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

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Message-ID: <id7sx5zsl6a7v7aoy2p4r6puxkvtlib6jlqv3rnt7pegbucbhx@5plupkfwo6en>
Date: Fri, 19 Dec 2025 16:58:26 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Qi Zheng <qi.zheng@...ux.dev>, 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 14/28] mm: mglru: prevent memory cgroup release in
 mglru

On Wed, Dec 17, 2025 at 05:18:40PM -0500, Johannes Weiner wrote:
> 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?

This function is called under ptl lock, so at least for non-RT kernels,
the preemption is disabled across the function. Anyways no strong
opinion either way.