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 <20211220085649.8196-14-songmuchun@bytedance.com> <20220105170348.GA21070@blackbody.suse.cz>
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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Thu, 6 Jan 2022 11:34:58 +0800
Message-ID: <CAMZfGtWhYOFCKtFoHB7vsKmpEdj9F-axkba6p9EpuPUp3BK-ag@mail.gmail.com>
Subject: Re: [PATCH v5 13/16] mm: memcontrol: reuse memory cgroup ID for kmem ID
To:     =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        =?UTF-8?Q?Mika_Penttil=C3=A4?= <mika.penttila@nextfour.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Shakeel Butt <shakeelb@google.com>,
        Roman Gushchin <guro@fb.com>, Yang Shi <shy828301@gmail.com>,
        Alex Shi <alexs@kernel.org>,
        Wei Yang <richard.weiyang@gmail.com>,
        Dave Chinner <david@fromorbit.com>,
        trond.myklebust@hammerspace.com, anna.schumaker@netapp.com,
        jaegeuk@kernel.org, chao@kernel.org,
        Kari Argillander <kari.argillander@gmail.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-nfs@vger.kernel.org, Qi Zheng <zhengqi.arch@bytedance.com>,
        Xiongchun duan <duanxiongchun@bytedance.com>,
        Fam Zheng <fam.zheng@bytedance.com>,
        Muchun Song <smuchun@gmail.com>
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On Thu, Jan 6, 2022 at 1:03 AM Michal Koutn=C3=BD <mkoutny@suse.com> wrote:
>
> On Mon, Dec 20, 2021 at 04:56:46PM +0800, Muchun Song <songmuchun@bytedan=
ce.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.