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Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <421a1af6-df06-e4a6-b34f-526ac123bc4a@redhat.com>
Date:   Wed, 31 Jul 2019 16:49:32 +0800
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On 2019/7/30 下午11:08, Michael S. Tsirkin wrote:
> On Tue, Jul 30, 2019 at 03:44:47PM +0800, Jason Wang wrote:
>> On 2019/7/29 下午10:44, Michael S. Tsirkin wrote:
>>> On Mon, Jul 29, 2019 at 10:24:43PM +0800, Jason Wang wrote:
>>>> On 2019/7/29 下午4:59, Michael S. Tsirkin wrote:
>>>>> On Mon, Jul 29, 2019 at 01:54:49PM +0800, Jason Wang wrote:
>>>>>> On 2019/7/26 下午9:49, Michael S. Tsirkin wrote:
>>>>>>>>> Ok, let me retry if necessary (but I do remember I end up with deadlocks
>>>>>>>>> last try).
>>>>>>>> Ok, I play a little with this. And it works so far. Will do more testing
>>>>>>>> tomorrow.
>>>>>>>>
>>>>>>>> One reason could be I switch to use get_user_pages_fast() to
>>>>>>>> __get_user_pages_fast() which doesn't need mmap_sem.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>> OK that sounds good. If we also set a flag to make
>>>>>>> vhost_exceeds_weight exit, then I think it will be all good.
>>>>>> After some experiments, I came up two methods:
>>>>>>
>>>>>> 1) switch to use vq->mutex, then we must take the vq lock during range
>>>>>> checking (but I don't see obvious slowdown for 16vcpus + 16queues). Setting
>>>>>> flags during weight check should work but it still can't address the worst
>>>>>> case: wait for the page to be swapped in. Is this acceptable?
>>>>>>
>>>>>> 2) using current RCU but replace synchronize_rcu() with vhost_work_flush().
>>>>>> The worst case is the same as 1) but we can check range without holding any
>>>>>> locks.
>>>>>>
>>>>>> Which one did you prefer?
>>>>>>
>>>>>> Thanks
>>>>> I would rather we start with 1 and switch to 2 after we
>>>>> can show some gain.
>>>>>
>>>>> But the worst case needs to be addressed.
>>>> Yes.
>>>>
>>>>
>>>>> How about sending a signal to
>>>>> the vhost thread?  We will need to fix up error handling (I think that
>>>>> at the moment it will error out in that case, handling this as EFAULT -
>>>>> and we don't want to drop packets if we can help it, and surely not
>>>>> enter any error states.  In particular it might be especially tricky if
>>>>> we wrote into userspace memory and are now trying to log the write.
>>>>> I guess we can disable the optimization if log is enabled?).
>>>> This may work but requires a lot of changes.
>>> I agree.
>>>
>>>> And actually it's the price of
>>>> using vq mutex.
>>> Not sure what's meant here.
>>
>> I mean if we use vq mutex, it means the critical section was increased and
>> we need to deal with swapping then.
>>
>>
>>>> Actually, the critical section should be rather small, e.g
>>>> just inside memory accessors.
>>> Also true.
>>>
>>>> I wonder whether or not just do synchronize our self like:
>>>>
>>>> static void inline vhost_inc_vq_ref(struct vhost_virtqueue *vq)
>>>> {
>>>>           int ref = READ_ONCE(vq->ref);
>>>>
>>>>           WRITE_ONCE(vq->ref, ref + 1);
>>>> smp_rmb();
>>>> }
>>>>
>>>> static void inline vhost_dec_vq_ref(struct vhost_virtqueue *vq)
>>>> {
>>>>           int ref = READ_ONCE(vq->ref);
>>>>
>>>> smp_wmb();
>>>>           WRITE_ONCE(vq->ref, ref - 1);
>>>> }
>>>>
>>>> static void inline vhost_wait_for_ref(struct vhost_virtqueue *vq)
>>>> {
>>>>           while (READ_ONCE(vq->ref));
>>>> mb();
>>>> }
>>> Looks good but I'd like to think of a strategy/existing lock that let us
>>> block properly as opposed to spinning, that would be more friendly to
>>> e.g. the realtime patch.
>>
>> Does it make sense to disable preemption in the critical section? Then we
>> don't need to block and we have a deterministic time spent on memory
>> accssors?
> Hmm maybe. I'm getting really nervious at this point - we
> seem to be using every trick in the book.
>

Yes, looking at the synchronization implemented by other MMU notifiers. 
Vhost is even the simplest.


>>>> Or using smp_load_acquire()/smp_store_release() instead?
>>>>
>>>> Thanks
>>> These are cheaper on x86, yes.
>>
>> Will use this.
>>
>> Thanks
>>
>>
> This looks suspiciously like a seqlock though.
> Can that be used somehow?
>

seqlock does not provide a way to synchronize with readers. But I did 
borrow some ideas from seqlock and post a new version.

Please review.

Thanks