Add support for building a squashed initramfs

With all files stored in ramfs, and most of them are not compressed,
the initramfs will take up a lot of memory. Besides, if the file number
is large, each file will waste some memory due to page fragmetation.
This is due to ramfs' design, at least one page will be allocated for
one file however small the file is. On machine with large page size,
this will become worse and waste too many memory.

One approach to reducing the memory usage is to reduce the number of
files that got directly loaded into the root ramfs, and compress files
by put most files will into a read-only squash image and keep a minimum
set of executable and libraries outside as the loader for the squash
image. After the squash image is mounted, the real 'init' will be
executed and then everything behaves as usual.

This patch will introduce a '99squash' module which will never be
included by default. User can force add it, and if it is included,
dracut will perform some extra steps before creating the final image:

For now, "/etc" and "/usr" will be moved into the squashfs image.
"/init" will be renamed to "/init.stock" and replaced by "/init.squash".
Files and folders need to be accessible before mounting the image will
be still avaliable at their original place. And due to squashfs is
readonly, an overlayfs layer will be created on top of squashfs mount
point, as many dracut module require readwrite access to "/etc" and
"/usr", "init.squash" will ultimately call "/init.stock".

An extra systemd service will be installed. This service will umount all
squashfs related mount points right before switch-root to release
resources properly. This service will not actually do anything if
switch-root is not used.

This is very helpful when mem resource is very limited, like Kdump.
According to my tests, this squash module can help save about 35MB of
memory with 64K page size, or about 15MB with 4K page size on an
ordinary kdump capture routine. This module could also help reduce
memory usage for normal boot up process.

Won't change any behavior if squash module is not enabled.

Signed-off-by: Kairui Song <kasong@redhat.com>

dracut.sh

@@ -1747,6 +1747,120 @@ fi
 
 dinfo "*** Creating image file '$outfile' ***"
 
+if dracut_module_included "squash"; then
+    if ! check_kernel_config CONFIG_SQUASHFS; then
+        dfatal "CONFIG_SQUASHFS have to be enabled for dracut squash module to work"
+        exit 1
+    fi
+    if ! check_kernel_config CONFIG_OVERLAY_FS; then
+        dfatal "CONFIG_OVERLAY_FS have to be enabled for dracut squash module to work"
+        exit 1
+    fi
+    if ! check_kernel_config CONFIG_DEVTMPFS; then
+        dfatal "CONFIG_DEVTMPFS have to be enabled for dracut squash module to work"
+        exit 1
+    fi
+
+    readonly squash_dir="${DRACUT_TMPDIR}/squashfs"
+    readonly squash_img=$initdir/squash/root.img
+
+    # Currently only move "usr" "etc" to squashdir
+    readonly squash_candidate=( "usr" "etc" )
+
+    mkdir -m 0755 -p $squash_dir
+    for folder in "${squash_candidate[@]}"; do
+        mv $initdir/$folder $squash_dir/$folder
+    done
+
+    # Reinstall required files, because we have moved some important folders to $squash_dir
+    inst_multiple "echo" "sh" "mount" "modprobe" "mkdir" \
+        "systemctl" "udevadm" "$systemdutildir/systemd"
+    hostonly="" instmods "loop" "squashfs" "overlay"
+
+    for folder in "${squash_candidate[@]}"; do
+        # Remove duplicated files in squashfs image, save some more space
+        [[ ! -d $initdir/$folder/ ]] && continue
+        for file in $(find $initdir/$folder/ -not -type d);
+        do
+            if [[ -e $squash_dir${file#$initdir} ]]; then
+                mv $squash_dir${file#$initdir} $file
+            fi
+        done
+    done
+
+    # Move some files out side of the squash image, including:
+    # - Files required to boot and mount the squashfs image
+    # - Files need to be accessable without mounting the squash image
+    required_in_root() {
+        local file=$1
+        local _sqsh_file=$squash_dir/$file
+        local _init_file=$initdir/$file
+
+        if [[ -e $_init_file ]]; then
+            return
+        fi
+
+        if [[ ! -e $_sqsh_file ]] && [[ ! -L $_sqsh_file ]]; then
+            derror "$file is required to boot a squashed initramfs but it's not installed!"
+            return
+        fi
+
+        if [[ ! -d $(dirname $_init_file) ]]; then
+            required_in_root $(dirname $file)
+        fi
+
+        if [[ -d $_sqsh_file ]]; then
+            if [[ -L $_sqsh_file ]]; then
+                cp --preserve=all -P $_sqsh_file $_init_file
+            else
+                mkdir $_init_file
+            fi
+        else
+            if [[ -L $_sqsh_file ]]; then
+                cp --preserve=all -P $_sqsh_file $_init_file
+                _sqsh_file=$(realpath $_sqsh_file 2>/dev/null)
+                if [[ -e $_sqsh_file ]] && [[ "$_sqsh_file" == "$squash_dir"* ]]; then
+                    # Relative symlink
+                    required_in_root ${_sqsh_file#$squash_dir/}
+                    return
+                fi
+                if [[ -e $squash_dir$_sqsh_file ]]; then
+                    # Absolute symlink
+                    required_in_root ${_sqsh_file#/}
+                    return
+                fi
+                required_in_root ${module_spec#$squash_dir/}
+            else
+                mv $_sqsh_file $_init_file
+            fi
+        fi
+    }
+
+    required_in_root etc/initrd-release
+
+    for module_spec in $squash_dir/usr/lib/modules/*/modules.*;
+    do
+        required_in_root ${module_spec#$squash_dir/}
+    done
+
+    for dracut_spec in $squash_dir/usr/lib/dracut/*;
+    do
+        required_in_root ${dracut_spec#$squash_dir/}
+    done
+
+    mv $initdir/init $initdir/init.stock
+    mv $initdir/shutdown $initdir/shutdown.stock
+    ln -s squash/init.sh $initdir/init
+    ln -s squash/shutdown.sh $initdir/shutdown
+
+    mksquashfs $squash_dir $squash_img -comp xz -b 64K -Xdict-size 100% &> /dev/null
+
+    if [[ $? != 0 ]]; then
+        dfatal "dracut: Failed making squash image"
+        exit 1
+    fi
+fi
+
 if [[ $uefi = yes ]]; then
     readonly uefi_outdir="$DRACUT_TMPDIR/uefi"
     mkdir "$uefi_outdir"

modules.d/99squash/clear-squash.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+SQUASH_MNT_REC=/squash/mounts
+SQUASH_MNTS=( )
+
+while read mnt; do
+    SQUASH_MNTS+=( "$mnt" )
+done <<< "$(cat $SQUASH_MNT_REC)"
+
+umount --lazy -- ${SQUASH_MNTS[@]}

modules.d/99squash/init.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+/squash/setup-squash.sh
+
+exec /init.stock
+
+echo "Something went wrong when trying to start original init executable!"
+exit 1

modules.d/99squash/module-setup.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+
+check() {
+    return 255
+}
+
+depends() {
+    echo "bash systemd systemd-initrd"
+    return 0
+}
+
+installkernel() {
+    hostonly="" instmods squashfs loop overlay
+}
+
+install() {
+    if ! type -P mksquashfs >/dev/null || ! type -P unsquashfs >/dev/null ; then
+        derror "squash module requires squashfs-tools to be installed."
+        return 1
+    fi
+
+    inst_multiple kmod modprobe mount mkdir ln echo
+    inst $moddir/setup-squash.sh /squash/setup-squash.sh
+    inst $moddir/clear-squash.sh /squash/clear-squash.sh
+    inst $moddir/init.sh /squash/init.sh
+
+    inst "$moddir/squash-mnt-clear.service" "$systemdsystemunitdir/squash-mnt-clear.service"
+    ln_r "$systemdsystemunitdir/squash-mnt-clear.service" "$systemdsystemunitdir/initrd.target.wants/squash-mnt-clear.service"
+}

modules.d/99squash/setup-squash.sh

@@ -0,0 +1,61 @@
+#!/bin/sh
+PATH=/bin:/sbin
+
+SQUASH_IMG=/squash/root.img
+SQUASH_MNT=/squash/root
+SQUASH_MNT_REC=/squash/mounts
+SQUASHED_MNT="usr etc"
+
+echo $SQUASH_MNT > $SQUASH_MNT_REC
+
+# Following mount points are neccessary for mounting a squash image
+
+[ ! -d /proc/self ] && \
+    mount -t proc -o nosuid,noexec,nodev proc /proc
+
+[ ! -d /sys/kernel ] && \
+    mount -t sysfs -o nosuid,noexec,nodev sysfs /sys
+
+[ ! -e /dev/loop-control ] && \
+    mount -t devtmpfs -o mode=0755,noexec,nosuid,strictatime devtmpfs /dev
+
+# Need a loop device backend, overlayfs, and squashfs module
+modprobe loop
+if [ $? != 0 ]; then
+    echo "Unable to setup loop module"
+fi
+
+modprobe squashfs
+if [ $? != 0 ]; then
+    echo "Unable to setup squashfs module"
+fi
+
+modprobe overlay
+if [ $? != 0 ]; then
+    echo "Unable to setup overlay module"
+fi
+
+[ ! -d "$SQUASH_MNT" ] && \
+	mkdir -m 0755 -p $SQUASH_MNT
+
+# Mount the squashfs image
+mount -t squashfs -o ro,loop $SQUASH_IMG $SQUASH_MNT
+
+if [ $? != 0 ]; then
+    echo "Unable to mount squashed initramfs image"
+fi
+
+for file in $SQUASHED_MNT; do
+	lowerdir=$SQUASH_MNT/$file
+	workdir=/squash/overlay-work/$file
+	upperdir=/$file
+	mntdir=/$file
+
+	mkdir -m 0755 -p $workdir
+	mkdir -m 0755 -p $mntdir
+
+	mount -t overlay overlay -o\
+		lowerdir=$lowerdir,upperdir=$upperdir,workdir=$workdir $mntdir
+
+	echo $mntdir >> $SQUASH_MNT_REC
+done

modules.d/99squash/shutdown.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+/squash/setup-squash.sh
+
+exec /shutdown.stock
+
+echo "Something went wrong when trying to start original shutdown executable!"
+exit 1

modules.d/99squash/squash-mnt-clear.service

@@ -0,0 +1,19 @@
+#  This file is part of dracut.
+#
+
+[Unit]
+Description=Cleanup squashfs mounts when switch root
+DefaultDependencies=no
+After=initrd.target
+After=dracut-initqueue.service dracut-pre-pivot.service
+Before=initrd-cleanup.service
+ConditionPathExists=/squash/root
+Conflicts=initrd-switch-root.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+StandardInput=null
+StandardOutput=syslog+console
+StandardError=syslog+console
+ExecStop=/squash/clear-squash.sh