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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: <3dfa2269-60ba-7dd8-99af-5aef8552bd98@redhat.com>
Date:   Wed, 24 Jul 2019 18:08:05 +0800
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On 2019/7/24 下午4:05, Michael S. Tsirkin wrote:
> On Wed, Jul 24, 2019 at 10:17:14AM +0800, Jason Wang wrote:
>> On 2019/7/23 下午11:02, Michael S. Tsirkin wrote:
>>> On Tue, Jul 23, 2019 at 09:34:29PM +0800, Jason Wang wrote:
>>>> On 2019/7/23 下午6:27, Michael S. Tsirkin wrote:
>>>>>> 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
>>>>> I don't think this helps at all.
>>>>>
>>>>> There's no lock between checking the invalidate counter and
>>>>> get user pages fast within vhost_map_prefetch. So it's possible
>>>>> that get user pages fast reads PTEs speculatively before
>>>>> invalidate is read.
>>>>>
>>>>> -- 
>>>> In vhost_map_prefetch() we do:
>>>>
>>>>           spin_lock(&vq->mmu_lock);
>>>>
>>>>           ...
>>>>
>>>>           err = -EFAULT;
>>>>           if (vq->invalidate_count)
>>>>                   goto err;
>>>>
>>>>           ...
>>>>
>>>>           npinned = __get_user_pages_fast(uaddr->uaddr, npages,
>>>>                                           uaddr->write, pages);
>>>>
>>>>           ...
>>>>
>>>>           spin_unlock(&vq->mmu_lock);
>>>>
>>>> Is this not sufficient?
>>>>
>>>> Thanks
>>> So what orders __get_user_pages_fast wrt invalidate_count read?
>>
>> So in invalidate_end() callback we have:
>>
>> spin_lock(&vq->mmu_lock);
>> --vq->invalidate_count;
>>          spin_unlock(&vq->mmu_lock);
>>
>>
>> So even PTE is read speculatively before reading invalidate_count (only in
>> the case of invalidate_count is zero). The spinlock has guaranteed that we
>> won't read any stale PTEs.
>>
>> Thanks
> I'm sorry I just do not get the argument.
> If you want to order two reads you need an smp_rmb
> or stronger between them executed on the same CPU.
>
> Executing any kind of barrier on another CPU
> will have no ordering effect on the 1st one.
>
>
> So if CPU1 runs the prefetch, and CPU2 runs invalidate
> callback, read of invalidate counter on CPU1 can bypass
> read of PTE on CPU1 unless there's a barrier
> in between, and nothing CPU2 does can affect that outcome.
>
>
> What did I miss?


It doesn't harm if PTE is read before invalidate_count, this is because:

1) This speculation is serialized with invalidate_range_end() because of 
the spinlock

2) This speculation can only make effect when we read invalidate_count 
as zero.

3) This means the speculation is done after the last 
invalidate_range_end() and because of the spinlock, when we enter the 
critical section of spinlock in prefetch, we can not see any stale PTE 
that was unmapped before.

Am I wrong?

Thanks