How to boot into root btrfs file system with minimal initramfs without udev hook?

Question

Arch Linux is UEFI booted from USB flash drive with GPT and three partitions:

1. EFI (vfat file system)
2. root (btrfs file system, was converted from ext4)
3. home (btrfs file system, was converted from ext4)

The btrfs partitions have no sub-volumes and are on a single disk (the USB flash memory drive). LVM is not in use here.

Task

Trying to create a minimal initramfs by removing udev and a lot of other hooks. Optimizing Bootup With mkinitcpio was used as inspiration too.
The effective mkinitcpio hooks are: base, autodetect and modconf.

Btrfs hook

The btrfs hook is not enabled because mkinitcpio hooks documentation lists for btrfs hook:

This hook is not required for using Btrfs on a single device.

Regression

1. I have tried to remove udev -> boot error
2. I have tried to add the btrfs module -> boot error
3. I have added btrfs hook -> boot error
4. Changed root=PARTUUID= to root=UUID= notation -> boot error
5. adding paramters rootfstype=btrfs -> boot error
6. rootdelay=0 -> boot error
7. rootdelay=10 -> boot error
8. Mounting using /dev/sda2 from emergency shell -> ok

Error

Only after inserting udev or systemd hooks the system will root into the btrfs root partition, otherwise this error appears:

ERROR: device 'PARTUUID=c2...c13' not found. Skipping fsck.
:: mounting 'PARTUUID=c2...c13' on real root 
mount: can't find 'PARTUUID=c2...c13'
You are now being dropped into an emergency shell.

Runtime init debug/log output

Enabling boot parameters rd.debug and rd.log shows that "premount" calls the resolve_device function and that returns an empty lookup.

resolve_device PARTUUID=c2...c13
local major minor dev tag device=PARTUUID=c2...c13
blkid -lt PARTUUID=c2...c13 -o device
dev=

The last empty dev is causing the device not found error.

initramfs mount command

mount_handler=default_mount_handler
...
# Mount root at /new_root
"$mount_handler" /new_root

source: https://git.archlinux.org/mkinitcpio.git/tree/init

default_mount_handler() {
 msg ":: mounting '$root' on real root"
 mount ${rootfstype:+-t $rootfstype} -o ${rwopt:-ro}${rootflags:+,$rootflags} "$root" "1ドル"

source: https://git.archlinux.org/mkinitcpio.git/tree/init_functions#n375

initramfs mount version

[rootfs ]# mount -V
mount from util-linux 2.29.2 (libmount 2.29.2: btrfs, assert, debug)

initramfs contents

$ lsinitcpio -a /boot/initramfs-linux-tiny.img
==> Image: /boot/initramfs-linux-tiny.img
==> Created with mkinitcpio 23
==> Kernel: 4.10.3-1-ARCH
==> Size: 3.53 MiB
==> Compressed with: lz4 -l
 -> Uncompressed size: 8.32 MiB (.424 ratio)
 -> Estimated extraction time: 0.028s
==> Included modules:
 ahci [explicit] hid-generic [explicit] raid6_pq usbcore
 atkbd [explicit] i8042 [explicit] scsi_mod usbhid [explicit]
 btrfs [explicit] libahci sd_mod [explicit] xhci-hcd
 crc32c-intel [explicit] libata serio xhci-pci [explicit]
 crc32c_generic libcrc32c serio_raw [explicit] xor
 ehci-hcd libps2 uas [explicit]
 ehci-pci [explicit] ohci-hcd usb-common
 hid ohci-pci [explicit] usb-storage
==> Included binaries:
 blkid busybox dosfsck fsck fsck.vfat kmod mount switch_root

The emergency shell its blkid command lists the correct (PART)UUID value. Could the mounting using using (PART)UUID fail because there is no /dev/disk/?

Question

What is necessary to boot into a non-raid non-subvolume single-drive root btrfs partition located on a USB flash drive without udev?

---

PS This error might be caused by a RACE condition, UUID/PARTUUID not yet available when initramfs/init executes the mount ... UUID=... command.

Answer 1

Cause

In version 23, the mkinitcpio resolve_device() function is called only once. When at execution time the drive labels are not yet read, blkid can't lookup the kernel drive (/dev/...) name for the requested label.

Solution

By adding the "without-udev" hook, as listed below, the resolve_device function is left untouched. Though standard available mkinitcpio functionality to (削除) override the mount_handler (削除ここまで) add a run_hook is used to poll until blkid returns a value, or (a timeout of) 10 seconds has passed. Thus the "udev" hook can be removed from mkinitcpio config.

Notes

• This solution was created with the help of falconindy .
• There was an error message in the early boot phase involving fsck. To remove that message the without-udev hook has been rewritten to use a run_hook instead of a (削除) mount_handler (削除ここまで). The newer code is even shorter.

---

$ cat /usr/lib/initcpio/hooks/without-udev
#!/bin/ash
# Minimal initramfs files are created without udev.
# This hooks provides a polling disk mount replacement for udev.
# Udev hook can be removed, resulting in smaller initramfs files.
run_hook () {
 local dev timeout sleepval device=$root
 # if udev is running then exit
 [ "$udevd_running" -eq 1 ] && return
 # try for (timeout * sleepval =) 10 seconds to handle slow (USB) devices
 timeout=1000
 sleepval=0.01
 case $device in
 # label to resolve, when resolved the kernel block device also exists
 UUID=*|LABEL=*|PARTUUID=*|PARTLABEL=*)
 while [ $timeout -gt 0 ]; do
 timeout=$((timeout - 1))
 dev=$(blkid -lt "$device" -o device)
 [ -n "$dev" ] && timeout=0 || sleep $sleepval
 done
 ;;
 # kernel named block device, poll for existence
 /dev/*)
 while [ $timeout -gt 0 ]; do
 timeout=$((timeout -1))
 if [ -b "$device" ]; then
 dev=$device
 timeout=0
 else
 sleep $sleepval
 fi
 done
 ;;
 esac
}
# vim:set syntax=sh:

---

$ cat /usr/lib/initcpio/install/without-udev
#!/bin/bash
build() {
 add_runscript
}
help() {
 cat <<HELPEOF
This hook provides support for booting without the "udev" hook,
including support for UUID, LABEL, PARTUUID, PARTLABEL.
HELPEOF
}
# vim: set ft=sh ts=4 sw=4 et:

• ZeroPhase: Should I be getting ERROR: Device 'PARTUUID:XXXX..' not found. Skipping fsck. with this workaround? I boot fine, but that error gets listed.
• Pro Backup: @ZeroPhase I my initcpio-without-udev setup, I do indeed also get the message ERROR: Device 'PARTUUID:XXXX..' not found. Skipping fsck. during boot.
• ZeroPhase: That's not going to cause issues with fsck?
• Pro Backup: @ZeroPhase I don't think that ERROR: Device PARTUUID message will cause issues with fsck. A little later in code fsck is done again on /dev/sd(a). That is the line prepended with the comment: # call fsck again because in main loop $root device was unknown at that time.
• Pro Backup: @ZeroPhase The without-udev hook code has been updated to no longer output the ERROR: Device 'PARTUUID:XXXX..' not found. Skipping fsck. message. As a side effect the code is even shorter.