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

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Muchun, can you please reduce the CC list to mm/memcg folks only for
the next submission? I think probably 80% of the current recipients
don't care ;-)

On Mon, Mar 01, 2021 at 10:11:45AM -0800, Shakeel Butt wrote:
> On Sun, Feb 28, 2021 at 10:25 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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?

The next patch removes page_memcg_kmem() again to replace it with
page_objcg(). That should (luckily) remove the need for this
distinction and keep page_memcg() simple and obvious.

It would be better to not introduce page_memcg_kmem() in the first
place in this patch, IMO.



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