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

[Muchun Song]

On Tue, Mar 2, 2021 at 3:09 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> 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 ;-)

At first, I just used scripts/get_maintainer.pl to get the
CC list. I will reduce the CC list in the next version.
Thanks.

>
> 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.

OK. I will follow your suggestion. Thanks.