linux-kernel - RE: [EXT] [systemd-devel] [PATCH 0/3] Add the ability to mount filesystems during initramfs expansion

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Message-ID: <7F889C39-4D60-4A12-9F60-D4F4B7B75474@zytor.com>
Date: Tue, 03 Feb 2026 08:11:54 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: "Windl, Ulrich" <u.windl@....de>, Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>
CC: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Lennart Poettering <lennart@...ttering.net>,
        "systemd-devel@...ts.freedesktop.org" <systemd-devel@...ts.freedesktop.org>
Subject: RE: [EXT] [systemd-devel] [PATCH 0/3] Add the ability to mount filesystems during initramfs expansion

On February 1, 2026 11:38:23 PM PST, "Windl, Ulrich" <u.windl@....de> wrote:
>Hi!
>
>I wonder: wouldn't it be nicer to use a subdirectory like ".systemd-magic" to place such magic files there that are interpreted by systemd? Then "!!!MOUNT!!!" would become a simple "mount" or maybe "fstab" or "mountttab", ...
>
>Kind regards,
>Ulrich Windl
>
>> -----Original Message-----
>> From: systemd-devel <systemd-devel-bounces@...ts.freedesktop.org> On
>> Behalf Of H. Peter Anvin
>> Sent: Saturday, January 24, 2026 1:40 AM
>> To: Alexander Viro <viro@...iv.linux.org.uk>; Christian Brauner
>> <brauner@...nel.org>; Jan Kara <jack@...e.cz>; Jonathan Corbet
>> <corbet@....net>; H. Peter Anvin <hpa@...or.com>
>> Cc: linux-fsdevel@...r.kernel.org; linux-doc@...r.kernel.org; linux-
>> kernel@...r.kernel.org; Lennart Poettering <lennart@...ttering.net>;
>> systemd-devel@...ts.freedesktop.org
>> Subject: [EXT] [systemd-devel] [PATCH 0/3] Add the ability to mount
>> filesystems during initramfs expansion
>> 
>> 
>> At Plumber's 2024, Lennart Poettering of the systemd project requested
>> the ability to overmount the rootfs with a separate tmpfs before
>> initramfs expansion, so the populated tmpfs can be unmounted.
>> 
>> This patchset takes this request and goes one step further: it allows
>> (mostly) arbitrary filesystems mounts during initramfs processing.
>> 
>> This is done by having the initramfs expansion code detect the special
>> filename "!!!MOUNT!!!" which is then parsed into a simplified
>> fstab-type mount specification and the directory in which the
>> !!!MOUNT!!! entry is used as the mount point.
>> 
>> This specific method was chosen for the following reasons:
>> 
>> 1. This information is specific to the expectations of the initramfs;
>>    therefore using kernel command line options is not
>>    appropriate. This way the information is fully contained within the
>>    initramfs itself.
>> 2. The sequence !!! is already special in cpio, due to the "TRAILER!!!"
>>    entries.
>> 3. The filename "!!!MOUNT!!!" will typically be sorted first, which
>>    means using standard find+cpio tools to create the initramfs still
>>    work.
>> 4. Similarly, standard cpio can still expand the initramfs.
>> 5. If run on a legacy kernel, the !!!MOUNT!!! file is created, which
>>    is easy to detect in the initramfs code which can then activate
>>    some fallback code.
>> 6. It allows for multiple filesystems to be mounted, possibly of
>>    different types and in different locations, e.g. the initramfs can
>>    get started with /dev, /proc, and /sys already booted.
>> 
>> The patches are:
>> 
>>     1/3: fs/init: move creating the mount data_page into init_mount()
>>     2/3: initramfs: support mounting filesystems during initramfs expansion
>>     3/3: Documentation/initramfs: document mount points in initramfs
>> 
>> ---
>>  .../driver-api/early-userspace/buffer-format.rst   | 60 +++++++++++++-
>>  fs/init.c                                          | 23 +++++-
>>  include/linux/init_syscalls.h                      |  3 +-
>>  init/do_mounts.c                                   | 17 +---
>>  init/initramfs.c                                   | 95 +++++++++++++++++++++-
>>  5 files changed, 175 insertions(+), 23 deletions(-)
>

The point is that this is done during initramfs deencapsulation.

Either way, it doesn't seem like there is interest.