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Date:   Wed, 18 Mar 2020 13:32:02 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: memcg: make memory.oom.group tolerable to task
 migration
Message-ID: <20200318123202.GL21362@dhcp22.suse.cz>
References: <20200316223510.3176148-1-guro@fb.com>
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On Mon 16-03-20 15:35:10, Roman Gushchin wrote:
> If a task is getting moved out of the OOMing cgroup, it might
> result in unexpected OOM killings if memory.oom.group is used
> anywhere in the cgroup tree.
> 
> Imagine the following example:
> 
>           A (oom.group = 1)
>          / \
>   (OOM) B   C
> 
> Let's say B's memory.max is exceeded and it's OOMing. The OOM killer
> selects a task in B as a victim, but someone asynchronously moves
> the task into C. mem_cgroup_get_oom_group() will iterate over all
> ancestors of C up to the root cgroup. In theory it had to stop
> at the oom_domain level - the memory cgroup which is OOMing.
> But because B is not an ancestor of C, it's not happening.
> Instead it chooses A (because it's oom.group is set), and kills
> all tasks in A. This behavior is wrong because the OOM happened in B,
> so there is no reason to kill anything outside.
> 
> Fix this by checking it the memory cgroup to which the task belongs
> is a descendant of the oom_domain. If not, memory.oom.group should
> be ignored, and the OOM killer should kill only the victim task.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Reported-by: Dan Schatzberg <dschatzberg@fb.com>

After the follow up discussion I do agree that this should be sufficient
for now.
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memcontrol.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index daa399be4688..d8c4b7aa4e73 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1930,6 +1930,14 @@ struct mem_cgroup *mem_cgroup_get_oom_group(struct task_struct *victim,
>  	if (memcg == root_mem_cgroup)
>  		goto out;
>  
> +	/*
> +	 * If the victim task has been asynchronously moved to a different
> +	 * memory cgroup, we might end up killing tasks outside oom_domain.
> +	 * In this case it's better to ignore memory.group.oom.
> +	 */
> +	if (unlikely(!mem_cgroup_is_descendant(memcg, oom_domain)))
> +		goto out;
> +
>  	/*
>  	 * Traverse the memory cgroup hierarchy from the victim task's
>  	 * cgroup up to the OOMing cgroup (or root) to find the
> -- 
> 2.24.1

-- 
Michal Hocko
SUSE Labs