Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page

[Muchun Song]

On Fri, Feb 5, 2021 at 4:36 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Fri 05-02-21 14:27:19, Muchun Song wrote:
> > The get_mem_cgroup_from_page() is called under page lock, so the page
> > memcg cannot be changed under us.
>
> Where is the page lock enforced?

Because it is called from alloc_page_buffers(). This path is under
page lock.

>
> > Also, css_get is enough because page
> > has a reference to the memcg.
>
> tryget used to be there to guard against offlined memcg but we have
> concluded this is impossible in this path. tryget stayed there to catch
> some unexpected cases IIRC.

Yeah, it can catch some unexpected cases. But why is this path
special so that we need a tryget?

>
> > If we really want to make the get_mem_cgroup_from_page() suitable for
> > arbitrary page, we should use page_memcg_rcu() instead of page_memcg()
> > and call it after rcu_read_lock().
>
> What is the primary motivation to change this code? is the overhead of
> tryget/RCU something that needs optimizing?

Actually, the rcu_read_lock() is not necessary here. So it is better to
remove it (indeed reduce some code).

Thanks.


>
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  mm/memcontrol.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 87f01bc05d1f..6c7f1ea3955e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1063,16 +1063,15 @@ EXPORT_SYMBOL(get_mem_cgroup_from_mm);
> >   */
> >  struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
> >  {
> > -     struct mem_cgroup *memcg = page_memcg(page);
> > +     struct mem_cgroup *memcg;
> >
> >       if (mem_cgroup_disabled())
> >               return NULL;
> >
> > -     rcu_read_lock();
> >       /* Page should not get uncharged and freed memcg under us. */
> > -     if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
> > -             memcg = root_mem_cgroup;
> > -     rcu_read_unlock();
> > +     memcg = page_memcg(page) ? : root_mem_cgroup;
> > +     css_get(&memcg->css);
> > +
> >       return memcg;
> >  }
> >  EXPORT_SYMBOL(get_mem_cgroup_from_page);
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs