linux-kernel - Re: [PATCH cgroup] cgroup: set the correct return code if hierarchy limits are reached

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Date:   Tue, 28 Jun 2022 06:59:06 +0300
From:   Vasily Averin <vvs@...nvz.org>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Michal Koutný <mkoutny@...e.com>,
        Michal Hocko <mhocko@...e.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, 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 cgroup] cgroup: set the correct return code if hierarchy
 limits are reached

On 6/28/22 03:44, Roman Gushchin wrote:
> On Mon, Jun 27, 2022 at 05:12:55AM +0300, Vasily Averin wrote:
>> When cgroup_mkdir reaches the limits of the cgroup hierarchy, it should
>> not return -EAGAIN, but instead react similarly to reaching the global
>> limit.
>>
>> Signed-off-by: Vasily Averin <vvs@...nvz.org>
>> ---
>>  kernel/cgroup/cgroup.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index 1be0f81fe8e1..243239553ea3 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -5495,7 +5495,7 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
>>  		return -ENODEV;
>>  
>>  	if (!cgroup_check_hierarchy_limits(parent)) {
>> -		ret = -EAGAIN;
>> +		ret = -ENOSPC;
> 
> I'd not argue whether ENOSPC is better or worse here, but I don't think we need
> to change it now. It's been in this state for a long time and is a part of ABI.
> EAGAIN is pretty unique as a mkdir() result, so systemd can handle it well.

I would agree with you, however in my opinion EAGAIN is used to restart an
interrupted system call. Thus, I worry its return can loop the user space without
any chance of continuation.

However, maybe I'm confusing something?

Thank you,
	Vasily Averin