linux-kernel - Re: [PATCH RFC] memcg: notify about global mem_cgroup

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Date:   Sat, 25 Jun 2022 18:56:04 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Vasily Averin <vvs@...nvz.org>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Michal Koutný <mkoutny@...e.com>,
        Michal Hocko <mhocko@...e.com>, kernel@...nvz.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Muchun Song <songmuchun@...edance.com>, cgroups@...r.kernel.org
Subject: Re: [PATCH RFC] memcg: notify about global mem_cgroup_id space
 depletion

On Sat, Jun 25, 2022 at 05:04:27PM +0300, Vasily Averin wrote:
> Currently host owner is not informed about the exhaustion of the
> global mem_cgroup_id space. When this happens, systemd cannot
> start a new service, but nothing points to the real cause of
> this failure.
> 
> Signed-off-by: Vasily Averin <vvs@...nvz.org>
> ---
>  mm/memcontrol.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d4c606a06bcd..5229321636f2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5317,6 +5317,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>  				 1, MEM_CGROUP_ID_MAX + 1, GFP_KERNEL);
>  	if (memcg->id.id < 0) {
>  		error = memcg->id.id;
> +		pr_notice_ratelimited("mem_cgroup_id space is exhausted\n");
>  		goto fail;
>  	}

Hm, in this case it should return -ENOSPC and it's a very unique return code.
If it's not returned from the mkdir() call, we should fix this.
Otherwise it's up to systemd to handle it properly.

I'm not opposing for adding a warning, but parsing dmesg is not how
the error handling should be done.

Thanks!