netdev - Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

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Date:   Thu, 7 Mar 2019 10:47:22 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, linux-mm@...ck.org, aarcange@...hat.com
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address

On Wed, Mar 06, 2019 at 02:18:12AM -0500, Jason Wang wrote:
> +static const struct mmu_notifier_ops vhost_mmu_notifier_ops = {
> +	.invalidate_range = vhost_invalidate_range,
> +};
> +
>  void vhost_dev_init(struct vhost_dev *dev,
>  		    struct vhost_virtqueue **vqs, int nvqs, int iov_limit)
>  {

I also wonder here: when page is write protected then
it does not look like .invalidate_range is invoked.

E.g. mm/ksm.c calls

mmu_notifier_invalidate_range_start and
mmu_notifier_invalidate_range_end but not mmu_notifier_invalidate_range.

Similarly, rmap in page_mkclean_one will not call
mmu_notifier_invalidate_range.

If I'm right vhost won't get notified when page is write-protected since you
didn't install start/end notifiers. Note that end notifier can be called
with page locked, so it's not as straight-forward as just adding a call.
Writing into a write-protected page isn't a good idea.

Note that documentation says:
	it is fine to delay the mmu_notifier_invalidate_range
	call to mmu_notifier_invalidate_range_end() outside the page table lock.
implying it's called just later.

-- 
MST