linux-kernel - Re: [PATCH v1 07/26] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from

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Message-ID: <aR16bLuvj-W0AAM1@hyeyoo>
Date: Wed, 19 Nov 2025 17:06:04 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
        roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
        muchun.song@...ux.dev, david@...hat.com, lorenzo.stoakes@...cle.com,
        ziy@...dia.com, imran.f.khan@...cle.com, kamalesh.babulal@...cle.com,
        axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
        akpm@...ux-foundation.org, 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 v1 07/26] mm: memcontrol: prevent memory cgroup release
 in get_mem_cgroup_from_folio()

On Tue, Oct 28, 2025 at 09:58:20PM +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>
> ---

This patch looks correct to me, so:
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

with a few side question on current implementation rather than
this change...

>  mm/memcontrol.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d484b632c790f..1da3ad77054d3 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)))

I have no idea why we're already calling rcu_read_lock() when a folio pins
the memcg :)

> -		memcg = root_mem_cgroup;
> +retry:
> +	memcg = folio_memcg(folio);
> +	if (unlikely(!css_tryget(&memcg->css)))
> +		goto retry;
>  	rcu_read_unlock();
>  	return memcg;
>  }
> -- 
> 2.20.1

-- 
Cheers,
Harry / Hyeonggon