Re: [PATCH v2 17/28] mm: thp: prevent memory cgroup release in folio_split_queue_lock{_irqsave}()

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

On 12/20/25 9:11 AM, Shakeel Butt wrote:
> On Wed, Dec 17, 2025 at 05:27:17PM -0500, Johannes Weiner wrote:
>> On Wed, Dec 17, 2025 at 03:27:41PM +0800, Qi Zheng wrote:
>>> From: Qi Zheng <zhengqi.arch@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_split_queue_lock{_irqsave}().
>>>
>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
>>> ---
>>>   mm/huge_memory.c | 16 ++++++++++++++--
>>>   1 file changed, 14 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 12b46215b30c1..b9e6855ec0b6a 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -1154,13 +1154,25 @@ split_queue_lock_irqsave(int nid, struct mem_cgroup *memcg, unsigned long *flags
>>>   
>>>   static struct deferred_split *folio_split_queue_lock(struct folio *folio)
>>>   {
>>> -	return split_queue_lock(folio_nid(folio), folio_memcg(folio));
>>> +	struct deferred_split *queue;
>>> +
>>> +	rcu_read_lock();
>>> +	queue = split_queue_lock(folio_nid(folio), folio_memcg(folio));
>>> +	rcu_read_unlock();
>>
>> Ah, the memcg destruction path is acquiring the split queue lock for
>> reparenting. Once you have it locked, it's safe to drop the rcu lock.
> 
> Qi, please add the above explanation in a comment and with that:

OK, will do.

> 
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Thanks!