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 15:37:34 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Jason Wang <jasowang@...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 Wed, Jul 31, 2019 at 04:32:52PM -0300, Jason Gunthorpe wrote:
> On Wed, Jul 31, 2019 at 09:29:28PM +0800, Jason Wang wrote:
> > 
> > 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.
> 
> I'm not sure what that is talking about
> 
> But this fixes what I described:
> 
> https://lkml.org/lkml/2019/7/22/554
> 
> Jason

These are two reasons for a possible counter imbalance.
Unsurprisingly they are both fixed if you reset the counter to 0.

-- 
MST