To obtain the pointer to the next memcg position, mem_cgroup_iter()
currently holds css->refcnt during memcg traversal only to put
css->refcnt at the end of the routine. This isn't necessary as an
rcu_read_lock is already held throughout the function. The use of
the RCU read lock with css_next_descendant_pre() guarantees that
sibling linkage is safe without holding a ref on the passed-in @css.

Remove css->refcnt usage during traversal by leveraging RCU.

Signed-off-by: Kinsey Ho <kinseyho@google.com>
---
 include/linux/memcontrol.h |  2 +-
 mm/memcontrol.c            | 18 +-----------------
 2 files changed, 2 insertions(+), 18 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 90ecd2dbca06..1aaed2f1f6ae 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -75,7 +75,7 @@ struct lruvec_stats_percpu;
 struct lruvec_stats;
 
 struct mem_cgroup_reclaim_iter {
-	struct mem_cgroup *position;
+	struct mem_cgroup __rcu *position;
 	/* scan generation, increased every round-trip */
 	unsigned int generation;
 };
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dacf4fec4541..1688aae3b1b4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1052,20 +1052,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		else if (reclaim->generation != iter->generation)
 			goto out_unlock;
 
-		while (1) {
-			pos = READ_ONCE(iter->position);
-			if (!pos || css_tryget(&pos->css))
-				break;
-			/*
-			 * css reference reached zero, so iter->position will
-			 * be cleared by ->css_released. However, we should not
-			 * rely on this happening soon, because ->css_released
-			 * is called from a work queue, and by busy-waiting we
-			 * might block it. So we clear iter->position right
-			 * away.
-			 */
-			(void)cmpxchg(&iter->position, pos, NULL);
-		}
+		pos = rcu_dereference(iter->position);
 	} else if (prev) {
 		pos = prev;
 	}
@@ -1106,9 +1093,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		 */
 		(void)cmpxchg(&iter->position, pos, memcg);
 
-		if (pos)
-			css_put(&pos->css);
-
 		if (!memcg)
 			iter->generation++;
 	}
-- 
2.46.0.76.ge559c4bf1a-goog

On Tue, Aug 13, 2024 at 08:47:12PM GMT, Kinsey Ho <kinseyho@google.com> wrote:
> To obtain the pointer to the next memcg position, mem_cgroup_iter()
> currently holds css->refcnt during memcg traversal only to put
> css->refcnt at the end of the routine. This isn't necessary as an
> rcu_read_lock is already held throughout the function. The use of
> the RCU read lock with css_next_descendant_pre() guarantees that
> sibling linkage is safe without holding a ref on the passed-in @css.
> 
> Remove css->refcnt usage during traversal by leveraging RCU.
> 
> Signed-off-by: Kinsey Ho <kinseyho@google.com>
> ---
>  include/linux/memcontrol.h |  2 +-
>  mm/memcontrol.c            | 18 +-----------------
>  2 files changed, 2 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 90ecd2dbca06..1aaed2f1f6ae 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -75,7 +75,7 @@ struct lruvec_stats_percpu;
>  struct lruvec_stats;
>  
>  struct mem_cgroup_reclaim_iter {
> -	struct mem_cgroup *position;
> +	struct mem_cgroup __rcu *position;

I'm not sure about this annotation.
This pointer could be modified concurrently with RCU read sections with
the cmpxchg which would assume that's equivalent with
rcu_assign_pointer(). (Which it might be but it's not idiomatic, so it
causes some head wrapping.)
Isn't this situation covered with a regular pointer and
READ_ONCE()+cmpxchg?

Regards,
Michal

Hi Michal,

Thank you for reviewing this patchset!

On Wed, Aug 14, 2024 at 5:00 AM Michal Koutný <mkoutny@suse.com> wrote:
>
> On Tue, Aug 13, 2024 at 08:47:12PM GMT, Kinsey Ho <kinseyho@google.com> wrote:
> > To obtain the pointer to the next memcg position, mem_cgroup_iter()
> > currently holds css->refcnt during memcg traversal only to put
> > css->refcnt at the end of the routine. This isn't necessary as an
> > rcu_read_lock is already held throughout the function. The use of
> > the RCU read lock with css_next_descendant_pre() guarantees that
> > sibling linkage is safe without holding a ref on the passed-in @css.
> >
> > Remove css->refcnt usage during traversal by leveraging RCU.
> >
> > Signed-off-by: Kinsey Ho <kinseyho@google.com>
> > ---
> >  include/linux/memcontrol.h |  2 +-
> >  mm/memcontrol.c            | 18 +-----------------
> >  2 files changed, 2 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 90ecd2dbca06..1aaed2f1f6ae 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -75,7 +75,7 @@ struct lruvec_stats_percpu;
> >  struct lruvec_stats;
> >
> >  struct mem_cgroup_reclaim_iter {
> > -     struct mem_cgroup *position;
> > +     struct mem_cgroup __rcu *position;
>
> I'm not sure about this annotation.
> This pointer could be modified concurrently with RCU read sections with
> the cmpxchg which would assume that's equivalent with
> rcu_assign_pointer(). (Which it might be but it's not idiomatic, so it
> causes some head wrapping.)
> Isn't this situation covered with a regular pointer and
> READ_ONCE()+cmpxchg?

Yes, that's a good point – this situation is covered with a regular
pointer and READ_ONCE() + cmpxchg(). I'll make the change to remove
the __rcu tag and replace rcu_dereference() with READ_ONCE() and send
it out in v3. (This also rids of the sparse errors seen in v1)

Thanks for pointing this out.

Best,
Kinsey