In some cases, creating a cgroup allocates a noticeable amount of memory. This operation can be executed from inside memory-limited container, but currently this memory is not accounted to memcg and can be misused. This allow container to exceed the assigned memory limit and avoid memcg OOM. Moreover, in case of global memory shortage on the host, the OOM-killer may not find a real memory eater and start killing random processes on the host. This is especially important for OpenVZ and LXC used on hosting, where containers are used by untrusted end users. Below is tracing results of mkdir /sys/fs/cgroup/vvs.test on 4cpu VM with Fedora and self-complied upstream kernel. The calculations are not precise, it depends on kernel config options, number of cpus, enabled controllers, ignores possible page allocations etc. However this is enough to clarify the general situation. All allocations are splitted into: - common part, always called for each cgroup type - per-cgroup allocations In each group we consider 2 corner cases: - usual allocations, important for 1-2 CPU nodes/Vms - percpu allocations, important for 'big irons' common part: ~11Kb + 318 bytes percpu memcg: ~17Kb + 4692 bytes percpu cpu: ~2.5Kb + 1036 bytes percpu cpuset: ~3Kb + 12 bytes percpu blkcg: ~3Kb + 12 bytes percpu pid: ~1.5Kb + 12 bytes percpu perf: ~320b + 60 bytes percpu ------------------------------------------- total: ~38Kb + 6142 bytes percpu currently accounted: 4668 bytes percpu - it's important to account usual allocations called in common part, because almost all of cgroup-specific allocations are small. One exception here is memory cgroup, it allocates a few huge objects that should be accounted. - Percpu allocation called in common part, in memcg and cpu cgroups should be accounted, rest ones are small an can be ignored. - KERNFS objects are allocated both in common part and in most of cgroups Details can be found here: https://lore.kernel.org/all/d28233ee-bccb-7bc3-c2ec-461fd7f95e6a@xxxxxxxxxx/ I checked other cgroups types was found that they all can be ignored. Additionally I found allocation of struct rt_rq called in cpu cgroup if CONFIG_RT_GROUP_SCHED was enabled, it allocates huge (~1700 bytes) percpu structure and should be accounted too. v4: 1) re-based to linux-next (next-20220610) now psi_group is not a part of struct cgroup and is allocated on demand 2) added received approval from Muchun Song 3) improved cover letter description according to akpm@ request v3: 1) re-based to current upstream (v5.18-11267-gb00ed48bb0a7) 2) fixed few typos 3) added received approvals v2: 1) re-split to simplify possible bisect, re-ordered 2) added accounting for percpu psi_group_cpu and cgroup_rstat_cpu, allocated in common part 3) added accounting for percpu allocation of struct rt_rq (actual if CONFIG_RT_GROUP_SCHED is enabled) 4) improved patches descriptions Vasily Averin (9): memcg: enable accounting for struct cgroup memcg: enable accounting for kernfs nodes memcg: enable accounting for kernfs iattrs memcg: enable accounting for struct simple_xattr memcg: enable accounting for percpu allocation of struct psi_group_cpu memcg: enable accounting for percpu allocation of struct cgroup_rstat_cpu memcg: enable accounting for large allocations in mem_cgroup_css_alloc memcg: enable accounting for allocations in alloc_fair_sched_group memcg: enable accounting for perpu allocation of struct rt_rq fs/kernfs/mount.c | 6 ++++-- fs/xattr.c | 2 +- kernel/cgroup/cgroup.c | 2 +- kernel/cgroup/rstat.c | 3 ++- kernel/sched/fair.c | 4 ++-- kernel/sched/psi.c | 5 +++-- kernel/sched/rt.c | 2 +- mm/memcontrol.c | 4 ++-- 8 files changed, 16 insertions(+), 12 deletions(-) -- 2.36.1
