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

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Date:   Thu, 11 Aug 2022 10:49:13 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, jolsa@...nel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH bpf-next 05/15] bpf: Fix incorrect mem_cgroup_put

On Thu, Aug 11, 2022 at 1:07 AM Shakeel Butt <shakeelb@...gle.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().
Maybe the change below would be more reasonable ?

+static void bpf_map_put_memcg(const struct bpf_map *map, struct
mem_cgroup *memcg)
+{
+       if (map->objcg)
+           mem_cgroup_put(memcg);
+}

-- 
Regards
Yafang