> On Jul 31, 2024, at 18:06, Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx> wrote:
>
> On 7/24/24 4:23 AM, Muchun Song wrote:
>>
>>
>>> On Jul 24, 2024, at 08:45, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> On Thu, 18 Jul 2024 16:36:07 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>>>
>>>> The mem_cgroup_from_slab_obj() is supposed to be called under rcu
>>>> lock or cgroup_mutex or others which could prevent returned memcg
>>>> from being freed. Fix it by adding missing rcu read lock.
>>>
>>> "or others" is rather vague. What others?
>>
>> Like objcg_lock. I have added this one into obj_cgroup_memcg().
>>
>>>
>>>> @@ -109,14 +110,20 @@ EXPORT_SYMBOL_GPL(list_lru_add);
>>>>
>>>> bool list_lru_add_obj(struct list_lru *lru, struct list_head *item)
>>>> {
>>>> + bool ret;
>>>> int nid = page_to_nid(virt_to_page(item));
>>>> - struct mem_cgroup *memcg = list_lru_memcg_aware(lru) ?
>>>> - mem_cgroup_from_slab_obj(item) : NULL;
>>>> + struct mem_cgroup *memcg;
>>>>
>>>> - return list_lru_add(lru, item, nid, memcg);
>>>> + rcu_read_lock();
>>>> + memcg = list_lru_memcg_aware(lru) ? mem_cgroup_from_slab_obj(item) : NULL;
>>>> + ret = list_lru_add(lru, item, nid, memcg);
>>>> + rcu_read_unlock();
>>>
>>> We don't need rcu_read_lock() to evaluate NULL.
>>>
>>> memcg = NULL;
>>> if (list_lru_memcg_aware(lru)) {
>>> rcu_read_lock();
>>> memcg = mem_cgroup_from_slab_obj(item);
>>> rcu_read_unlock();
>>
>> Actually, the access to memcg is in list_lru_add(), so the rcu lock should
>> also cover this function rather than only mem_cgroup_from_slab_obj().
>> Something like:
>>
>> memcg = NULL;
>> if (list_lru_memcg_aware(lru)) {
>> rcu_read_lock();
>> memcg = mem_cgroup_from_slab_obj(item);
>> }
>> ret = list_lru_add(lru, item, nid, memcg);
>> if (list_lru_memcg_aware(lru))
>> rcu_read_unlock();
>>
>> Not concise. I don't know if this is good.
>
> At such point, it's probably best to just:
>
> if (list_lru_memcg_aware(lru)) {
> rcu_read_lock();
> ret = list_lru_add(lru, item, nid, mem_cgroup_from_slab_obj(item));
> rcu_read_unlock();
> } else {
> list_lru_add(lru, item, nid, NULL);
> }
Good. Will update v2.
Thanks.
>
> ?
>
>>
>>> }
>>>
>>> Seems worthwhile?
