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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Thu, Dec 14, 2017 at 5:17 PM, Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>>> Well, the patch does not fix hanging VMs, which have been shutdown and
>>> can't be killed any more.
>>> Because of the stack trace
>>>
>>> [<ffffffffc0d0e3c5>] vhost_net_ubuf_put_and_wait+0x35/0x60 [vhost_net]
>>> [<ffffffffc0d0f264>] vhost_net_ioctl+0x304/0x870 [vhost_net]
>>> [<ffffffff9b25460f>] do_vfs_ioctl+0x8f/0x5c0
>>> [<ffffffff9b254bb4>] SyS_ioctl+0x74/0x80
>>> [<ffffffff9b00365b>] do_syscall_64+0x5b/0x100
>>> [<ffffffff9b78e7ab>] entry_SYSCALL64_slow_path+0x25/0x25
>>> [<ffffffffffffffff>] 0xffffffffffffffff
>>>
>>> I was hoping, that the problems could be related - but that seems not to
>>> be true.
>>
>> However, it turned out, that reverting the complete patchset "Remove UDP
>> Fragmentation Offload support" prevent hanging qemu processes.
>
> That implies a combination of UFO and vhost zerocopy. Disabling
> experimental_zcopytx in vhost_net will probably work around the bug
> then.
>
> On the surface the two features are independent. Most of the relevant
> UFO code is reverted with the patch mentioned earlier. Missing from
> that is protocol stack support, but it is unlikely that your host OS is
> generating these UFO packets.
>
> They are coming from a guest over virtio_net, to which vhost_net then
> applies zerocopy. Then the packet(s) is/are either freed without calling
> uarg->callback() or queued somewhere for a very long time.
>
> Looking at the diff-of-diffs between my stable patch and your full revert,
> the majority of missing bits beside the procol layer is in device driver
> support. Removing that causes the UFO packets to be segmented at any
> dev_queue_xmit on their path. skb_segment ensures that when it segments
> a large zerocopy packet, all new segments also point to the zerocopy
> callback struct (ubuf_info), as the shared memory pages may not be
> released until all skbs pointing to them are freed.
>
> That may be wrong with vhost_zerocopy_callback, which does not use
> refcounting. I will look into that. It may be that before the msg_zerocopy
> patchsets large packets were copied before entering segmentation. It is
> safe to enter segmentation for msg_zerocopy skbs, but not legacy zerocopy
> skbs.

If this is the cause, then the following, while not a real solution, would
probably also solve resolve the observed issue.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index e140ba49b30a..8fe5bca1d6ae 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3655,10 +3655,10 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
                skb_copy_from_linear_data_offset(head_skb, offset,
                                                 skb_put(nskb, hsize), hsize);

+               if (unlikely(skb_orphan_frags_rx(head_skb, GFP_ATOMIC)))
+                       goto err;
                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;

This basically converts zerocopy TSO skbs to regular and calls their
uarg->callback just before segmenting them.