Re: WARNING in __mmdrop

From: Jason Wang
Date: Thu Jul 25 2019 - 14:03:56 EST

On 2019/7/25 äå1:52, Michael S. Tsirkin wrote:
On Tue, Jul 23, 2019 at 09:31:35PM +0800, Jason Wang wrote:
On 2019/7/23 äå5:26, Michael S. Tsirkin wrote:
On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote:
On 2019/7/23 äå4:10, Michael S. Tsirkin wrote:
On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote:
On 2019/7/23 äå3:23, 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
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.

This is how I want to go with RFC and V1. But I end up with deadlock between
vq locks and some MM internal locks. So I decide to use RCU which is 100%
under the control of vhost.

And I guess the deadlock is because GUP is taking mmu locks which are
taken on mmu notifier path, right?

Yes, but it's not the only lock. I don't remember the details, but I can confirm I meet issues with one or two other locks.

How about we add a seqlock and take
that in invalidate callbacks? We can then drop the VQ lock before GUP,
and take it again immediately after.

something like
if (!vq_meta_mapped(vq)) {

The problem is the vq address could be changed at this time.


/* recheck both sock->private_data and seqlock count. */
if changed - bail out

And also requires that VQ uaddrs is defined like this:
- writers must have both vq mutex and dev mutex
- readers must have either vq mutex or dev mutex

That's a big change though. For now, how about switching to a per-vq SRCU?
That is only a little bit more expensive than RCU, and we
can use synchronize_srcu_expedited.

Consider we switch to use kfree_rcu(), what's the advantage of per-vq SRCU?