diff --git a/man/systemd-fstab-generator.xml b/man/systemd-fstab-generator.xml
index ec8f5c988e1..3c5a5cc50a3 100644
--- a/man/systemd-fstab-generator.xml
+++ b/man/systemd-fstab-generator.xml
@@ -82,9 +82,20 @@
root=
- Takes the root filesystem to mount in the
- initrd. root= is honored by the
- initrd.
+ Configures the operating system's root filesystem to mount when running in the
+ initrd. This accepts a device node path (usually /dev/disk/by-uuid/… or
+ /dev/disk/by-label/… or similar), or the special values gpt-auto
+ and tmpfs.
+
+ Use gpt-auto to explicitly request automatic root file system discovery via
+ systemd-gpt-auto-generator8.
+
+ Use tmpfs in order to mount a tmpfs5 file
+ system as root file system of the OS. This is useful in combination with
+ mount.usr= (see below) in order to combine a volatile root file system with a
+ separate, immutable /usr/ file system. Also see
+ systemd.volatile= below.
@@ -193,10 +204,19 @@
or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.
- Note that with the exception of overlay mode, enabling this setting will only work
- correctly on operating systems that can boot up with only /usr/ mounted, and are able to
- automatically populate /etc/, and also /var/ in case of
- systemd.volatile=yes.
+ Note that with the exception of overlay mode, enabling this setting will
+ only work correctly on operating systems that can boot up with only /usr/
+ mounted, and are able to automatically populate /etc/, and also
+ /var/ in case of systemd.volatile=yes.
+
+ Also see root=tmpfs above, for a method to combine a
+ tmpfs file system with a regular /usr/ file system (as
+ configured via mount.usr=). The main distinction between
+ systemd.volatile=yes, and root=tmpfs in combination
+ mount.usr= is that the former operates on top of a regular root file system and
+ temporarily obstructs the files and directories above its /usr/ subdirectory,
+ while the latter does not hide any files, but simply mounts a unpopulated tmpfs as root file system
+ and combines it with a user picked /usr/ file system.
@@ -218,6 +238,7 @@
systemd.mount5,
systemd.swap5,
systemd-cryptsetup-generator8,
+ systemd-gpt-auto-generator8,
kernel-command-line7
diff --git a/src/fstab-generator/fstab-generator.c b/src/fstab-generator/fstab-generator.c
index a526d6e8fa8..6df7fa53289 100644
--- a/src/fstab-generator/fstab-generator.c
+++ b/src/fstab-generator/fstab-generator.c
@@ -671,7 +671,8 @@ static int parse_fstab(bool initrd) {
static int add_sysroot_mount(void) {
_cleanup_free_ char *what = NULL;
- const char *opts;
+ const char *opts, *fstype;
+ bool default_rw;
int r;
if (isempty(arg_root_what)) {
@@ -691,12 +692,29 @@ static int add_sysroot_mount(void) {
return 0;
}
- what = fstab_node_to_udev_node(arg_root_what);
- if (!what)
- return log_oom();
+ if (streq(arg_root_what, "tmpfs")) {
+ /* If root=tmpfs is specified, then take this as shortcut for a writable tmpfs mount as root */
+
+ what = strdup("rootfs"); /* just a pretty name, to show up in /proc/self/mountinfo */
+ if (!what)
+ return log_oom();
+
+ fstype = arg_root_fstype ?: "tmpfs"; /* tmpfs, unless overriden */
+
+ default_rw = true; /* writable, unless overriden */;
+ } else {
+
+ what = fstab_node_to_udev_node(arg_root_what);
+ if (!what)
+ return log_oom();
+
+ fstype = arg_root_fstype; /* if not specified explicitly, don't default to anything here */
+
+ default_rw = false; /* read-only, unless overriden */
+ }
if (!arg_root_options)
- opts = arg_root_rw > 0 ? "rw" : "ro";
+ opts = arg_root_rw > 0 || (arg_root_rw < 0 && default_rw) ? "rw" : "ro";
else if (arg_root_rw >= 0 ||
!fstab_test_option(arg_root_options, "ro\0" "rw\0"))
opts = strjoina(arg_root_options, ",", arg_root_rw > 0 ? "rw" : "ro");
@@ -715,7 +733,7 @@ static int add_sysroot_mount(void) {
what,
"/sysroot",
NULL,
- arg_root_fstype,
+ fstype,
opts,
is_device_path(what) ? 1 : 0, /* passno */
0, /* makefs off, growfs off, noauto off, nofail off, automount off */