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

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Date:   Mon, 10 Jul 2023 18:58:35 +0800
From:   Peng Zhang <zhangpeng.00@...edance.com>
To:     Alexander Potapenko <glider@...gle.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,
        Peng Zhang <zhangpeng.00@...edance.com>
Subject: Re: [PATCH] mm: kfence: allocate kfence_metadata at runtime



在 2023/7/10 18:37, Alexander Potapenko 写道:
> 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.
I don't know why the 2MB memory was saved, but it looks like it has to 
do with the .bss section, maybe removing this array affected the linker?