Re: [PATCH 2/5] mm: memcontrol: make page_memcg{_rcu} only applicable for non-kmem page

[Shakeel Butt <shakeelb@google.com>]

On Sun, Feb 28, 2021 at 10:25 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> We want to reuse the obj_cgroup APIs to reparent the kmem pages when
> the memcg offlined. If we do this, we should store an object cgroup
> pointer to page->memcg_data for the kmem pages.
>
> Finally, page->memcg_data can have 3 different meanings.
>
>   1) For the slab pages, page->memcg_data points to an object cgroups
>      vector.
>
>   2) For the kmem pages (exclude the slab pages), page->memcg_data
>      points to an object cgroup.
>
>   3) For the user pages (e.g. the LRU pages), page->memcg_data points
>      to a memory cgroup.
>
> Currently we always get the memcg associated with a page via page_memcg
> or page_memcg_rcu. page_memcg_check is special, it has to be used in
> cases when it's not known if a page has an associated memory cgroup
> pointer or an object cgroups vector. Because the page->memcg_data of
> the kmem page is not pointing to a memory cgroup in the later patch,
> the page_memcg and page_memcg_rcu cannot be applicable for the kmem
> pages. In this patch, we introduce page_memcg_kmem to get the memcg
> associated with the kmem pages. And make page_memcg and page_memcg_rcu
> no longer apply to the kmem pages.
>
> In the end, there are 4 helpers to get the memcg associated with a
> page. The usage is as follows.
>
>   1) Get the memory cgroup associated with a non-kmem page (e.g. the LRU
>      pages).
>
>      - page_memcg()
>      - page_memcg_rcu()

Can you rename these to page_memcg_lru[_rcu] to make them explicitly
for LRU pages?

>
>   2) Get the memory cgroup associated with a kmem page (exclude the slab
>      pages).
>
>      - page_memcg_kmem()
>
>   3) Get the memory cgroup associated with a page. It has to be used in
>      cases when it's not known if a page has an associated memory cgroup
>      pointer or an object cgroups vector. Returns NULL for slab pages or
>      uncharged pages, otherwise, returns memory cgroup for charged pages
>      (e.g. kmem pages, LRU pages).
>
>      - page_memcg_check()
>
> In some place, we use page_memcg to check whether the page is charged.
> Now we introduce page_memcg_charged helper to do this.
>
> This is a preparation for reparenting the kmem pages. To support reparent
> kmem pages, we just need to adjust page_memcg_kmem and page_memcg_check in
> the later patch.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
[snip]
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -855,10 +855,11 @@ void __mod_lruvec_page_state(struct page *page, enum node_stat_item idx,
>                              int val)
>  {
>         struct page *head = compound_head(page); /* rmap on tail pages */
> -       struct mem_cgroup *memcg = page_memcg(head);
> +       struct mem_cgroup *memcg;
>         pg_data_t *pgdat = page_pgdat(page);
>         struct lruvec *lruvec;
>
> +       memcg = PageMemcgKmem(head) ? page_memcg_kmem(head) : page_memcg(head);

Should page_memcg_check() be used here?

>         /* Untracked pages have no memcg, no lruvec. Update only the node */
>         if (!memcg) {
>                 __mod_node_page_state(pgdat, idx, val);
> @@ -3170,12 +3171,13 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
>   */
>  void __memcg_kmem_uncharge_page(struct page *page, int order)
>  {
> -       struct mem_cgroup *memcg = page_memcg(page);
> +       struct mem_cgroup *memcg;
>         unsigned int nr_pages = 1 << order;
>
> -       if (!memcg)
> +       if (!page_memcg_charged(page))
>                 return;
>
> +       memcg = page_memcg_kmem(page);
>         VM_BUG_ON_PAGE(mem_cgroup_is_root(memcg), page);
>         __memcg_kmem_uncharge(memcg, nr_pages);
>         page->memcg_data = 0;
> @@ -6831,24 +6833,25 @@ static void uncharge_batch(const struct uncharge_gather *ug)
>  static void uncharge_page(struct page *page, struct uncharge_gather *ug)
>  {
>         unsigned long nr_pages;
> +       struct mem_cgroup *memcg;
>
>         VM_BUG_ON_PAGE(PageLRU(page), page);
>
> -       if (!page_memcg(page))
> +       if (!page_memcg_charged(page))
>                 return;
>
>         /*
>          * Nobody should be changing or seriously looking at
> -        * page_memcg(page) at this point, we have fully
> -        * exclusive access to the page.
> +        * page memcg at this point, we have fully exclusive
> +        * access to the page.
>          */
> -
> -       if (ug->memcg != page_memcg(page)) {
> +       memcg = PageMemcgKmem(page) ? page_memcg_kmem(page) : page_memcg(page);

Same, should page_memcg_check() be used here?