linux-kernel - Re: [PATCH v2 23/28] mm: memcontrol: prepare for reparenting LRU pages for lruvec lock

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Message-ID: <jdhf6h7weedyentvyaqsr3n3v7dlj4n6k3mrryn2hkyhc255cl@kmexszfblh6s>
Date: Fri, 19 Dec 2025 18:03:53 -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, 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 23/28] mm: memcontrol: prepare for reparenting LRU
 pages for lruvec lock

On Wed, Dec 17, 2025 at 03:27:47PM +0800, Qi Zheng wrote:
> @@ -1232,14 +1221,20 @@ struct lruvec *folio_lruvec_lock(struct folio *folio)
>   * - folio frozen (refcount of 0)
>   *
>   * Return: The lruvec this folio is on with its lock held and interrupts
> - * disabled.
> + * disabled and rcu read lock held.
>   */
>  struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
>  {
> -	struct lruvec *lruvec = folio_lruvec(folio);
> +	struct lruvec *lruvec;
>  
> +	rcu_read_lock();
> +retry:
> +	lruvec = folio_lruvec(folio);
>  	spin_lock_irq(&lruvec->lru_lock);
> -	lruvec_memcg_debug(lruvec, folio);
> +	if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
> +		spin_unlock_irq(&lruvec->lru_lock);
> +		goto retry;
> +	}

So after this patch, all folio_lruvec_lock_irq() calls should be paired
with lruvec_unlock_irq() but lru_note_cost_refault() is calling
folio_lruvec_lock_irq() without the paired lruvec_unlock_irq(). It is
using lru_note_cost_unlock_irq() for unlocking lru lock and thus rcu
read unlock is missed.

Beside fixing this, I would suggest to add __acquire()/__release() tags
for both lru lock and rcu for all these functions.