Date:   Tue, 2 Mar 2021 11:03:05 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
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        Xiongchun duan <duanxiongchun@...edance.com>
Subject: Re: [External] Re: [PATCH 2/5] mm: memcontrol: make page_memcg{_rcu}
 only applicable for non-kmem page

On Tue, Mar 2, 2021 at 2:11 AM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Sun, Feb 28, 2021 at 10:25 PM Muchun Song <songmuchun@...edance.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?

Yes. Will do. Thanks.

>
> >
> >   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@...edance.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?

Yeah. page_memcg_check() can be used here.
But on the inside of the page_memcg_check(),
there is a READ_ONCE(). Actually, we do not
need READ_ONCE() here. So I use page_memcg
or page_memcg_kmem directly. Thanks.

>
> >         /* 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?

Same as above.

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