On Thu, Mar 12, 2020 at 11:58:09PM -0400, Joel Fernandes wrote: > 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. > OK. Let's setup something. Currently i am working based on dev.2020.02.16a branch. If you can, please do it and let me know so we can avoid of conflicts. > 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. > We can do dynamic attaching. Later on, i think we can implement "arrays" logic like we do for RCU-tree but without any batching. Simple array pointer store mechanism for head-less 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). > OK. Then, i think i will make kvfree_rcu(ptr, rcu) interface and all the rest on top. -- Vlad Rezki
