linux-kernel - Re: [PATCH v2 17/28] mm: thp: prevent memory cgroup release in folio_split_queue_lock{

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Message-ID: <75f31ec7-1605-4f50-9adb-6d84e9e81101@linux.dev>
Date: Mon, 22 Dec 2025 11:33:48 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Shakeel Butt <shakeel.butt@...ux.dev>,
 Johannes Weiner <hannes@...xchg.org>
Cc: hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev,
 muchun.song@...ux.dev, david@...nel.org, 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, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v2 17/28] mm: thp: prevent memory cgroup release in
 folio_split_queue_lock{_irqsave}()



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@...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_split_queue_lock{_irqsave}().
>>>
>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>>> Reviewed-by: Harry Yoo <harry.yoo@...cle.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@...ux.dev>

Thanks!