linux-kernel - Re: [PATCH] mm: kfence: allocate kfence

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Message-ID: <CAG_fn=XH8s8JbMKjsyyw_FZhLuoBqAwWU_+hCGyAXwe3wTBCWQ@mail.gmail.com>
Date:   Mon, 10 Jul 2023 12:37:44 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     elver@...gle.com, dvyukov@...gle.com, akpm@...ux-foundation.org,
        kasan-dev@...glegroups.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, muchun.song@...ux.dev
Subject: Re: [PATCH] mm: kfence: allocate kfence_metadata at runtime

On Mon, Jul 10, 2023 at 5:27 AM 'Peng Zhang' via kasan-dev
<kasan-dev@...glegroups.com> wrote:
>
> kfence_metadata is currently a static array. For the purpose of
> allocating scalable __kfence_pool, we first change it to runtime
> allocation of metadata. Since the size of an object of kfence_metadata
> is 1160 bytes, we can save at least 72 pages (with default 256 objects)
> without enabling kfence.
>
> Below is the numbers obtained in qemu (with default 256 objects).
> before: Memory: 8134692K/8388080K available (3668K bss)
> after: Memory: 8136740K/8388080K available (1620K bss)
> More than expected, it saves 2MB memory.

Do you have an understanding of where these 2MB come from?
According to your calculations (which seem valid) the gain should be
290K, so either 2MB is irrelevant to your change (then these numbers
should be omitted), or there's some hidden cost that we do not know
about.