Re: [PATCH v6 05/19] mm: memcontrol: decouple reference counting from page accounting

[Shakeel Butt]

On Thu, Jun 18, 2020 at 6:08 PM Roman Gushchin <guro@xxxxxx> wrote:
>
> On Thu, Jun 18, 2020 at 07:55:35AM -0700, Shakeel Butt wrote:
> > Not sure if my email went through, so, re-sending.
> >
> > On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@xxxxxx> wrote:
> > >
> > > From: Johannes Weiner <hannes@xxxxxxxxxxx>
> > >
> > [...]
> > > @@ -3003,13 +3004,16 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
> > >   */
> > >  void mem_cgroup_split_huge_fixup(struct page *head)
> > >  {
> > > +       struct mem_cgroup *memcg = head->mem_cgroup;
> > >         int i;
> > >
> > >         if (mem_cgroup_disabled())
> > >                 return;
> > >
> >
> > A memcg NULL check is needed here.
>
> Hm, it seems like the only way how it can be NULL is if mem_cgroup_disabled() is true:
>
> int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
> {
>         unsigned int nr_pages = hpage_nr_pages(page);
>         struct mem_cgroup *memcg = NULL;
>         int ret = 0;
>
>         if (mem_cgroup_disabled())
>                 goto out;
>
>         <...>
>
>         if (!memcg)
>                 memcg = get_mem_cgroup_from_mm(mm);
>
>         ret = try_charge(memcg, gfp_mask, nr_pages);
>         if (ret)
>                 goto out_put;
>
>         css_get(&memcg->css);
>         commit_charge(page, memcg);
>
>
> Did you hit this issue in reality? The only possible scenario I can imagine
> is if the page was allocated before enabling memory cgroups.
>
> Are you about this case?
>
> Otherwise we put root_mem_cgroup there.
>

Oh yes, you are right. I am confusing this with kmem pages for root
memcg where we don't set the page->mem_cgroup and this patch series
should be changing that.

Shakeel