Re: WARNING in __mmdrop

[Jason Wang <jasowang@redhat.com>]

On 2019/7/23 下午3:56, 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.
> Another thing I like about the patch I posted is that
> it removes 60 lines of code, instead of adding more :)
> Mostly because of unifying everything into
> a single cleanup function and using kfree_rcu.


Yes.


>
> So how about this: do exactly what you propose but as a 2 patch series:
> start with the slow safe patch, and add then return uaddr optimizations
> on top. We can then more easily reason about whether they are safe.


If you stick, I can do this.


> Basically you are saying this:
> 	- notifiers are only needed to invalidate maps
> 	- we make sure any uaddr change invalidates maps anyway
> 	- thus it's ok not to have notifiers since we do
> 	  not have maps
>
> All this looks ok but the question is why do we
> bother unregistering them. And the answer seems to
> be that this is so we can start with a balanced
> counter: otherwise we can be between _start and
> _end calls.


Yes, since there could be multiple co-current invalidation requests. We 
need count them to make sure we don't pin wrong pages.


>
> I also wonder about ordering. kvm has this:
>         /*
>           * Used to check for invalidations in progress, of the pfn that is
>           * returned by pfn_to_pfn_prot below.
>           */
>          mmu_seq = kvm->mmu_notifier_seq;
>          /*
>           * Ensure the read of mmu_notifier_seq isn't reordered with PTE reads in
>           * gfn_to_pfn_prot() (which calls get_user_pages()), so that we don't
>           * risk the page we get a reference to getting unmapped before we have a
>           * chance to grab the mmu_lock without mmu_notifier_retry() noticing.
>           *
>           * This smp_rmb() pairs with the effective smp_wmb() of the combination
>           * of the pte_unmap_unlock() after the PTE is zapped, and the
>           * spin_lock() in kvm_mmu_notifier_invalidate_<page|range_end>() before
>           * mmu_notifier_seq is incremented.
>           */
>          smp_rmb();
>
> does this apply to us? Can't we use a seqlock instead so we do
> not need to worry?


I'm not familiar with kvm MMU internals, but we do everything under of 
mmu_lock.

Thanks