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

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

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On 12/08/2017 at 09:44 PM Andreas Hartmann wrote:
> On 12/08/2017 at 09:11 PM Andreas Hartmann wrote:
>> On 12/08/2017 at 05:04 PM Willem de Bruijn wrote:
>>> On Fri, Dec 8, 2017 at 6:40 AM, Michal Kubecek <mkubecek@suse.cz> wrote:
>>>> On Fri, Dec 08, 2017 at 11:31:50AM +0100, Andreas Hartmann wrote:
>>>>> On 12/08/2017 at 09:47 AM Michal Kubecek wrote:
>>>>>> On Fri, Dec 08, 2017 at 08:21:16AM +0100, Andreas Hartmann wrote:
>>>>>>>
>>>>>>> All my VMs are using virtio_net. BTW: I couldn't see the problems
>>>>>>> (sometimes, the VM couldn't be stopped at all) if all my VMs are using
>>>>>>> e1000 as interface instead.
>>>>>>>
>>>>>>> This finding now matches pretty much the responsible UDP-package which
>>>>>>> caused the stall. I already mentioned it here [2].
>>>>>>>
>>>>>>> To prove it, I reverted from the patch series "[PATCH v2 RFC 0/13]
>>>>>>> Remove UDP Fragmentation Offload support" [3]
>>>>>>>
>>>>>>> 11/13 [v2,RFC,11/13] net: Remove all references to SKB_GSO_UDP. [4]
>>>>>>> 12/13 [v2,RFC,12/13] inet: Remove software UFO fragmenting code. [5]
>>>>>>> 13/13 [v2,RFC,13/13] net: Kill NETIF_F_UFO and SKB_GSO_UDP. [6]
>>>>>>>
>>>>>>> and applied it to Linux 4.14.4. It compiled fine and is running fine.
>>>>>>> The vnet doesn't die anymore. Yet, I can't say if the qemu stop hangs
>>>>>>> are gone, too.
>>>>>>>
>>>>>>> Obviously, there is something broken with the new UDP handling. Could
>>>>>>> you please analyze this problem? I could test some more patches ... .
>>>>>>
>>>>>> Any chance your VMs were live migrated from pre-4.14 host kernel?
>>>>>
>>>>> No - the VMs are not live migrated. They are always running on the same
>>>>> host - either with kernel < 4.14 or with kernel 4.14.x.
>>>>
>>>> This is disturbing... unless I'm mistaken, it shouldn't be possible to
>>>> have UFO enabled on a virtio device in a VM booted on a host with 4.14
>>>> kernel.
>>>
>>> Indeed. When working on that revert patch I verified that UFO in
>>> the guest virtio_net was off before the revert patch, on after.
>>>
>>> Qemu should check host support with tap_probe_has_ufo
>>> before advertising support to the guest. Indeed, this is exactly
>>> what broke live migration in virtio_net_load_device at
>>>
>>>     if (qemu_get_byte(f) && !peer_has_ufo(n)) {
>>>         error_report("virtio-net: saved image requires TUN_F_UFO support");
>>>         return -1;
>>>     }
>>>
>>> Which follows
>>>
>>>    peer_has_ufo
>>>      qemu_has_ufo
>>>        tap_has_ufo
>>>          s->has_ufo
>>>
>>> where s->has_ufo was set by tap_probe_has_ufo in net_tap_fd_init.
>>>
>>> Now, checking my qemu git branch, I ran pretty old 2.7.0-rc3. But this
>>> codepath does not seem to have changed between then and 2.10.1.
>>>
>>> I cherry-picked the revert onto 4.14.3. It did not apply cleanly, but the
>>> fix-up wasn't too hard. Compiled and booted, but untested otherwise. At
>>>
>>>   https://github.com/wdebruij/linux/commits/v4.14.3-aargh-ufo
>>
>> I'm just running it at the moment. I didn't face any network hang until
>> now - although the critical UDP packages have been gone through.
>> Therefore: looks nice.
> 
> 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. I tested
today and the whole weekend - I didn't face any problem. Before that, I
could see nearly immediately hanging qemu processes after shutdown w/
libvirt.

Tested w/ 4.14.4, qemu 2.6.2 and libvirt 2.0.0 and 4 VMs.

I'll be back if the problem comes up again while the patchset is reverted.


Thanks,
Andreas

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