linux-kernel - Re: [PATCH mm-unstable v2 2/5] mm: don't hold css->refcnt during traversal

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Message-ID: <CAF6N3nV3RPaiS9E4=-ABXQ-F++J=E8goSquN1cq2S_TuftUNxg@mail.gmail.com>
Date: Fri, 16 Aug 2024 12:15:01 -0400
From: Kinsey Ho <kinseyho@...gle.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	Yosry Ahmed <yosryahmed@...gle.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>
Subject: Re: [PATCH mm-unstable v2 2/5] mm: don't hold css->refcnt during traversal

Hi Michal,

Thank you for reviewing this patchset!

On Wed, Aug 14, 2024 at 5:00 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Tue, Aug 13, 2024 at 08:47:12PM GMT, Kinsey Ho <kinseyho@...gle.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@...gle.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