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

From: Johannes Weiner <hannes@cmpxchg.org>
To: Shakeel Butt <shakeelb@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>,
	Amir Goldstein <amir73il@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	john.fastabend@gmail.com, kpsingh@kernel.org,
	Ingo Molnar <mingo@redhat.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	dietmar.eggemann@arm.com, Steven Rostedt <rostedt@goodmis.org>,
	Benjamin Segall <bsegall@google.com>,
	Mel Gorman <mgorman@suse.de>,
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	Suren Baghdasaryan <surenb@google.com>,
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	duanxiongchun@bytedance.com
Subject: Re: [PATCH 2/5] mm: memcontrol: make page_memcg{_rcu} only applicable for non-kmem page
Date: Mon, 1 Mar 2021 14:09:55 -0500	[thread overview]
Message-ID: <YD08A+fEp/4pw10I@cmpxchg.org> (raw)
In-Reply-To: <CALvZod7sysj0+wrzLTXnwn7s_Gf-V2eFPJ6cLcoRmR0LdAFk0Q@mail.gmail.com>

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@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?

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.