On Tue, Oct 20, 2020 at 06:27:14PM +0200, Michal Koutny wrote: > Hi. > > On Mon, Oct 19, 2020 at 03:28:45PM -0700, Roman Gushchin <guro@xxxxxx> wrote: > > Currently the root memory cgroup is never charged directly, but > > if an ancestor cgroup is charged, the charge is propagated up to the > s/ancestor/descendant/ Oops, will fix, thanks! > > > The root memory cgroup doesn't show the charge to a user, neither it > > does allow to set any limits/protections. > An appealing claim, I'd like this to be true... > > > Please, note, that cgroup v1 provides root level memory.usage_in_bytes. > > However, it's not based on page counters (refer to mem_cgroup_usage()). > ...and it almost is. But there are still exposed kmem and tcpmem counters. Hm, I wonder what do they show given that we never set sk->sk_memcg to the root_mem_cgroup (see mem_cgroup_sk_alloc()) and we never charge the root_mem_cgroup for !slab kmem allocations (see __memcg_kmem_charge_page()). So yeah, it's quite a mess now, and it looks like it has been broken in multiple places and for a while. If we want these counter to function properly, then we should go into the opposite direction and remove the special handling of the root memory cgroup in many places. > > To avoid multiple identical checks over the page counters > > code, for_each_nonroot_ancestor() macro is introduced. > If the assumptions behind this patch's idea were true, I think the > implementation would be simpler by merely (not)connecting the root > counters and keep the traversal as is. We use some fields in root page counters to calculate protections: see propagate_protected_usage(). Thanks!
