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Date:   Sun, 26 Jun 2022 10:11:31 +0300
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Subject: Re: [PATCH RFC] memcg: notify about global mem_cgroup_id space
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To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Shakeel Butt <shakeelb@google.com>,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        Michal Hocko <mhocko@suse.com>, kernel@openvz.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Vlastimil Babka <vbabka@suse.cz>,
        Muchun Song <songmuchun@bytedance.com>, cgroups@vger.kernel.org
References: <YrXDV7uPpmDigh3G@dhcp22.suse.cz>
 <c53e1df0-5174-66de-23cc-18797f0b512d@openvz.org> <Yre8tNUY8vBrO0yl@castle>
From:   Vasily Averin <vvs@openvz.org>
In-Reply-To: <Yre8tNUY8vBrO0yl@castle>
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On 6/26/22 04:56, Roman Gushchin wrote:
> 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@openvz.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.

I'm agree,  I think it's a good idea. Moreover I think it makes sense to
use -ENOSPC  when the local cgroup's limit is reached.
Currently cgroup_mkdir() returns -EAGAIN, this looks strange for me.

        if (!cgroup_check_hierarchy_limits(parent)) {
                ret = -EAGAIN;
                goto out_unlock;
        }

Thank you,
	Vasily Averin