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

From: Michael S. Tsirkin
Date: Thu Mar 07 2019 - 10:47:29 EST

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

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.