Re: [PATCH v2] memcg: Only record foreign writebacks with dirty pages when memcg is not disabled

[Baoquan He]

On 09/24/19 at 02:27pm, Michal Hocko wrote:
> On Tue 24-09-19 19:11:51, Baoquan He wrote:
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index f3c15bb07cce..84e3fdb1ccb4 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4317,6 +4317,9 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
> >  
> >  	trace_track_foreign_dirty(page, wb);
> >  
> > +	if (mem_cgroup_disabled())
> > +		return;
> > +
> 
> This doesn't seem correct. We shouldn't even enter the slowpath with
> memcg disabled AFAIC. The check should be done at mem_cgroup_track_foreign_dirty
> level.

You mean the way in v1 patch, right? It's also fine to me.

I am worried about the case that memcg is enabled, the checking by
calling mem_cgroup_disabled() will lower efficiency. And it entering
into mem_cgroup_track_foreign_dirty_slowpath() should be a rare event.

Thanks
Baoquan