On 2019/7/23 下午6:27, Michael S. Tsirkin wrote:

quoted

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

quoted

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


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