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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: <e0c91b89-d1e8-9831-00fe-23fe92d79fa2@redhat.com>
Date:   Wed, 24 Jul 2019 10:17:14 +0800
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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


>