Re: [PATCH bpf-next 05/15] bpf: Fix incorrect mem_cgroup_put

[Muchun Song <muchun.song@linux.dev>]

> On Aug 12, 2022, at 08:27, Yafang Shao <laoar.shao@gmail.com> wrote:
> 
> On Thu, Aug 11, 2022 at 11:47 PM Shakeel Butt <shakeelb@google.com> 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@google.com> 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