linux-kernel - Re: [PATCH v5 13/16] mm: memcontrol: reuse memory cgroup ID for kmem ID

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Message-ID: <CAMZfGtWhYOFCKtFoHB7vsKmpEdj9F-axkba6p9EpuPUp3BK-ag@mail.gmail.com>
Date:   Thu, 6 Jan 2022 11:34:58 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Koutný <mkoutny@...e.com>,
        Mika Penttilä <mika.penttila@...tfour.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <guro@...com>, Yang Shi <shy828301@...il.com>,
        Alex Shi <alexs@...nel.org>,
        Wei Yang <richard.weiyang@...il.com>,
        Dave Chinner <david@...morbit.com>,
        trond.myklebust@...merspace.com, anna.schumaker@...app.com,
        jaegeuk@...nel.org, chao@...nel.org,
        Kari Argillander <kari.argillander@...il.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-nfs@...r.kernel.org, Qi Zheng <zhengqi.arch@...edance.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        Fam Zheng <fam.zheng@...edance.com>,
        Muchun Song <smuchun@...il.com>
Subject: Re: [PATCH v5 13/16] mm: memcontrol: reuse memory cgroup ID for kmem ID

On Thu, Jan 6, 2022 at 1:03 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Mon, Dec 20, 2021 at 04:56:46PM +0800, Muchun Song <songmuchun@...edance.com> wrote:
> > There are two idrs being used by memory cgroup, one is for kmem ID,
> > another is for memory cgroup ID. The maximum ID of both is 64Ki.
> > Both of them can limit the total number of memory cgroups.
> > Actually, we can reuse memory cgroup ID for kmem ID to simplify the
> > code.
>
> An interesting improvement.
>
> I'm a bit dense -- what's the purpose the MEM_CGROUP_ID_DIFF offset?

Hi Michal and Mika,

MEM_CGROUP_ID_DIFF is introduced to be consistent with before
that kmem ID starts with -1 and has no holes. Actually, it can be dropped
and make memcg->kmemcg_id equal to memcg->id.id directly.

> Couldn't this deduplication be extended to only use mem_cgroup.id.id
> instead of mem_cgroup.kmemcg_id? (With a boolean telling whether kmem
> accounting is active.)
>

Not easy to completely remove memcg->kmemcg_id since this filed
will be used to sync list_lru reparenting which will change
memcg->kmemcg_id to its parent's kmem ID (more details refers to
memcg_drain_all_list_lrus() in patch 10 of this series).

Thanks.