Re: [External] Re: [PATCH v4 4/5] mm: memcontrol: use obj_cgroup APIs to charge kmem pages

[Muchun Song <songmuchun@bytedance.com>]

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?

>
> > +{
> > +       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.

>
> > -       if (memcg && !mem_cgroup_is_root(memcg)) {
> > -               ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
> > +       objcg = get_obj_cgroup_from_current();
> > +       if (objcg) {
> > +               ret = obj_cgroup_charge_pages(objcg, gfp, 1 << order);
> >                 if (!ret) {
> > -                       page->memcg_data = (unsigned long)memcg |
> > +                       page->memcg_data = (unsigned long)objcg |
> >                                 MEMCG_DATA_KMEM;
> >                         return 0;
> >                 }
> > -               css_put(&memcg->css);
> > +               obj_cgroup_put(objcg);
> >         }
> >         return ret;
> >  }
> [...]
> >  static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> >  {
> >         unsigned long nr_pages;
> > +       struct mem_cgroup *memcg;
> > +       struct obj_cgroup *objcg;
> >
> >         VM_BUG_ON_PAGE(PageLRU(page), page);
> >
> > -       if (!page_memcg(page))
> > -               return;
> > -
> >         /*
> >          * Nobody should be changing or seriously looking at
> > -        * page_memcg(page) at this point, we have fully
> > +        * page memcg or objcg at this point, we have fully
> >          * exclusive access to the page.
> >          */
> > +       if (PageMemcgKmem(page)) {
> > +               objcg = __page_objcg(page);
> > +               memcg = get_obj_cgroup_memcg(objcg);
>
> Can you add a comment that this get matches the put at the end of the
> function and kmem pages do not hold memcg references anymore.

Make sense. Will do.

Thanks for your suggestions.

>
> > +       } else {
> > +               memcg = __page_memcg(page);
> > +       }
> > +
> > +       if (!memcg)
> > +               return;
> >
> > -       if (ug->memcg != page_memcg(page)) {
> > +       if (ug->memcg != memcg) {
> >                 if (ug->memcg) {
> >                         uncharge_batch(ug);
> >                         uncharge_gather_clear(ug);
> >                 }
> > -               ug->memcg = page_memcg(page);
> > +               ug->memcg = memcg;
> >                 ug->dummy_page = page;
> >
> >                 /* pairs with css_put in uncharge_batch */
> > -               css_get(&ug->memcg->css);
> > +               css_get(&memcg->css);
> >         }
> >
> >         nr_pages = compound_nr(page);
> > -       ug->nr_pages += nr_pages;
> >
> > -       if (PageMemcgKmem(page))
> > +       if (PageMemcgKmem(page)) {
> > +               ug->nr_memory += nr_pages;
> >                 ug->nr_kmem += nr_pages;
> > -       else
> > +
> > +               page->memcg_data = 0;
> > +               obj_cgroup_put(objcg);
> > +       } else {
> > +               /* LRU pages aren't accounted at the root level */
> > +               if (!mem_cgroup_is_root(memcg))
> > +                       ug->nr_memory += nr_pages;
> >                 ug->pgpgout++;
> >
> > -       page->memcg_data = 0;
> > -       css_put(&ug->memcg->css);
> > +               page->memcg_data = 0;
> > +       }
> > +
> > +       css_put(&memcg->css);
> >  }
> >
> >  /**
> > --
> > 2.11.0
> >