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

Roman Gushchin
2020-06-19T01:08:58Z

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@fb.com> wrote:
> >
> > From: Johannes Weiner <hannes@cmpxchg.org>
> >
> [...]
> > @@ -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.

Thanks!