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

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

On 12/20/2017 at 11:44 PM Willem de Bruijn wrote:
> On Wed, Dec 20, 2017 at 10:56 AM, Andreas Hartmann
> <andihartmann@01019freenet.de> 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.
> 
> Thanks for testing.
> 
> I sent the first, simpler, one to net together with another fix.
> 
>    http://patchwork.ozlabs.org/patch/851715/
> 

If I'm using the second patch above (the more efficient one and not 
"[net,1/2] skbuff: orphan frags before zerocopy clone"), which I'm 
already testing here: Is it still necessary to apply this patch 
"[net,2/2] skbuff: skb_copy_ubufs must release uarg even without user 
frags"?


Thanks,
Andreas