linux-kernel - Re: [PATCH v2 14/28] mm: mglru: prevent memory cgroup release in mglru

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Message-ID: <aUMsQPjBtYtVWjwf@cmpxchg.org>
Date: Wed, 17 Dec 2025 17:18:40 -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 14/28] mm: mglru: prevent memory cgroup release in
 mglru

On Wed, Dec 17, 2025 at 03:27:38PM +0800, Qi Zheng wrote:
> @@ -4242,6 +4244,13 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
>  		}
>  	}
>  
> +	rcu_read_lock();
> +	memcg = folio_memcg(folio);
> +	lruvec = mem_cgroup_lruvec(memcg, pgdat);
> +	max_seq = READ_ONCE((lruvec)->lrugen.max_seq);
> +	gen = lru_gen_from_seq(max_seq);
> +	mm_state = get_mm_state(lruvec);
> +
>  	arch_enter_lazy_mmu_mode();
>  
>  	pte -= (addr - start) / PAGE_SIZE;
> @@ -4282,6 +4291,8 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
>  	if (mm_state && suitable_to_scan(i, young))
>  		update_bloom_filter(mm_state, max_seq, pvmw->pmd);
>  
> +	rcu_read_unlock();
> +
>  	return true;

This seems a bit long to be holding the rcu lock. Maybe do a get and a
put instead?