linux-kernel - Re: [PATCH v2 17/28] mm: thp: prevent memory cgroup release in folio_split_queue_lock{

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Message-ID: <aUMuRfPVxkfccdmp@cmpxchg.org>
Date: Wed, 17 Dec 2025 17:27:17 -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,
	Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v2 17/28] mm: thp: prevent memory cgroup release in
 folio_split_queue_lock{_irqsave}()

On Wed, Dec 17, 2025 at 03:27:41PM +0800, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@...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_split_queue_lock{_irqsave}().
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>  mm/huge_memory.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 12b46215b30c1..b9e6855ec0b6a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1154,13 +1154,25 @@ split_queue_lock_irqsave(int nid, struct mem_cgroup *memcg, unsigned long *flags
>  
>  static struct deferred_split *folio_split_queue_lock(struct folio *folio)
>  {
> -	return split_queue_lock(folio_nid(folio), folio_memcg(folio));
> +	struct deferred_split *queue;
> +
> +	rcu_read_lock();
> +	queue = split_queue_lock(folio_nid(folio), folio_memcg(folio));
> +	rcu_read_unlock();

Ah, the memcg destruction path is acquiring the split queue lock for
reparenting. Once you have it locked, it's safe to drop the rcu lock.

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