linux-kernel - Re: [PATCH v2 10/28] writeback: prevent memory cgroup release in writeback module

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Message-ID: <aUMp9cXFGMwgj8Pn@cmpxchg.org>
Date: Wed, 17 Dec 2025 17:08:53 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev,
	shakeel.butt@...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 10/28] writeback: prevent memory cgroup release in
 writeback module

On Wed, Dec 17, 2025 at 03:27:34PM +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 function get_mem_cgroup_css_from_folio()
> and the rcu read lock are employed to safeguard against the release
> of the memory cgroup.
> 
> 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>

Looks sane to me.

The roo_mem_cgroup handling in get_mem_cgroup_css_from_folio() is
unusual - usually we do NULL for mem_cgroup_disabled(). But that's a
quirk you're inheriting from the existing writeback code.

Acked-by: Johannes Weiner <hannes@...xchg.org>