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Date:   Tue, 23 Jul 2019 05:26:50 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
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Subject: Re: WARNING in __mmdrop
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On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote:
> 
> On 2019/7/23 下午4:10, Michael S. Tsirkin wrote:
> > On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote:
> > > On 2019/7/23 下午3:23, Michael S. Tsirkin wrote:
> > > > > > Really let's just use kfree_rcu. It's way cleaner: fire and forget.
> > > > > Looks not, you need rate limit the fire as you've figured out?
> > > > See the discussion that followed. Basically no, it's good enough
> > > > already and is only going to be better.
> > > > 
> > > > > And in fact,
> > > > > the synchronization is not even needed, does it help if I leave a comment to
> > > > > explain?
> > > > Let's try to figure it out in the mail first. I'm pretty sure the
> > > > current logic is wrong.
> > > 
> > > Here is what the code what to achieve:
> > > 
> > > - The map was protected by RCU
> > > 
> > > - Writers are: MMU notifier invalidation callbacks, file operations (ioctls
> > > etc), meta_prefetch (datapath)
> > > 
> > > - Readers are: memory accessor
> > > 
> > > Writer are synchronized through mmu_lock. RCU is used to synchronized
> > > between writers and readers.
> > > 
> > > The synchronize_rcu() in vhost_reset_vq_maps() was used to synchronized it
> > > with readers (memory accessors) in the path of file operations. But in this
> > > case, vq->mutex was already held, this means it has been serialized with
> > > memory accessor. That's why I think it could be removed safely.
> > > 
> > > Anything I miss here?
> > > 
> > So invalidate callbacks need to reset the map, and they do
> > not have vq mutex. How can they do this and free
> > the map safely? They need synchronize_rcu or kfree_rcu right?
> 
> 
> Invalidation callbacks need but file operations (e.g ioctl) not.
> 
> 
> > 
> > And I worry somewhat that synchronize_rcu in an MMU notifier
> > is a problem, MMU notifiers are supposed to be quick:
> 
> 
> Looks not, since it can allow to be blocked and lots of driver depends on
> this. (E.g mmu_notifier_range_blockable()).

Right, they can block. So why don't we take a VQ mutex and be
done with it then? No RCU tricks.

> 
> > they are on a read side critical section of SRCU.
> > 
> > If we could get rid of RCU that would be even better.
> > 
> > But now I wonder:
> > 	invalidate_start has to mark page as dirty
> > 	(this is what my patch added, current code misses this).
> 
> 
> Nope, current code did this but not the case when map need to be invalidated
> in the vhost control path (ioctl etc).
> 
> 
> > 
> > 	at that point kernel can come and make the page clean again.
> > 
> > 	At that point VQ handlers can keep a copy of the map
> > 	and change the page again.
> 
> 
> We will increase invalidate_count which prevent the page being used by map.
> 
> Thanks

OK I think I got it, thanks!


> 
> > 
> > 
> > At this point I don't understand how we can mark page dirty
> > safely.
> > 
> > > > > > > Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(),
> > > > > > > (just a little bit more hard to trigger):
> > > > > > AFAIK these never run in response to guest events.
> > > > > > So they can take very long and guests still won't crash.
> > > > > What if guest manages to escape to qemu?
> > > > > 
> > > > > Thanks
> > > > Then it's going to be slow. Why do we care?
> > > > What we do not want is synchronize_rcu that guest is blocked on.
> > > > 
> > > Ok, this looks like that I have some misunderstanding here of the reason why
> > > synchronize_rcu() is not preferable in the path of ioctl. But in kvm case,
> > > if rcu_expedited is set, it can triggers IPIs AFAIK.
> > > 
> > > Thanks
> > > 
> > Yes, expedited is not good for something guest can trigger.
> > Let's just use kfree_rcu if we can. Paul said even though
> > documentation still says it needs to be rate-limited, that
> > part is basically stale and will get updated.
> >