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

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Message-ID: <9f41a142-e640-4bf2-86da-234e2b758a0a@linux.dev>
Date: Fri, 21 Nov 2025 16:01:55 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Harry Yoo <harry.yoo@...cle.com>
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 11/21/25 11:15 AM, Harry Yoo wrote:
> 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

Right.

> section, and technically calling rcu_read_unlock() right after acquiring
> the spinlock is fine?

Right.

>