Re: [PATCH v1 13/26] mm: mglru: prevent memory cgroup release in mglru

Qi Zheng
2025-11-20T13:39:30Z

On 11/19/25 6:13 PM, Harry Yoo wrote:
> On Tue, Oct 28, 2025 at 09:58:26PM +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 mglru.
>>
>> 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>
>> ---
>> mm/vmscan.c | 23 +++++++++++++++++------
>> 1 file changed, 17 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 660cd40cfddd4..676e6270e5b45 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -4279,6 +4288,8 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
>>
>> arch_leave_lazy_mmu_mode();
>>
>> + rcu_read_unlock();
>> +
>> /* feedback from rmap walkers to page table walkers */
>> if (mm_state && suitable_to_scan(i, young))
>> update_bloom_filter(mm_state, max_seq, pvmw->pmd);
>
> mm_state has the same life cycle as mem_cgroup. So it should be
> protected by rcu read lock?

You are right. The mm_state is defined as follows:

struct lruvec {
struct lru_gen_mm_state mm_state;
};

will fix it in the next version.

Thanks,
Qi

>