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