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    SubjectRe: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address
    On Fri, Mar 08, 2019 at 04:58:44PM +0800, Jason Wang wrote:
    >
    > On 2019/3/8 上午3:17, Jerome Glisse wrote:
    > > On Thu, Mar 07, 2019 at 12:56:45PM -0500, Michael S. Tsirkin wrote:
    > > > On Thu, Mar 07, 2019 at 10:47:22AM -0500, Michael S. Tsirkin wrote:
    > > > > 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.
    > > > OK I missed the fact that _end actually calls
    > > > mmu_notifier_invalidate_range internally. So that part is fine but the
    > > > fact that you are trying to take page lock under VQ mutex and take same
    > > > mutex within notifier probably means it's broken for ksm and rmap at
    > > > least since these call invalidate with lock taken.
    > > >
    > > > And generally, Andrea told me offline one can not take mutex under
    > > > the notifier callback. I CC'd Andrea for why.
    > > Correct, you _can not_ take mutex or any sleeping lock from within the
    > > invalidate_range callback as those callback happens under the page table
    > > spinlock. You can however do so under the invalidate_range_start call-
    > > back only if it is a blocking allow callback (there is a flag passdown
    > > with the invalidate_range_start callback if you are not allow to block
    > > then return EBUSY and the invalidation will be aborted).
    > >
    > >
    > > > That's a separate issue from set_page_dirty when memory is file backed.
    > > If you can access file back page then i suggest using set_page_dirty
    > > from within a special version of vunmap() so that when you vunmap you
    > > set the page dirty without taking page lock. It is safe to do so
    > > always from within an mmu notifier callback if you had the page map
    > > with write permission which means that the page had write permission
    > > in the userspace pte too and thus it having dirty pte is expected
    > > and calling set_page_dirty on the page is allowed without any lock.
    > > Locking will happen once the userspace pte are tear down through the
    > > page table lock.
    >
    >
    > Can I simply can set_page_dirty() before vunmap() in the mmu notifier
    > callback, or is there any reason that it must be called within vumap()?
    >
    > Thanks


    I think this is what Jerome is saying, yes.
    Maybe add a patch to mmu notifier doc file, documenting this?


    >
    > >
    > > > It's because of all these issues that I preferred just accessing
    > > > userspace memory and handling faults. Unfortunately there does not
    > > > appear to exist an API that whitelists a specific driver along the lines
    > > > of "I checked this code for speculative info leaks, don't add barriers
    > > > on data path please".
    > > Maybe it would be better to explore adding such helper then remapping
    > > page into kernel address space ?
    > >
    > > Cheers,
    > > Jérôme

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