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

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Message-Id: <20220811154731.3smhom6v4qy2u6rd@google.com>
Date:   Thu, 11 Aug 2022 15:47:31 +0000
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yafang Shao <laoar.shao@...il.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 10:49:13AM +0800, Yafang Shao wrote:
> 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().

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. Or are you saying that root_mem_cgroup is NULL?

> 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