Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker

[Jason Gunthorpe]

On Sat, Aug 03, 2019 at 05:36:13PM -0400, Michael S. Tsirkin wrote:
> On Fri, Aug 02, 2019 at 02:24:18PM -0300, Jason Gunthorpe wrote:
> > On Fri, Aug 02, 2019 at 10:27:21AM -0400, Michael S. Tsirkin wrote:
> > > On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote:
> > > > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote:
> > > > > > This must be a proper barrier, like a spinlock, mutex, or
> > > > > > synchronize_rcu.
> > > > > 
> > > > > 
> > > > > I start with synchronize_rcu() but both you and Michael raise some
> > > > > concern.
> > > > 
> > > > I've also idly wondered if calling synchronize_rcu() under the various
> > > > mm locks is a deadlock situation.
> > > > 
> > > > > Then I try spinlock and mutex:
> > > > > 
> > > > > 1) spinlock: add lots of overhead on datapath, this leads 0 performance
> > > > > improvement.
> > > > 
> > > > I think the topic here is correctness not performance improvement
> > > 
> > > The topic is whether we should revert
> > > commit 7f466032dc9 ("vhost: access vq metadata through kernel virtual address")
> > > 
> > > or keep it in. The only reason to keep it is performance.
> > 
> > Yikes, I'm not sure you can ever win against copy_from_user using
> > mmu_notifiers?
> 
> Ever since copy_from_user started playing with flags (for SMAP) and
> added speculation barriers there's a chance we can win by accessing
> memory through the kernel address.

You think copy_to_user will be more expensive than the minimum two
atomics required to synchronize with another thread?

> > Also, why can't this just permanently GUP the pages? In fact, where
> > does it put_page them anyhow? Worrying that 7f466 adds a get_user page
> > but does not add a put_page??

You didn't answer this.. Why not just use GUP?

Jason