On Sun, May 09, 2021 at 05:54:03PM -0700, Andrew Morton wrote: > On Sun, 25 Apr 2021 15:54:10 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote: > > > The below scenario can cause the page counters of the root_mem_cgroup > > to be out of balance. > > > > CPU0: CPU1: > > > > objcg = get_obj_cgroup_from_current() > > obj_cgroup_charge_pages(objcg) > > memcg_reparent_objcgs() > > // reparent to root_mem_cgroup > > WRITE_ONCE(iter->memcg, parent) > > // memcg == root_mem_cgroup > > memcg = get_mem_cgroup_from_objcg(objcg) > > // do not charge to the root_mem_cgroup > > try_charge(memcg) > > > > obj_cgroup_uncharge_pages(objcg) > > memcg = get_mem_cgroup_from_objcg(objcg) > > // uncharge from the root_mem_cgroup > > refill_stock(memcg) > > drain_stock(memcg) > > page_counter_uncharge(&memcg->memory) > > > > get_obj_cgroup_from_current() never returns a root_mem_cgroup's objcg, > > so we never explicitly charge the root_mem_cgroup. And it's not > > going to change. It's all about a race when we got an obj_cgroup > > pointing at some non-root memcg, but before we were able to charge it, > > the cgroup was gone, objcg was reparented to the root and so we're > > skipping the charging. Then we store the objcg pointer and later use > > to uncharge the root_mem_cgroup. > > > > This can cause the page counter to be less than the actual value. > > Although we do not display the value (mem_cgroup_usage) so there > > shouldn't be any actual problem, but there is a WARN_ON_ONCE in > > the page_counter_cancel(). Who knows if it will trigger? So it > > is better to fix it. > > > > It sounds like Roman will be acking this, but some additional reviewer > attention would be helpful, please. > The patch is technically correct, so I'm ok to ack it: Acked-by: Roman Gushchin <guro@xxxxxx> I remember Michal was looking into it, so he can probably add something here. Thanks!
