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

[Yafang Shao <laoar.shao@gmail.com>]

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.

  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