Re: [External] Re: [PATCH 5/5] mm, memcg: always call __mod_node_page_state() with preempt disabled

Muchun Song
2021-07-30T02:33:59Z

On Fri, Jul 30, 2021 at 9:52 AM Miaohe Lin <linmiaohe-hv44wF8Li93QT0dZR+AlfA@public.gmane.org> wrote:
>
> On 2021/7/29 22:39, Shakeel Butt wrote:
> > On Thu, Jul 29, 2021 at 5:58 AM Miaohe Lin <linmiaohe-hv44wF8Li93QT0dZR+AlfA@public.gmane.org> wrote:
> >>
> >> We should always ensure __mod_node_page_state() is called with preempt
> >> disabled or percpu ops may manipulate the wrong cpu when preempt happened.
> >>
> >> Fixes: b4e0b68fbd9d ("mm: memcontrol: use obj_cgroup APIs to charge kmem pages")
> >> Signed-off-by: Miaohe Lin <linmiaohe-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
> >> ---
> >> mm/memcontrol.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index 70a32174e7c4..616d1a72ece3 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -697,8 +697,8 @@ void __mod_lruvec_page_state(struct page *page, enum node_stat_item idx,
> >> memcg = page_memcg(head);
> >> /* Untracked pages have no memcg, no lruvec. Update only the node */
> >> if (!memcg) {
> >> - rcu_read_unlock();
> >> __mod_node_page_state(pgdat, idx, val);
> >> + rcu_read_unlock();
> >
> > This rcu is for page_memcg. The preemption and interrupts are disabled
> > across __mod_lruvec_page_state().
> >
>
> I thought it's used to protect __mod_node_page_state(). Looks somewhat confusing for me.
> Many thanks for pointing this out!

Hi Miaohe,

git show b4e0b68fbd9d can help you find out why we add
the rcu read lock around it.

Thanks.

>
> >> return;
> >> }
> >>
> >> --
> >> 2.23.0
> >>
> > .
> >
>