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