rootless: cgroups v1: Failed to add conmon to cgroupfs sandbox cgroup: error creating cgroup for SOMETHING

[edsantiago]

Fallout from #8479 (changing log-level from error to warn): podman run, rootless, on a cgroups v1 system, emits unhelpful warnings:

$ podman run quay.io/libpod/testimage:20200929 date
WARN[0000] Failed to add conmon to cgroupfs sandbox cgroup: error creating cgroup for memory: mkdir /sys/fs/cgroup/memory/libpod_parent: permission denied
Mon Dec 14 21:39:58 UTC 2020

The command works fine, exits 0 and everything, but the warning is harmful to CI tests as well as to users who neither know nor care about libpod_parent and stuff like that.

The precise message differs. These are variants I see:

...creating cgroup for cpu: mkdir /sys/fs/cgroup/cpu/libpod_parent: permission denied
...creating cgroup for blkio: mkdir /sys/fs/cgroup/blkio/libpod_parent: permission denied
...creating cgroup for pids: mkdir /sys/fs/cgroup/pids/libpod_parent: permission denied

Please either fix the condition leading to the warning, or remove the warning.

[rhatdan]

@giuseppe @haircommander @mheon WDYT?

[giuseppe]

Proposed fix here: #8726