Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <8bc21e32-fdb0-e1a3-477f-dd660646ccdd@bytedance.com>
Date:   Mon, 10 Jul 2023 18:58:35 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0)
 Gecko/20100101 Thunderbird/102.12.0
Subject: Re: [PATCH] mm: kfence: allocate kfence_metadata at runtime
To:     Alexander Potapenko <glider@google.com>
Cc:     elver@google.com, dvyukov@google.com, akpm@linux-foundation.org,
        kasan-dev@googlegroups.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, muchun.song@linux.dev,
        Peng Zhang <zhangpeng.00@bytedance.com>
References: <20230710032714.26200-1-zhangpeng.00@bytedance.com>
 <CAG_fn=XH8s8JbMKjsyyw_FZhLuoBqAwWU_+hCGyAXwe3wTBCWQ@mail.gmail.com>
From:   Peng Zhang <zhangpeng.00@bytedance.com>
In-Reply-To: <CAG_fn=XH8s8JbMKjsyyw_FZhLuoBqAwWU_+hCGyAXwe3wTBCWQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk


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@googlegroups.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?