Re: [PATCH] mm: introduce entrance for root_mem_cgroup's current

[Shakeel Butt]

On Thu, Feb 2, 2023 at 12:27 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 02-02-23 12:32:57, zhaoyang.huang wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > Introducing memory.root_current for the memory charges on root_mem_cgroup.
>
> Charges are not currently accounted for the root memcg universally. See
> try_charge which is used for all user space and skmem charges. I am not
> 100% sure about objcg based accounting because there is no explicit
> check for the root memcg but this might be hidden somewhere as well.

Yes in __get_obj_cgroup_from_memcg(). However the reason to use
try_charge_memcg() to bypass root check was to avoid the race with
reparenting. More details in c5c8b16b596e ("mm: memcontrol: fix
root_mem_cgroup charging").

>
> That means that the patch as is doesn't really provide and usable value.
> The root exemption has been removed in the past but that has been
> reverted due to a regression. See ce00a967377b ("mm: memcontrol: revert
> use of root_mem_cgroup res_counter") for more.
>

One advantage I can see is if someone is looking for usage for all top
containers (alive or zombie) but I wanted to know if that was the real
motivation behind the patch.