Re: Is there a reason we don't have kvfree_rcu()?

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Mar 12, 2020 at 08:10:09PM +0100, Uladzislau Rezki wrote:
> On Thu, Mar 12, 2020 at 11:11:38AM -0700, Paul E. McKenney wrote:
> > On Thu, Mar 12, 2020 at 04:27:30PM +0000, George Spelvin wrote:
> > > kvfree() is a superset of kfree(), so there's nothing obvious stopping 
> > > kfree_rcu() from simply changing to kvfree() and everything will keep 
> > > working.
> > > 
> > > I'd probably add a kvfree_rcu() alias, just for documentation's sake and
> > > to make code that depends on the new feature explode at compile time, but
> > > it could be identical behind the scenes.
> > > 
> > > There's an existing user in mm/list_lru.c already.
> > > 
> > > I was just thinking of using kvmalloc() in a module, and realized that the 
> > > lack of a core kvfree_rcu() helper meant I'd have to synchronize_rcu() on 
> > > module unload.
> > 
> > There was a recent proposal to do just that, but current patches in -rcu
> > use kfree_bulk().  It doesn't look to me that this works for kfvree()
> > under the covers in its current form.  Could it be upgraded to handle
> > this case?
> > 
> > Adding Vlad on CC for his thoughts.
> > 
> Paul, see below my view:
> 
> Answering to topic's question it looks like we need kvfree_rcu() support :)
> 
> It is easy to add it actually. But if we are talking about the case when
> an object has rcu_head inside. From the other hand recent discussion showed
> that we would like to have head-less variant of the kvfree_rcu() functionality:
> 
> https://lkml.org/lkml/2020/2/18/566
> 
> for example, as Ted pointed, he would go with head-less case(for ext4) only.
> The reason is nobody wants to modify internal structures injecting rcu_head
> there. Also there are many other places in the kernel where it would be good
> to have kfree_rcu() head-less variant as well.
> 
> I spent some time implementing it together with Joel. It is ready from my
> side but only for RCU-tree case. Next step is RCU-tiny support, so i am
> working on it.  
> 
> I can send out an RFC for RCU-tree only support, so we can discuss it
> and agree on how to move forward. After that i or Joel or together can
> update RCU-tine.
> 
> Joel: What do you think?

Yes, your sending an RFC with what you have sounds good. I can prepare a tree
for both of us then and we can develop on that. I was actually waiting on
your patches so I can add more on top.

One more thing I want to add is the shrinker interface to prevent OOM during
kfree_rcu() flood. I sent patches to fix that. It works well. We can prepare
a tree with all these features and develop on that so there's no conflict.

For -tiny and lack of rcu_head, I think we discussed that we would
always dynamically allocate rcu_head for that case.

> Another thought. We can add kvfree_rcu(ptr, rcu) first, because it is
> easy and after that implement head-less case.

Yes, that is also fine. We can start simple and then keep improving it. I
think we have now 3 users who want head-less interface so ultimately we can
shoot for that goal (at later stage).

thanks,

 - Joel