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

[Jason Wang <jasowang@redhat.com>]

On 2019/3/8 下午10:58, Jerome Glisse wrote:
> On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote:
>> On 2019/3/8 上午3:16, Andrea Arcangeli 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.
>>> Yes this lock inversion needs more thoughts.
>>>
>>>> And generally, Andrea told me offline one can not take mutex under
>>>> the notifier callback. I CC'd Andrea for why.
>>> Yes, the problem then is the ->invalidate_page is called then under PT
>>> lock so it cannot take mutex, you also cannot take the page_lock, it
>>> can at most take a spinlock or trylock_page.
>>>
>>> So it must switch back to the _start/_end methods unless you rewrite
>>> the locking.
>>>
>>> The difference with _start/_end, is that ->invalidate_range avoids the
>>> _start callback basically, but to avoid the _start callback safely, it
>>> has to be called in between the ptep_clear_flush and the set_pte_at
>>> whenever the pfn changes like during a COW. So it cannot be coalesced
>>> in a single TLB flush that invalidates all sptes in a range like we
>>> prefer for performance reasons for example in KVM. It also cannot
>>> sleep.
>>>
>>> In short ->invalidate_range must be really fast (it shouldn't require
>>> to send IPI to all other CPUs like KVM may require during an
>>> invalidate_range_start) and it must not sleep, in order to prefer it
>>> to _start/_end.
>>>
>>> I.e. the invalidate of the secondary MMU that walks the linux
>>> pagetables in hardware (in vhost case with GUP in software) has to
>>> happen while the linux pagetable is zero, otherwise a concurrent
>>> hardware pagetable lookup could re-instantiate a mapping to the old
>>> page in between the set_pte_at and the invalidate_range_end (which
>>> internally calls ->invalidate_range). Jerome documented it nicely in
>>> Documentation/vm/mmu_notifier.rst .
>>
>> Right, I've actually gone through this several times but some details were
>> missed by me obviously.
>>
>>
>>> Now you don't really walk the pagetable in hardware in vhost, but if
>>> you use gup_fast after usemm() it's similar.
>>>
>>> For vhost the invalidate would be really fast, there are no IPI to
>>> deliver at all, the problem is just the mutex.
>>
>> Yes. A possible solution is to introduce a valid flag for VA. Vhost may only
>> try to access kernel VA when it was valid. Invalidate_range_start() will
>> clear this under the protection of the vq mutex when it can block. Then
>> invalidate_range_end() then can clear this flag. An issue is blockable is
>> always false for range_end().
>>
> Note that there can be multiple asynchronous concurrent invalidate_range
> callbacks. So a flag does not work but a counter of number of active
> invalidation would. See how KSM is doing it for instance in kvm_main.c
>
> The pattern for this kind of thing is:
>      my_invalidate_range_start(start,end) {
>          ...
>          if (mystruct_overlap(mystruct, start, end)) {
>              mystruct_lock();
>              mystruct->invalidate_count++;
>              ...
>              mystruct_unlock();
>          }
>      }
>
>      my_invalidate_range_end(start,end) {
>          ...
>          if (mystruct_overlap(mystruct, start, end)) {
>              mystruct_lock();
>              mystruct->invalidate_count--;
>              ...
>              mystruct_unlock();
>          }
>      }
>
>      my_access_va(mystruct) {
>      again:
>          wait_on(!mystruct->invalidate_count)
>          mystruct_lock();
>          if (mystruct->invalidate_count) {
>              mystruct_unlock();
>              goto again;
>          }
>          GUP();
>          ...
>          mystruct_unlock();
>      }
>
> Cheers,
> Jérôme


Yes, this should work.

Thanks