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Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
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From:   Jason Wang <jasowang@redhat.com>
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Date:   Tue, 23 Jul 2019 11:55:28 +0800
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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


>
>