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

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Message-ID: <4effa243-bae3-45e4-8662-dca86a7e5d12@linux.dev>
Date: Thu, 18 Dec 2025 19:40:43 +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 5:43 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?

> 
> 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).
>