linux-kernel - Re: [PATCH v2 08/28] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from

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Message-ID: <ot5kji77yk6sqsjhe3fm4hufryovs7in4bivwu6xplqc4btar3@ngl5r7clogkr>
Date: Thu, 18 Dec 2025 18:09:50 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Qi Zheng <qi.zheng@...ux.dev>, hughd@...gle.com, mhocko@...e.com, 
	roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...nel.org, 
	lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com, imran.f.khan@...cle.com, 
	kamalesh.babulal@...cle.com, axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com, 
	chenridong@...weicloud.com, mkoutny@...e.com, akpm@...ux-foundation.org, 
	hamzamahfooz@...ux.microsoft.com, apais@...ux.microsoft.com, lance.yang@...ux.dev, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	Muchun Song <songmuchun@...edance.com>, Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v2 08/28] mm: memcontrol: prevent memory cgroup release
 in get_mem_cgroup_from_folio()

On Wed, Dec 17, 2025 at 04:45:06PM -0500, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:32PM +0800, Qi Zheng wrote:
> > From: Muchun Song <songmuchun@...edance.com>
> > 
> > In the near future, a folio will no longer pin its corresponding
> > memory cgroup. To ensure safety, it will only be appropriate to
> > hold the rcu read lock or acquire a reference to the memory cgroup
> > returned by folio_memcg(), thereby preventing it from being released.
> > 
> > In the current patch, the rcu read lock is employed to safeguard
> > against the release of the memory cgroup in get_mem_cgroup_from_folio().
> > 
> > This serves as a preparatory measure for the reparenting of the
> > LRU pages.
> > 
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> > Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> > ---
> >  mm/memcontrol.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 21b5aad34cae7..431b3154c70c5 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
> >   */
> >  struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
> >  {
> > -	struct mem_cgroup *memcg = folio_memcg(folio);
> > +	struct mem_cgroup *memcg;
> >  
> >  	if (mem_cgroup_disabled())
> >  		return NULL;
> >  
> > +	if (!folio_memcg_charged(folio))
> > +		return root_mem_cgroup;
> > +
> >  	rcu_read_lock();
> > -	if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
> > -		memcg = root_mem_cgroup;
> > +retry:
> > +	memcg = folio_memcg(folio);
> > +	if (unlikely(!css_tryget(&memcg->css)))
> > +		goto retry;
> 
> So starting in patch 27, the tryget can fail if the memcg is offlined,

offlined or on its way to free? It is css_tryget() without online.

> and the folio's objcg is reparented concurrently. We'll retry until we
> find a memcg that isn't dead yet. There's always root_mem_cgroup.
> 
> It makes sense, but a loop like this begs the question of how it is
> bounded. I pieced it together looking ahead. Since this is a small
> diff, it would be nicer to fold it into 27. I didn't see anything in
> between depending on it, but correct me if I'm wrong.

I agree to fold it in the patch where it is needed. Currently at this
point in series I don't see how css_tryget() can fail here.

> 
> Minor style preference:
> 
> 	/* Comment explaining the above */
> 	do {
> 		memcg = folio_memcg(folio);
> 	} while (!css_tryget(&memcg->css));