linux-kernel - Re: [PATCH V2 4/9] vhost: reset invalidate_count in vhost_set_vring_num

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Date:   Wed, 31 Jul 2019 21:29:28 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH V2 4/9] vhost: reset invalidate_count in
 vhost_set_vring_num_addr()


On 2019/7/31 下午8:41, Jason Gunthorpe wrote:
> On Wed, Jul 31, 2019 at 04:46:50AM -0400, Jason Wang wrote:
>> The vhost_set_vring_num_addr() could be called in the middle of
>> invalidate_range_start() and invalidate_range_end(). If we don't reset
>> invalidate_count after the un-registering of MMU notifier, the
>> invalidate_cont will run out of sync (e.g never reach zero). This will
>> in fact disable the fast accessor path. Fixing by reset the count to
>> zero.
>>
>> Reported-by: Michael S. Tsirkin <mst@...hat.com>
> Did Michael report this as well?


Correct me if I was wrong. I think it's point 4 described in 
https://lkml.org/lkml/2019/7/21/25.

Thanks


>
>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>   drivers/vhost/vhost.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 2a3154976277..2a7217c33668 100644
>> +++ b/drivers/vhost/vhost.c
>> @@ -2073,6 +2073,10 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
>>   		d->has_notifier = false;
>>   	}
>>   
>> +	/* reset invalidate_count in case we are in the middle of
>> +	 * invalidate_start() and invalidate_end().
>> +	 */
>> +	vq->invalidate_count = 0;
>>   	vhost_uninit_vq_maps(vq);
>>   #endif
>>