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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: <9a9a7ac0-2657-ff09-2644-f8d7ae0f9222@redhat.com>
Date:   Tue, 23 Jul 2019 15:55:47 +0800
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On 2019/7/23 下午3:25, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 01:48:52PM +0800, Jason Wang wrote:
>> 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
> Oh I didn't know one can push a test to syzbot and get back
> a result. How does one do that?


See here https://github.com/google/syzkaller/blob/master/docs/syzbot.md

Just reply this thread by attaching a fix with command like: "#syz test: 
git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git 
7f466032dc9e5a61217f22ea34b2df932786bbfc"

Btw, I've let syzbot test you patch, and it passes.

Thanks


>