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SubjectRe: WARNING in __mmdrop

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 <>
>>>> Date: Fri May 24 08:12:18 2019 +0000
>>>> vhost: access vq metadata through kernel virtual address
>>>> bisection log:
>>>> start commit: 6d21a41b Add linux-next specific files for 20190718
>>>> git tree: linux-next
>>>> final crash:
>>>> console output:
>>>> kernel config:
>>>> dashboard link:
>>>> syz repro:
>>>> Reported-by:
>>>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual
>>>> address")
>>>> For information about bisection process see:
>>> 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.



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