Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate_mapping()

[Qi Zheng <qi.zheng@linux.dev>]

On 12/18/25 9:04 PM, David Hildenbrand (Red Hat) wrote:
> On 12/18/25 14:00, Qi Zheng wrote:
>>
>>
>> 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@bytedance.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@bytedance.com>
>>>>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>>>>>>> Reviewed-by: Harry Yoo <harry.yoo@oracle.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?
> 
> What I mean is:
> 
> memcg = folio_memcg_begin(folio);
> do_some_thing(memcg);
> folio_memcg_end(folio);
> 
> but do the rcu_read_lock() in folio_memcg_begin() and the 
> rcu_read_unlock() in folio_memcg_end().
> 
> You could also have (expensive) variants, as you describe, that mess 
> with getting/dopping the memcg.

Or simple use folio_memcg_begin(memcg)/folio_memcg_end(memcg) in all cases.

Or add a parameter to them:

struct mem_cgroup *folio_memcg_begin(struct folio *folio, bool get_refcnt)
{
	struct mem_cgroup *memcg;

	if (get_refcnt)
		memcg = get_mem_cgroup_from_folio(folio);
	else {
		rcu_read_lock();
		memcg = folio_memcg(folio);
	}

	return memcg;
}

void folio_memcg_end(struct mem_cgroup *memcg, bool get_refcnt)
{
	if (get_refcnt)
		mem_cgroup_put(memcg);
	else
		rcu_read_unlock();
}



> 
> But my points was about hiding the rcu details in a set of helpers.
> 
> Sorry if what I say is confusing.
>