Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | 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
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