linux-kernel - Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate

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Message-ID: <3a6ab69e-a2cc-4c61-9de1-9b0958c72dda@kernel.org>
Date: Thu, 18 Dec 2025 10:09:21 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Qi Zheng <qi.zheng@...ux.dev>, hannes@...xchg.org, hughd@...gle.com,
 mhocko@...e.com, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
 muchun.song@...ux.dev, 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
Cc: 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 13/28] mm: migrate: prevent memory cgroup release in
 folio_migrate_mapping()

On 12/17/25 08:27, 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 folio_migrate_mapping().

We usually avoid talking about "patches".

In __folio_migrate_mapping(), the rcu read lock ...

> 
> 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/migrate.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 5169f9717f606..8bcd588c083ca 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -671,6 +671,7 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>   		struct lruvec *old_lruvec, *new_lruvec;
>   		struct mem_cgroup *memcg;
>   
> +		rcu_read_lock();
>   		memcg = folio_memcg(folio);

In general, LGTM

I wonder, though, whether we should embed that in the ABI.

Like "lock RCU and get the memcg" in one operation, to the "return memcg 
and unock rcu" in another operation.

Something like "start / end" semantics.

--
Cheers

David