On 12/20/25 9:05 AM, Shakeel Butt wrote:
> On Wed, Dec 17, 2025 at 03:27:39PM +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 mem_cgroup_swap_full().
>>
>> 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/memcontrol.c | 10 +++++++---
>> 1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 131f940c03fa0..f2c891c1f49d5 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5267,17 +5267,21 @@ bool mem_cgroup_swap_full(struct folio *folio)
>> if (do_memsw_account())
>> return false;
>>
>> - memcg = folio_memcg(folio);
>> - if (!memcg)
>> + if (!folio_memcg_charged(folio))
>> return false;
>>
>> + rcu_read_lock();
>> + memcg = folio_memcg(folio);
>> for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
>> unsigned long usage = page_counter_read(&memcg->swap);
>>
>> if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
>> - usage * 2 >= READ_ONCE(memcg->swap.max))
>> + usage * 2 >= READ_ONCE(memcg->swap.max)) {
>> + rcu_read_unlock();
>> return true;
>> + }
>> }
>> + rcu_read_unlock();
>>
>> return false;
>> }
>
> How about the following?
>
>
> bool mem_cgroup_swap_full(struct folio *folio)
> {
> struct mem_cgroup *memcg;
> + bool ret = false;
>
> VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>
> if (vm_swap_full())
> return true;
> - if (do_memsw_account())
> - return false;
>
> - if (!folio_memcg_charged(folio))
> - return false;
> + if (do_memsw_account() || !folio_memcg_charged(folio))
> + return ret;
>
> rcu_read_lock();
> memcg = folio_memcg(folio);
> @@ -5277,13 +5276,13 @@ bool mem_cgroup_swap_full(struct folio *folio)
>
> if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
> usage * 2 >= READ_ONCE(memcg->swap.max)) {
> - rcu_read_unlock();
> - return true;
> + ret = true;
> + break;
> }
> }
> rcu_read_unlock();
>
> - return false;
> + return ret;
> }
LGTM, will do.
>
>
> Anyways LGTM.
>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Thanks!