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

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Date:   Fri, 2 Aug 2019 14:24:18 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Michael S. Tsirkin" <mst@...hat.com>
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 7/9] vhost: do not use RCU to synchronize MMU notifier
 with worker

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?  The synchronization requirements are likely always
more expensive unless large and scattered copies are being done..

The rcu is about the only simple approach that could be less
expensive, and that gets back to the question if you can block an
invalidate_start_range in synchronize_rcu or not..

So, frankly, I'd revert it until someone could prove the rcu solution is
OK..

BTW, how do you get copy_from_user to work outside a syscall?

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??

Jason