linux-kernel - Re: [PATCH v1 21/26] mm: memcontrol: prepare for reparenting LRU pages for lruvec lock

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Message-ID: <aR_ZQjoAA9CFwcKG@hyeyoo>
Date: Fri, 21 Nov 2025 12:15:14 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
        roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
        muchun.song@...ux.dev, david@...hat.com, lorenzo.stoakes@...cle.com,
        ziy@...dia.com, imran.f.khan@...cle.com, kamalesh.babulal@...cle.com,
        axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
        akpm@...ux-foundation.org, 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 v1 21/26] mm: memcontrol: prepare for reparenting LRU
 pages for lruvec lock

On Tue, Oct 28, 2025 at 09:58:34PM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@...edance.com>
> 
> The following diagram illustrates how to ensure the safety of the folio
> lruvec lock when LRU folios undergo reparenting.
> 
> In the folio_lruvec_lock(folio) function:
> ```
>     rcu_read_lock();
> retry:
>     lruvec = folio_lruvec(folio);
>     /* There is a possibility of folio reparenting at this point. */
>     spin_lock(&lruvec->lru_lock);
>     if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
>         /*
>          * The wrong lruvec lock was acquired, and a retry is required.
>          * This is because the folio resides on the parent memcg lruvec
>          * list.
>          */
>         spin_unlock(&lruvec->lru_lock);
>         goto retry;
>     }
> 
>     /* Reaching here indicates that folio_memcg() is stable. */

Does that mean we call rcu_read_unlock() in lruvec_unlock() instead of
in folio_lruvec_lock() only to avoid false warnings inside the critical
section, and technically calling rcu_read_unlock() right after acquiring
the spinlock is fine?

-- 
Cheers,
Harry / Hyeonggon

> ```
> 
> In the memcg_reparent_objcgs(memcg) function:
> ```
>     spin_lock(&lruvec->lru_lock);
>     spin_lock(&lruvec_parent->lru_lock);
>     /* Transfer folios from the lruvec list to the parent's. */
>     spin_unlock(&lruvec_parent->lru_lock);
>     spin_unlock(&lruvec->lru_lock);
> ```
> 
> After acquiring the lruvec lock, it is necessary to verify whether
> the folio has been reparented. If reparenting has occurred, the new
> lruvec lock must be reacquired. During the LRU folio reparenting
> process, the lruvec lock will also be acquired (this will be
> implemented in a subsequent patch). Therefore, folio_memcg() remains
> unchanged while the lruvec lock is held.
> 
> Given that lruvec_memcg(lruvec) is always equal to folio_memcg(folio)
> after the lruvec lock is acquired, the lruvec_memcg_debug() check is
> redundant. Hence, it is removed.
> 
> This patch serves as a preparation for the reparenting of LRU folios.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---