linux-kernel - Re: [PATCH 2/5] mm: memcontrol: make page_memcg{

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Date:   Mon, 1 Mar 2021 14:09:55 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john.fastabend@...il.com, kpsingh@...nel.org,
        Ingo Molnar <mingo@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, bristot@...hat.com,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        alexander.h.duyck@...ux.intel.com,
        Chris Down <chris@...isdown.name>,
        Wei Yang <richard.weiyang@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Oskolkov <posk@...gle.com>, Jann Horn <jannh@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, daniel.vetter@...ll.ch,
        Waiman Long <longman@...hat.com>,
        Michel Lespinasse <walken@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>, krisman@...labora.com,
        esyr@...hat.com, Suren Baghdasaryan <surenb@...gle.com>,
        Marco Elver <elver@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        duanxiongchun@...edance.com
Subject: Re: [PATCH 2/5] mm: memcontrol: make page_memcg{_rcu} only
 applicable for non-kmem page

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@...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?

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.