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

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Message-ID: <11a60eba-3447-47de-9d59-af5842f5dc5e@kernel.org>
Date: Thu, 18 Dec 2025 12:56:13 +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 12:40, Qi Zheng wrote:
> 
> 
> On 12/18/25 5:43 PM, David Hildenbrand (Red Hat) wrote:
>> 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);
> 
> For some longer or might-sleep critical sections (such as those pointed
> by Johannes), perhaps it can be defined like this:
> 
> struct mem_cgroup *folio_memcg_begin(struct folio *folio)
> {
> 	return get_mem_cgroup_from_folio(folio);
> }
> 
> void folio_memcg_end(struct mem_cgroup *memcg)
> {
> 	mem_cgroup_put(memcg);
> }
> 
> But for some short critical sections, using RCU lock directly might
> be the most convention option?
> 

Then put the rcu read locking in there instead?

-- 
Cheers

David