> 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.
> }
>
> Seems worthwhile?
>
>
