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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 19 Mar 2021 06:59:13 -0700
Message-ID: <CALvZod5H-hL8myH1hy6YRis5vGT5JSYTQv5AeM0zagymCg0mcA@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v4 4/5] mm: memcontrol: use obj_cgroup APIs
 to charge kmem pages
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Roman Gushchin <guro@fb.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Xiongchun duan <duanxiongchun@bytedance.com>
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On Thu, Mar 18, 2021 at 9:05 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Fri, Mar 19, 2021 at 11:40 AM Shakeel Butt <shakeelb@google.com> wrote:
> >
> > On Thu, Mar 18, 2021 at 4:08 AM Muchun Song <songmuchun@bytedance.com> wrote:
> > >
> > [...]
> > >
> > > +static inline struct mem_cgroup *get_obj_cgroup_memcg(struct obj_cgroup *objcg)
> >
> > I would prefer get_mem_cgroup_from_objcg().
>
> Inspired by obj_cgroup_memcg() which returns the memcg from objcg.
> So I introduce get_obj_cgroup_memcg() which obtains a reference of
> memcg on the basis of obj_cgroup_memcg().
>
> So that the names are more consistent. Just my thought.
>
> So should I rename it to get_mem_cgroup_from_objcg?
>

If you look at other functions which get reference on mem_cgroup, they
have the format of get_mem_cgroup_*. Similarly the current function to
get a reference on obj_cgroup is get_obj_cgroup_from_current().

So, from the name get_obj_cgroup_memcg(), it seems like we are getting
reference on obj_cgroup but the function is getting reference on
mem_cgroup.

> >
> > > +{
> > > +       struct mem_cgroup *memcg;
> > > +
> > > +       rcu_read_lock();
> > > +retry:
> > > +       memcg = obj_cgroup_memcg(objcg);
> > > +       if (unlikely(!css_tryget(&memcg->css)))
> > > +               goto retry;
> > > +       rcu_read_unlock();
> > > +
> > > +       return memcg;
> > > +}
> > > +
> > >  #ifdef CONFIG_MEMCG_KMEM
> > >  int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
> > >                                  gfp_t gfp, bool new_page)
> > > @@ -3070,15 +3088,8 @@ static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp,
> > >         struct mem_cgroup *memcg;
> > >         int ret;
> > >
> > > -       rcu_read_lock();
> > > -retry:
> > > -       memcg = obj_cgroup_memcg(objcg);
> > > -       if (unlikely(!css_tryget(&memcg->css)))
> > > -               goto retry;
> > > -       rcu_read_unlock();
> > > -
> > > +       memcg = get_obj_cgroup_memcg(objcg);
> > >         ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
> >
> > Why not manually inline __memcg_kmem_charge() here? This is the only user.
> >
> > Similarly manually inline __memcg_kmem_uncharge() into
> > obj_cgroup_uncharge_pages() and call obj_cgroup_uncharge_pages() in
> > obj_cgroup_release().
>
> Good point. I will do this.
>
> >
> > > -
> > >         css_put(&memcg->css);
> > >
> > >         return ret;
> > > @@ -3143,18 +3154,18 @@ static void __memcg_kmem_uncharge(struct mem_cgroup *memcg, unsigned int nr_page
> > >   */
> > >  int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
> > >  {
> > > -       struct mem_cgroup *memcg;
> > > +       struct obj_cgroup *objcg;
> > >         int ret = 0;
> > >
> > > -       memcg = get_mem_cgroup_from_current();
> >
> > This was the only use of get_mem_cgroup_from_current(). Why not remove it?
>
> I saw a potential user.
>
>     [PATCH v10 0/3] Charge loop device i/o to issuing cgroup
>
> To avoid reintroducing them. So I did not remove it.
>

Don't worry about that. Most probably that user would be changing this
function, so it would to better to introduce from scratch.