Re: [External] Re: [PATCH] mm: memcontrol: fix memsw uncharge for root_mem_cgroup

[Muchun Song]

On Wed, Mar 24, 2021 at 4:33 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 24-03-21 12:11:35, Muchun Song wrote:
> > On Tue, Mar 23, 2021 at 11:04 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > >
> > > The pages aren't accounted at the root level, so we cannot uncharge the
> > > page to the memsw counter for the root memcg. Fix this.
> > >
> > > Fixes: 1f47b61fb407 ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
> > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >
> > I am very sorry. I should repent. I suddenly realise the fix is totally
> > wrong. Because the @memcg cannot be root memcg when
> > @memcg != @swap_memcg.
>
> I am probably blind but I do not see why this would be the case.
> We have memcg != swap_memcg in this branch but we do not know the
> neither of the two is root_mem_cgroup, no? If we did knot that we
> wouldn't have to check for swap_memcg != root_mem_cgroup. Or do I miss
> something?

I look at the mem_cgroup_id_get_online() closely. If memcg is root, this
function always returns root memcg. So memcg will equal swap_memcg.
I apologize for not carefully reviewing the code myself.

Thanks.

> --
> Michal Hocko
> SUSE Labs