> On Aug 12, 2022, at 08:27, Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> On Thu, Aug 11, 2022 at 11:47 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>>
>> On Thu, Aug 11, 2022 at 10:49:13AM +0800, Yafang Shao wrote:
>>> On Thu, Aug 11, 2022 at 1:07 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>>>>
>>>> On Wed, Aug 10, 2022 at 03:18:30PM +0000, Yafang Shao wrote:
>>>>> The memcg may be the root_mem_cgroup, in which case we shouldn't put it.
>>>>
>>>> No, it is ok to put root_mem_cgroup. css_put already handles the root
>>>> cgroups.
>>>>
>>>
>>> Ah, this commit log doesn't describe the issue clearly. I should improve it.
>>> The issue is that in bpf_map_get_memcg() it doesn't get the objcg if
>>> map->objcg is NULL (that can happen if the map belongs to the root
>>> memcg), so we shouldn't put the objcg if map->objcg is NULL neither in
>>> bpf_map_put_memcg().
>>
>> Sorry I am still not understanding. We are not 'getting' objcg in
>> bpf_map_get_memcg(). We are 'getting' memcg from map->objcg and if that
>> is NULL the function is returning root memcg and putting root memcg is
>> totally fine.
>
> When the map belongs to root_mem_cgroup, the map->objcg is NULL, right ?
> See also bpf_map_save_memcg() and it describes clearly in the comment -
>
> static void bpf_map_save_memcg(struct bpf_map *map)
> {
> /* Currently if a map is created by a process belonging to the root
> * memory cgroup, get_obj_cgroup_from_current() will return NULL.
> * So we have to check map->objcg for being NULL each time it's
> * being used.
> */
> map->objcg = get_obj_cgroup_from_current();
> }
>
> So for the root_mem_cgroup case, bpf_map_get_memcg() will return
> root_mem_cgroup directly without getting its css, right ? See below,
>
> static struct mem_cgroup *bpf_map_get_memcg(const struct bpf_map *map)
> {
>
> if (map->objcg)
> return get_mem_cgroup_from_objcg(map->objcg);
>
> return root_mem_cgroup; // without css_get(&memcg->css);
> }
>
> But it will put the css unconditionally. See below,
>
> memcg = bpf_map_get_memcg(map);
> ...
> mem_cgroup_put(memcg);
>
> So we should put it *conditionally* as well.
Hi,
No. We could put root_mem_cgroup unconditionally since the root css
is treated as no reference css. See css_put().
static inline void css_put(struct cgroup_subsys_state *css)
{
// The root memcg’s css has been set with CSS_NO_REF.
if (!(css->flags & CSS_NO_REF))
percpu_ref_put(&css->refcnt);
}
Muchun,
Thanks.
>
> memcg = bpf_map_get_memcg(map);
> ...
> + if (map->objcg)
> mem_cgroup_put(memcg);
>
> Is it clear to you ?
>
>> Or are you saying that root_mem_cgroup is NULL?
>>
>
> No
>
> --
> Regards
> Yafang
