linux-kernel - Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate

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Message-ID: <02c3be32-4826-408d-8b96-1db51dcababf@linux.dev>
Date: Thu, 18 Dec 2025 17:36:00 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>, hannes@...xchg.org,
 hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev,
 shakeel.butt@...ux.dev, muchun.song@...ux.dev, 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
Cc: 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 13/28] mm: migrate: prevent memory cgroup release in
 folio_migrate_mapping()



On 12/18/25 5:09 PM, David Hildenbrand (Red Hat) wrote:
> On 12/17/25 08:27, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@...edance.com>
>>
>> In the near future, a folio will no longer pin its corresponding
>> memory cgroup. To ensure safety, it will only be appropriate to
>> hold the rcu read lock or acquire a reference to the memory cgroup
>> returned by folio_memcg(), thereby preventing it from being released.
>>
>> In the current patch, the rcu read lock is employed to safeguard
>> against the release of the memory cgroup in folio_migrate_mapping().
> 
> We usually avoid talking about "patches".

Got it.

> 
> In __folio_migrate_mapping(), the rcu read lock ...

Will do.

> 
>>
>> This serves as a preparatory measure for the reparenting of the
>> LRU pages.
>>
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
>> ---
>>   mm/migrate.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 5169f9717f606..8bcd588c083ca 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -671,6 +671,7 @@ static int __folio_migrate_mapping(struct 
>> address_space *mapping,
>>           struct lruvec *old_lruvec, *new_lruvec;
>>           struct mem_cgroup *memcg;
>> +        rcu_read_lock();
>>           memcg = folio_memcg(folio);
> 
> In general, LGTM
> 
> I wonder, though, whether we should embed that in the ABI.
> 
> Like "lock RCU and get the memcg" in one operation, to the "return memcg 
> and unock rcu" in another operation.

Do you mean adding a helper function like get_mem_cgroup_from_folio()?

> 
> Something like "start / end" semantics.
> 
> -- 
> Cheers
> 
> David