raise EPOLLOUT on device up



On 2018年05月18日 22:46, Michael S. Tsirkin wrote:
> On Fri, May 18, 2018 at 10:11:54PM +0800, Jason Wang wrote:
>>
>> On 2018年05月18日 22:06, Michael S. Tsirkin wrote:
>>> On Fri, May 18, 2018 at 10:00:31PM +0800, Jason Wang wrote:
>>>> On 2018年05月18日 21:26, Jason Wang wrote:
>>>>> On 2018年05月18日 21:13, Michael S. Tsirkin wrote:
>>>>>> On Fri, May 18, 2018 at 09:00:43PM +0800, Jason Wang wrote:
>>>>>>> We return -EIO on device down but can not raise EPOLLOUT after it was
>>>>>>> up. This may confuse user like vhost which expects tuntap to raise
>>>>>>> EPOLLOUT to re-enable its TX routine after tuntap is down. This could
>>>>>>> be easily reproduced by transmitting packets from VM while down and up
>>>>>>> the tap device. Fixing this by set SOCKWQ_ASYNC_NOSPACE on -EIO.
>>>>>>>
>>>>>>> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
>>>>>>> Cc: Eric Dumazet <edumazet@...gle.com>
>>>>>>> Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
>>>>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>>>>>> ---
>>>>>>>     drivers/net/tun.c | 4 +++-
>>>>>>>     1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>>>>>> index d45ac37..1b29761 100644
>>>>>>> --- a/drivers/net/tun.c
>>>>>>> +++ b/drivers/net/tun.c
>>>>>>> @@ -1734,8 +1734,10 @@ static ssize_t tun_get_user(struct
>>>>>>> tun_struct *tun, struct tun_file *tfile,
>>>>>>>         int skb_xdp = 1;
>>>>>>>         bool frags = tun_napi_frags_enabled(tun);
>>>>>>>     -    if (!(tun->dev->flags & IFF_UP))
>>>>>>> +    if (!(tun->dev->flags & IFF_UP)) {
>>>>>> Isn't this racy?  What if flag is cleared at this point?
>>>>> I think you mean "set at this point"? Then yes, so we probably need to
>>>>> set the bit during tun_net_close().
>>>>>
>>>>> Thanks
>>>> Looks no need, vhost will poll socket after it see EIO. So we are ok here?
>>>>
>>>> Thanks
>>> In fact I don't even understand why does this help any longer.
>>>
>> We disable tx polling and only enable it on demand for a better rx
>> performance. You may want to have a look at :
>>
>> commit feb8892cb441c742d4220cf7ced001e7fa070731
>> Author: Jason Wang <jasowang@...hat.com>
>> Date:   Mon Nov 13 11:45:34 2017 +0800
>>
>>      vhost_net: conditionally enable tx polling
>>
>> Thanks
>
> Question is, what looks at SOCKWQ_ASYNC_NOSPACE.
> I think it's tested when packet is transmitted,
> but there is no guarantee here any packet will
> ever be transmitted.
>

Well, actually, I do plan to disable vq polling from the beginning. But 
looks like you do not want this:

See https://patchwork.kernel.org/patch/10034025/

Thanks