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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>
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Date:   Mon, 29 Jul 2019 13:54:49 +0800
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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