Re: Linux 4.14 - regression: broken tun/tap / bridge network with virtio - bisected

[Andreas Hartmann <andihartmann@01019freenet.de>]

On 12/20/2017 at 04:56 PM Andreas Hartmann wrote:
> On 12/18/2017 at 06:11 PM Andreas Hartmann wrote:
>> On 12/17/2017 at 11:33 PM Willem de Bruijn wrote:
> [...]
>>> I have been able to reproduce the hang by sending a UFO packet
>>> between two guests running v4.13 on a host running v4.15-rc1.
>>>
>>> The vhost_net_ubuf_ref refcount indeed hits overflow (-1) from
>>> vhost_zerocopy_callback being called for each segment of a
>>> segmented UFO skb. This refcount is decremented then on each
>>> segment, but incremented only once for the entire UFO skb.
>>>
>>> Before v4.14, these packets would be converted in skb_segment to
>>> regular copy packets with skb_orphan_frags and the callback function
>>> called once at this point. v4.14 added support for reference counted
>>> zerocopy skb that can pass through skb_orphan_frags unmodified and
>>> have their zerocopy state safely cloned with skb_zerocopy_clone.
>>>
>>> The call to skb_zerocopy_clone must come after skb_orphan_frags
>>> to limit cloning of this state to those skbs that can do so safely.
>>>
>>> Please try a host with the following patch. This fixes it for me. I intend to
>>> send it to net.
>>>
>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>> index a592ca025fc4..d2d985418819 100644
>>> --- a/net/core/skbuff.c
>>> +++ b/net/core/skbuff.c
>>> @@ -3654,8 +3654,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>>
>>>                 skb_shinfo(nskb)->tx_flags |= skb_shinfo(head_skb)->tx_flags &
>>>                                               SKBTX_SHARED_FRAG;
>>> -               if (skb_zerocopy_clone(nskb, head_skb, GFP_ATOMIC))
>>> -                       goto err;
>>>
>>>                 while (pos < offset + len) {
>>>                         if (i >= nfrags) {
>>> @@ -3681,6 +3679,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>>
>>>                         if (unlikely(skb_orphan_frags(frag_skb, GFP_ATOMIC)))
>>>                                 goto err;
>>> +                       if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
>>> +                               goto err;
>>>
>>>                         *nskb_frag = *frag;
>>>                         __skb_frag_ref(nskb_frag);
>>>
>>>
>>> This is relatively inefficient, as it calls skb_zerocopy_clone for each frag
>>> in the frags[] array. I will follow-up with a patch to net-next that only
>>> checks once per skb:
>>>
>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>> index 466581cf4cdc..a293a33604ec 100644
>>> --- a/net/core/skbuff.c
>>> +++ b/net/core/skbuff.c
>>> @@ -3662,7 +3662,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>>
>>>                 skb_shinfo(nskb)->tx_flags |= skb_shinfo(head_skb)->tx_flags &
>>>                                               SKBTX_SHARED_FRAG;
>>> -               if (skb_zerocopy_clone(nskb, head_skb, GFP_ATOMIC))
>>> +               if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
>>> +                   skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
>>>                         goto err;
>>>
>>>                 while (pos < offset + len) {
>>> @@ -3676,6 +3677,11 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>>
>>>                                 BUG_ON(!nfrags);
>>>
>>> +                               if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
>>> +                                   skb_zerocopy_clone(nskb, frag_skb,
>>> +                                                      GFP_ATOMIC))
>>> +                                       goto err;
>>> +
>>>                                 list_skb = list_skb->next;
>>>                         }
>>>
>>> @@ -3687,9 +3693,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>>>                                 goto err;
>>>                         }
>>>
>>> -                       if (unlikely(skb_orphan_frags(frag_skb, GFP_ATOMIC)))
>>> -                               goto err;
>>> -
>>
>> I'm currently testing this one.
>>
> 
> Test is in progress. I'm testing w/ 4.14.7, which already contains "net:
> accept UFO datagrams from tuntap and packet".
> 
> At first, I tested an unpatched 4.14.7 - the problem (no more killable
> qemu-process) did occur promptly on shutdown of the machine. This was
> expected.
> 
> Next, I applied the above patch (the second one). Until now, I didn't
> face any problem any more on shutdown of VMs. Looks promising.

Ok, I didn't face any problem any more! Many thanks for your effort and
your 2 patches to get 4.14. working again w/ qemu and virtual networks /
virtio!


Regards,
Andreas