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

[Uladzislau Rezki <urezki@gmail.com>]

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?

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

--
Vlad Rezki