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

[Shakeel Butt]

On Thu, Mar 18, 2021 at 4:08 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
[...]
>
> +static inline struct mem_cgroup *get_obj_cgroup_memcg(struct obj_cgroup *objcg)

I would prefer 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().

> -
>         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?

> -       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.

> +       } 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
>