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

[Jason Wang <jasowang@redhat.com>]

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


>
>> 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