feature/runtime-squashfs: New feature to create a squashed initramfs

This feature introduces support for building a squashed initramfs to
significantly reduce memory usage during system boot, especially in
memory-constrained environments. By consolidating most files into a
compressed, read-only squashfs image, the number of files loaded
directly into ramfs is minimized, reducing both memory consumption and
page fragmentation.

Signed-off-by: Alexey Gladkov <[email protected]>

Author: legionus

Makefile.in

@@ -368,6 +368,11 @@ ifeq ($(BUILD_OS),linux-musl)
 BBCFG_SED_ARGS += -e "s,^CONFIG_FEATURE_VI_REGEX_SEARCH=.*,\# CONFIG_FEATURE_VI_REGEX_SEARCH is not set,"
 endif
 
+# We want to have this functionality in busybox, but we don't want symlinks to
+# exist because links won't allow us to install other implementations of these
+# utilities.
+BB_SYMLINK_DISABLE = ash insmod mount
+
 $(BBCFG): $(external_srcdir)/busybox/config
 	$(Q)mkdir -p -- "$(OBJDIR)/external/busybox"
 	$(Q)mkdir -p -- $(dest_runtimedir)
@@ -383,6 +388,7 @@ $(dest_data_bindir)/busybox: $(BBCFG)
 	$(Q)mkdir -p -- "$(dest_data_bindir)" "$(dest_data_sbindir)" "$(dest_data_libdir)"
 	$(Q)cp -a -- "$(OBJDIR)/external/busybox/busybox" "$@"
 	$(Q)while read -r busybox_link; do \
+	 for ign in $(BB_SYMLINK_DISABLE); do [ -n "$${busybox_link##*/$$ign}" ] || continue 2; done; \
 	 ln -srf -- "$@" "$(dest_runtimedir)$$busybox_link"; \
 	done < "$(OBJDIR)/external/busybox/busybox.links"
 

external/busybox/config

@@ -570,7 +570,7 @@ CONFIG_SULOGIN=y
 #
 # CONFIG_MODPROBE_SMALL is not set
 # CONFIG_DEPMOD is not set
-# CONFIG_INSMOD is not set
+CONFIG_INSMOD=y
 # CONFIG_LSMOD is not set
 # CONFIG_FEATURE_LSMOD_PRETTY_2_6_OUTPUT is not set
 # CONFIG_MODINFO is not set
@@ -645,7 +645,7 @@ CONFIG_XXD=y
 # CONFIG_IPCS is not set
 # CONFIG_LAST is not set
 # CONFIG_FEATURE_LAST_FANCY is not set
-# CONFIG_LOSETUP is not set
+CONFIG_LOSETUP=y
 # CONFIG_LSPCI is not set
 # CONFIG_LSUSB is not set
 CONFIG_MDEV=y
@@ -667,14 +667,14 @@ CONFIG_FEATURE_MDEV_DAEMON=y
 # CONFIG_MKSWAP is not set
 # CONFIG_FEATURE_MKSWAP_UUID is not set
 # CONFIG_MORE is not set
-# CONFIG_MOUNT is not set
-# CONFIG_FEATURE_MOUNT_FAKE is not set
-# CONFIG_FEATURE_MOUNT_VERBOSE is not set
+CONFIG_MOUNT=y
+CONFIG_FEATURE_MOUNT_FAKE=y
+CONFIG_FEATURE_MOUNT_VERBOSE=y
 # CONFIG_FEATURE_MOUNT_HELPERS is not set
 # CONFIG_FEATURE_MOUNT_LABEL is not set
 # CONFIG_FEATURE_MOUNT_NFS is not set
 # CONFIG_FEATURE_MOUNT_CIFS is not set
-# CONFIG_FEATURE_MOUNT_FLAGS is not set
+CONFIG_FEATURE_MOUNT_FLAGS=y
 # CONFIG_FEATURE_MOUNT_FSTAB is not set
 # CONFIG_FEATURE_MOUNT_OTHERTAB is not set
 CONFIG_MOUNTPOINT=y
@@ -1133,7 +1133,7 @@ CONFIG_SH_IS_NONE=y
 # CONFIG_BASH_IS_HUSH is not set
 CONFIG_BASH_IS_NONE=y
 # CONFIG_SHELL_ASH is not set
-# CONFIG_ASH is not set
+CONFIG_ASH=y
 # CONFIG_ASH_OPTIMIZE_FOR_SIZE is not set
 # CONFIG_ASH_INTERNAL_GLOB is not set
 # CONFIG_ASH_BASH_COMPAT is not set

features/runtime-squashfs/README.md

@@ -0,0 +1,27 @@
+# Feature: Squashed Runtime
+
+## Overview
+
+This feature introduces support for building a **squashed initramfs** to
+significantly reduce memory usage during system boot, especially in
+memory-constrained environments. By consolidating most files into a compressed,
+read-only squashfs image, the number of files loaded directly into ramfs is
+minimized, reducing both memory consumption and page fragmentation.
+
+## Background
+
+- **Problem:** Traditional initramfs stores all files in ramfs, which is
+  uncompressed and allocates at least one page per file, regardless of file
+  size. This leads to:
+  - High memory usage, especially with many small files.
+  - Increased waste on systems with large page sizes due to page fragmentation.
+
+- **Solution:** Move most files into a compressed squashfs image, keeping only a
+  minimal set of executables and libraries outside as a loader. This reduces the
+  number of files in ramfs and compresses their contents.
+
+## Parameters
+
+- **RUNTIME_SQUASHFS_COMPRESS** -- The parameter specifies a compression
+  algorithm (default: `xz`).
+- **RUNTIME_SQUASHFS_ARGS** -- Extra arguments for mksquashfs.

features/runtime-squashfs/bin/pack-squashfs

@@ -0,0 +1,78 @@
+#!/bin/bash -eu
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+. sh-functions
+
+if [ -f "$KERNEL_CONFIG" ]; then
+	rc=0
+	for cfgname in CONFIG_SQUASHFS CONFIG_BLK_DEV_LOOP CONFIG_OVERLAY_FS; do
+		if ! grep -qs "^$cfgname=[my]" "$KERNEL_CONFIG"; then
+			message "The kernel lacks support for needed parameters: $cfgname"
+			rc=1
+		fi
+	done
+	[ "$rc" -eq 0 ]
+fi
+
+mkdir "$rootdir/squash"
+
+find -P "$rootdir" -type f -execdir touch -c -m --date="1970-01-01 00:00:00 +0000" '{}' '+'
+
+mv -- "$rootdir" "$rootdir.raw"
+mkdir -- "$rootdir"
+
+mksquashfs "$rootdir.raw" "$rootdir"/squash.img -quiet -no-progress -no-xattrs \
+	${RUNTIME_SQUASHFS_ARGS} \
+	${RUNTIME_SQUASHFS_COMPRESS:+-comp "$RUNTIME_SQUASHFS_COMPRESS"} \
+	-all-root \
+	-p '/dev/ram     b 0644 0 0 1 1' \
+	-p '/dev/null    c 0666 0 0 1 3' \
+	-p '/dev/zero    c 0666 0 0 1 5' \
+	-p '/dev/full    c 0666 0 0 1 7' \
+	-p '/dev/random  c 0666 0 0 1 8' \
+	-p '/dev/systty  c 0666 0 0 4 0' \
+	-p '/dev/tty0    c 0666 0 0 4 0' \
+	-p '/dev/tty1    c 0666 0 0 4 1' \
+	-p '/dev/tty     c 0666 0 0 5 0' \
+	-p '/dev/console c 0600 0 0 5 1' \
+	-p '/dev/ptmx    c 0666 0 0 5 2' \
+#
+
+mkdir -p -- \
+	"$rootdir"/bin \
+	"$rootdir"/dev \
+	"$rootdir"/root \
+	"$rootdir"/squash \
+	"$rootdir"/usr \
+#
+
+ln -s -- ../bin  "$rootdir"/usr/bin
+ln -s -- ../sbin "$rootdir"/usr/bin
+
+put-file -r "$rootdir.raw" "$rootdir" "$rootdir.raw"/bin/busybox
+
+cat > "$rootdir"/init <<EOF
+#!/bin/busybox ash
+EOF
+
+depinfo --set-version="$KERNEL" --no-prefix --no-builtin \
+	$RUNTIME_SQUASHFS_MODULES |
+while read -r modname; do
+	mkdir -p -- "$rootdir/${modname%/*}"
+	cp -- "$rootdir.raw/$modname" "$rootdir/$modname"
+	printf '/bin/busybox insmod %s\n' "$modname"
+done >> "$rootdir/init"
+
+cat >> "$rootdir"/init <<EOF
+/bin/busybox mknod /dev/loop0 b 7 0
+/bin/busybox losetup /dev/loop0 /squash.img
+/bin/busybox mount -t ramfs ramfs /squash
+/bin/busybox mkdir /squash/root /squash/upper /squash/work
+/bin/busybox mount -t squashfs /dev/loop0 /squash/root
+/bin/busybox mount -t overlay overlay -o lowerdir=/squash/root,upperdir=/squash/upper,workdir=/squash/work /root
+/bin/busybox mount --move /squash /root/squash
+exec /bin/busybox switch_root /root /init
+EOF
+chmod +x "$rootdir"/init
+
+rm -rf -- "$rootdir.raw"

features/runtime-squashfs/config.mk

@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+RUNTIME_SQUASHFS_ARGS     ?= -b 1M
+RUNTIME_SQUASHFS_COMPRESS ?= xz

features/runtime-squashfs/rules.mk

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+RUNTIME_SQUASHFS_MODULES = loop squashfs overlay
+
+MODULES_ADD += $(RUNTIME_SQUASHFS_MODULES)
+
+pre-pack::
+	@$(VMSG) "Creating squashfs image ..."
+	@$(FEATURESDIR)/runtime-squashfs/bin/pack-squashfs