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);
}
?
>
>> }
>>
>> Seems worthwhile?
>>
>>
>
>
