Understand the Initial RAM Filesystem (initramfs)

Topic Overview

Initial RAM filesystem

An initramfs is a cpio archive the bootloader loads into memory before the kernel starts. Early kernel code mounts it as a tmpfs root so drivers and scripts can run before the real root disk is available. For example, a typical Linux image ships initramfs-linux.img containing module-loading helpers and init glue that finds the LUKS or LVM root.

What lives inside

• /init: Early userspace (BusyBox, systemd, or dracut script)
• Kernel modules: Storage, filesystem, and crypto drivers needed to mount root
• Firmware blobs: Sometimes copied for late-loaded devices
• Hooks: Distro-specific scripts (cryptsetup, btrfs snapshots)

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• Dracut and mkinitcpio rebuild the image when kernels or modules change
• Unlike initrd (deprecated block device), initramfs is always a cpio archive in RAM
• Size affects boot time: large archives take longer to decompress

Early userspace flow

The kernel mounts tmpfs, extracts the cpio archive into rootfs, and executes /init. Dracut-generated images include udev rules to modprobe storage drivers before pivot_root. If /init exits, the kernel panics; keep the process alive until switch_root completes. Inspect an image with lsinitrd or zcat initramfs.img | cpio -itv to see which helpers your distro ships.

Why for this exercise

You will trace how the bootloader passes an initramfs blob to the kernel and what early /init must accomplish. This exercise requires describing why root-on-LUKS or NFS roots depend on initramfs content before pivot_root.

Your Task

Before the real root filesystem is mounted, /init in the initramfs loads the kernel modules it needs, dependencies first. Simulate it.

For each required module, in input order, load it depth-first: first its dependencies, in their listed order, each with its own dependencies, then the module itself. A module loads only once. Track each module's state (not visited, loading, loaded), so a cycle is detected instead of recursing forever.

Input

1. M, then M lines NAME DEPCOUNT dep1 dep2 ...: the modules the initramfs ships.
2. R, then R required module names.

Output

• LOAD <name> each time a module finishes loading.
• A dependency not in the initramfs: MISSING <dep> (needed by <module>), then EMERGENCY SHELL, and stop.
• A required module not in the initramfs: MISSING <name> (not in initramfs), then EMERGENCY SHELL, and stop.
• Reaching a module that is still loading: DEPENDENCY CYCLE at <name>, then EMERGENCY SHELL, and stop.
• When every required module loaded: ROOT MOUNTED and SWITCH_ROOT on separate lines.