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

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Message-ID: <d62756d4-c249-474e-ae80-478d3c7cf34d@kernel.org>
Date: Thu, 18 Dec 2025 10:43:27 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Qi Zheng <qi.zheng@...ux.dev>, 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 10:36, Qi Zheng wrote:
> 
> 
> 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()?

Right, something like

memcg = folio_memcg_begin(folio);
folio_memcg_end(memcg);

Maybe someone reading along has a better idea. Then you can nicely 
document the requirements in the kerneldocs, and it is clear why the RCU 
lock is used (internally).

-- 
Cheers

David