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From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Yafang Shao <laoar.shao@gmail.com>, Michal Hocko <mhocko@suse.com>,
        Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Muchun Song <songmuchun@bytedance.com>,
        Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH] mm: memcontrol: do not miss MEMCG_MAX events for
 enforced allocations
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On Thu, Jul 07, 2022 at 03:41:11PM -0700, Alexei Starovoitov wrote:
> On Tue, Jul 5, 2022 at 9:24 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> >
> > Anyway, here is the patch for reparenting bpf maps:
> > https://github.com/rgushchin/linux/commit/f57df8bb35770507a4624fe52216b6c14f39c50c
> >
> > I gonna post it to bpf@ after some testing.
> 
> Please do. It looks good.
> It needs #ifdef CONFIG_MEMCG_KMEM
> because get_obj_cgroup_from_current() is undefined otherwise.
> Ideally just adding a static inline to a .h ?

Actually all call sites are already under CONFIG_MEMCG_KMEM.

> 
> and
> if (map->objcg)
>    memcg = get_mem_cgroup_from_objcg(map->objcg);
> 
> or !NULL check inside get_mem_cgroup_from_objcg()
> which would be better.

Yes, you're right, as now we need to handle it specially.

In the near future it won't be necessary. There are patches in
mm-unstable which make objcg API useful outside of CONFIG_MEMCG_KMEM.
In particular it means that objcg will be created for the root_mem_cgroup.
So map->objcg can always point at a valid objcg and we will be able
to drop this check.

Will post an updated version shortly.

Thanks!