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 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()?

> 
> Something like "start / end" semantics.
> 
> -- 
> Cheers
> 
> David