Re: [PATCH v2] mm: memcontrol: fix root_mem_cgroup charging

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 25 Apr 2021 15:54:10 +0800 Muchun Song <songmuchun@bytedance.com> 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.