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

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Date:   Wed, 29 Jun 2022 09:13:02 +0300
From:   Vasily Averin <vvs@...nvz.org>
To:     Tejun Heo <tj@...nel.org>,
        Michal Koutný <mkoutny@...e.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        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 12:22, Tejun Heo wrote:
> On Tue, Jun 28, 2022 at 11:16:48AM +0200, Michal Koutný wrote:
>> The mkdir(2) manpage doesn't list EAGAIN at all. ENOSPC makes better
>> sense here. (And I suspect the dependency on this particular value won't
>> be very wide spread.)
> 
> Given how we use these system calls as triggers for random kernel
> operations, I don't think adhering to posix standard is necessary or
> possible. Using an error code which isn't listed in the man page isn't
> particularly high in the list of discrepancies.
> 
> Again, I'm not against changing it but I'd like to see better
> rationales. On one side, we have "it's been this way for a long time
> and there's nothing particularly broken about it". I'm not sure the
> arguments we have for the other side is strong enough yet.

I would like to recall this patch.

I experimented on fedora36 node with LXC and centos stream 9 container.
and I did not noticed any critical systemd troubles with original -EAGAIN.
When cgroup's limit is reached systemd cannot start new services, 
for example lxc-attach generates following output:

[root@...4-vvs ~]# lxc-attach c9s
lxc-attach: c9s: cgroups/cgfsng.c: cgroup_attach_leaf: 2084 Resource temporarily unavailable - Failed to create leaf cgroup ".lxc"
lxc-attach: c9s: cgroups/cgfsng.c: __cgroup_attach_many: 3517 Resource temporarily unavailable - Failed to attach to cgroup fd 11
lxc-attach: c9s: attach.c: lxc_attach: 1679 Resource temporarily unavailable - Failed to attach cgroup
lxc-attach: c9s: attach.c: do_attach: 1237 No data available - Failed to receive lsm label fd
lxc-attach: c9s: attach.c: do_attach: 1375 Failed to attach to container

I did not found any loop in userspace caused by EAGAIN.
Messages looks unclear, however situation with the patched kernel is not much better:

[root@...4-vvs ~]# lxc-attach c9s
lxc-attach: c9s: cgroups/cgfsng.c: cgroup_attach_leaf: 2084 No space left on device - Failed to create leaf cgroup ".lxc"
lxc-attach: c9s: cgroups/cgfsng.c: __cgroup_attach_many: 3517 No space left on device - Failed to attach to cgroup fd 11
lxc-attach: c9s: attach.c: lxc_attach: 1679 No space left on device - Failed to attach cgroup
lxc-attach: c9s: attach.c: do_attach: 1237 No data available - Failed to receive lsm label fd
lxc-attach: c9s: attach.c: do_attach: 1375 Failed to attach to container

Thank you,
	Vasily Averin