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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: <4d43c094-44ed-dbac-b863-48fc3d754378@redhat.com>
Date:   Mon, 29 Jul 2019 22:24:43 +0800
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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. And actually it's the price 
of using vq mutex. Actually, the critical section should be rather 
small, e.g just inside memory accessors.

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();
}


Or using smp_load_acquire()/smp_store_release() instead?

Thanks

>