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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>
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Date:   Tue, 23 Jul 2019 21:34:29 +0800
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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