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:

quoted

On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote:

quoted

On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote:

quoted

quoted

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.

quoted

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