Linux Virtualization - Aug 2019 - [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

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.


Another reason would be to access it from e.g. softirq
context. copy_from_user will only work if the
correct mmu is active.

> 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..
I have it all disabled at compile time, so reverting isn't urgent
anymore. I'll wait a couple more days to decide what's cleanest.
> BTW, how do you get copy_from_user to work outside a syscall?
By switching to the correct mm.
> 
> 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

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