Re: WARNING in __mmdrop

[Jason Wang]

On 2019/7/23 äå6:27, 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:

ÂÂÂÂÂÂÂ spin_lock(&vq->mmu_lock);

    ...

ÂÂÂÂÂÂÂ err = -EFAULT;
ÂÂÂÂÂÂÂ if (vq->invalidate_count)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto err;

ÂÂÂÂÂÂÂ ...

ÂÂÂÂÂÂÂ npinned = __get_user_pages_fast(uaddr->uaddr, npages,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ uaddr->write, pages);

ÂÂÂÂÂÂÂ ...

ÂÂÂ ÂÂÂ spin_unlock(&vq->mmu_lock);

Is this not sufficient?

Thanks