linux-kernel - Re: [PATCH v3 08/30] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from

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Message-ID: <hu7mfsqyeuh2txp6vt5dvfsdrl37jeualew6slefyfqslg36nn@hysym264wfof>
Date: Sat, 17 Jan 2026 12:00:55 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, 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, yosry.ahmed@...ux.dev, 
	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 v3 08/30] mm: memcontrol: prevent memory cgroup release
 in get_mem_cgroup_from_folio()

On Wed, Jan 14, 2026 at 07:32:35PM +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.

This patch is adding a retry loop as css_tryget() on folio_memcg() can
fail because after reparenting functionality the folio no longer holds
down the memcg. Please clarify this reasoning in the commit.

With that:

Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>