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Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <124be1a2-1c53-8e65-0f06-ee2294710822@redhat.com>
Date:   Tue, 23 Jul 2019 13:48:52 +0800
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On 2019/7/23 下午1:02, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 11:55:28AM +0800, Jason Wang wrote:
>> On 2019/7/22 下午4:02, Michael S. Tsirkin wrote:
>>> On Mon, Jul 22, 2019 at 01:21:59PM +0800, Jason Wang wrote:
>>>> On 2019/7/21 下午6:02, Michael S. Tsirkin wrote:
>>>>> On Sat, Jul 20, 2019 at 03:08:00AM -0700, syzbot wrote:
>>>>>> syzbot has bisected this bug to:
>>>>>>
>>>>>> commit 7f466032dc9e5a61217f22ea34b2df932786bbfc
>>>>>> Author: Jason Wang <jasowang@redhat.com>
>>>>>> Date:   Fri May 24 08:12:18 2019 +0000
>>>>>>
>>>>>>        vhost: access vq metadata through kernel virtual address
>>>>>>
>>>>>> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=149a8a20600000
>>>>>> start commit:   6d21a41b Add linux-next specific files for 20190718
>>>>>> git tree:       linux-next
>>>>>> final crash:    https://syzkaller.appspot.com/x/report.txt?x=169a8a20600000
>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=129a8a20600000
>>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=3430a151e1452331
>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=e58112d71f77113ddb7b
>>>>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10139e68600000
>>>>>>
>>>>>> Reported-by: syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com
>>>>>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual
>>>>>> address")
>>>>>>
>>>>>> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>>>>> OK I poked at this for a bit, I see several things that
>>>>> we need to fix, though I'm not yet sure it's the reason for
>>>>> the failures:
>>>>>
>>>>>
>>>>> 1. mmu_notifier_register shouldn't be called from vhost_vring_set_num_addr
>>>>>       That's just a bad hack,
>>>> This is used to avoid holding lock when checking whether the addresses are
>>>> overlapped. Otherwise we need to take spinlock for each invalidation request
>>>> even if it was the va range that is not interested for us. This will be very
>>>> slow e.g during guest boot.
>>> KVM seems to do exactly that.
>>> I tried and guest does not seem to boot any slower.
>>> Do you observe any slowdown?
>>
>> Yes I do.
>>
>>
>>> Now I took a hard look at the uaddr hackery it really makes
>>> me nervious. So I think for this release we want something
>>> safe, and optimizations on top. As an alternative revert the
>>> optimization and try again for next merge window.
>>
>> Will post a series of fixes, let me know if you're ok with that.
>>
>> Thanks
> I'd prefer you to take a hard look at the patch I posted
> which makes code cleaner,


I did. But it looks to me a series that is only about 60 lines of code 
can fix all the issues we found without reverting the uaddr optimization.


>   and ad optimizations on top.
> But other ways could be ok too.


I'm waiting for the test result from syzbot and will post. Let's see if 
you are OK with that.

Thanks


>>>