Re: [PATCH v1 07/26] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio()

Qi Zheng
2025-11-20T13:32:28Z

On 11/19/25 4:06 PM, Harry Yoo wrote:
> On Tue, Oct 28, 2025 at 09:58:20PM +0800, 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 get_mem_cgroup_from_folio().
>>
>> 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>
>> ---
>
> This patch looks correct to me, so:
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
>
> with a few side question on current implementation rather than
> this change...
>
>> mm/memcontrol.c | 11 ++++++++---
>> 1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index d484b632c790f..1da3ad77054d3 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
>> */
>> struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
>> {
>> - struct mem_cgroup *memcg = folio_memcg(folio);
>> + struct mem_cgroup *memcg;
>>
>> if (mem_cgroup_disabled())
>> return NULL;
>>
>> + if (!folio_memcg_charged(folio))
>> + return root_mem_cgroup;
>> +
>> rcu_read_lock();
>> - if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
>
> I have no idea why we're already calling rcu_read_lock() when a folio pins
> the memcg :)

+1, this was introduced by
https://github.com/torvalds/linux/commit/f77f0c7514789577125c1b2df145703161736359,
may be some defensive programming.

>
>> - memcg = root_mem_cgroup;
>> +retry:
>> + memcg = folio_memcg(folio);
>> + if (unlikely(!css_tryget(&memcg->css)))
>> + goto retry;
>> rcu_read_unlock();
>> return memcg;
>> }
>> --
>> 2.20.1
>