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From: Askar Safin <safinaskar@gmail.com>
Date: Wed, 20 Dec 2023 05:19:25 +0300
Message-ID: <CAPnZJGDcNwPLbzC99qNQ+bRMwxPU-Z0xe=TD6DWQU=0MNyeftA@mail.gmail.com>
Subject: Re: [PATCH v3] rootfs: Fix support for rootfstype= when root= is given
To: stefanb@linux.ibm.com
Cc: gregkh@linuxfoundation.org, initramfs@vger.kernel.org, 
	linux-kernel@vger.kernel.org, rob@landley.net, stable@vger.kernel.org, 
	zohar@linux.ibm.com
Content-Type: text/plain; charset="UTF-8"

On Sun, Nov 19, 2023 at 08:12:48PM -0500, Stefan Berger wrote:
> Documentation/filesystems/ramfs-rootfs-initramfs.rst states:
>
>   If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
>   default.  To force ramfs, add "rootfstype=ramfs" to the kernel command
>   line.
>
> This currently does not work when root= is provided since then
> saved_root_name contains a string and rootfstype= is ignored. Therefore,
> ramfs is currently always chosen when root= is provided.

Maybe it is a good idea to just fully remove ramfs? initramfs will
always be tmpfs. And tmpfs will always be enabled.

As well as I understand, ramfs was originally introduced, because
tmpfs seemed too big. So, it seemed to be a good idea to have small fs
(ramfs), which is always enabled.

I just did an experiment. I compiled the kernel with a very small
config. And without TMPFS and SHMEM. I got 1059440 bytes image. Then I
enabled TMPFS and SHMEM, and I got 1072976 bytes. So tmpfs adds 13536
bytes, i. e. 14k, which is a very small amount. It adds 1.3 % to the
kernel even with very small config.

So I propose to remove ramfs and always enable tmpfs. This will
decrease complexity.

Here are my configs (x86_64). Just enough to run busybox in "qemu -serial stdio"

make KCONFIG_ALLCONFIG="$FILE" allnoconfig

CONFIG_64BIT=y
CONFIG_PRINTK=y
CONFIG_SERIAL_8250=y
CONFIG_TTY=y
CONFIG_SERIAL_8250_CONSOLE=y
CONFIG_BLK_DEV_INITRD=y
CONFIG_RD_GZIP=y
CONFIG_BINFMT_ELF=y
CONFIG_EMBEDDED=y
CONFIG_EXPERT=y
CONFIG_TMPFS=n # Try to change this to "y"
CONFIG_SHMEM=n # Try to change this to "y"

Here is full docker reproducer:

# Reproducible
# 20230227 = 20230227T000000Z = 20230226T090712Z
FROM debian:sid-20230227
ENV LC_ALL C.UTF-8
RUN sed -i 's~^URIs:.*$~URIs:
http://snapshot.debian.org/archive/debian/20230226T090712Z~'
/etc/apt/sources.list.d/debian.sources
RUN echo 'Acquire::Check-Valid-Until "false";' >
/etc/apt/apt.conf.d/02acquire-check-valid-until
RUN apt-get update && apt-get install -y apt-utils whiptail
RUN apt-get update && apt-get install -y busybox-static
qemu-system-x86 make gcc git flex bison bc libelf-dev less nano cpio
RUN git clone --depth=1 -b v6.2
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
RUN : > /.config
# See Linux f8f0d06438e5c810d1e13b5f8c2fed501fe36e9c
RUN echo 'CONFIG_64BIT=y' >> /.config
RUN echo 'CONFIG_PRINTK=y' >> /.config
RUN echo 'CONFIG_SERIAL_8250=y' >> /.config
RUN echo 'CONFIG_TTY=y' >> /.config
RUN echo 'CONFIG_SERIAL_8250_CONSOLE=y' >> /.config
RUN echo 'CONFIG_BLK_DEV_INITRD=y' >> /.config
RUN echo 'CONFIG_RD_GZIP=y' >> /.config
RUN echo 'CONFIG_BINFMT_ELF=y' >> /.config
RUN echo 'CONFIG_EMBEDDED=y' >> /.config
RUN echo 'CONFIG_EXPERT=y' >> /.config
RUN echo 'CONFIG_TMPFS=y' >> /.config # try "n"
RUN echo 'CONFIG_SHMEM=y' >> /.config # try "n"
RUN cd linux && make KCONFIG_ALLCONFIG=/.config allnoconfig
RUN cd linux && make -j4
RUN mkdir /initramfs && cp /bin/busybox /initramfs && cd /initramfs &&
ln -s busybox sh && find . | cpio --create --format=newc --quiet |
gzip > /initramfs.cpio.gz
RUN echo "qemu-system-x86_64 -M microvm -m 64M -serial stdio -display
none -kernel /linux/arch/x86/boot/bzImage -initrd /initramfs.cpio.gz
-append 'quiet console=ttyS0 earlyprintk=ttyS0 rdinit=/sh' -nodefaults
-no-user-config" > /root/.bash_history

-- 
Askar Safin