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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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.

But my points was about hiding the rcu details in a set of helpers.

Sorry if what I say is confusing.

-- 
Cheers

David