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

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Message-ID: <3c32d80a-ba0e-4ed2-87ae-fb80fc3374f7@linux.dev>
Date: Thu, 18 Dec 2025 21:00:53 +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 7:56 PM, David Hildenbrand (Red Hat) wrote:
> 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?

So for some longer or might-sleep critical sections, using:

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

for some short critical sections, using:

rcu_read_lock();
memcg = folio_memcg(folio);
do_some_thing(memcg);
rcu_read_unlock();

Right?


>