[RFC PATCH] Introduce a 'recovery' command line option

[Janne Karhunen]

Recovery option can be used to define a secondary rootfs
in case mounting of the primary root fails. This allows
the kernel to automatically switch to a recovery
filesystem without the initrd or the bootloader support
for the switch.

Signed-off-by: Janne Karhunen <Janne.Karhunen@xxxxxxxxx>
---
 Documentation/kernel-parameters.txt |  4 ++++
 init/do_mounts.c                    | 47 +++++++++++++++++++++++++++++++++----
 2 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 46c030a..febbd3e 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3587,6 +3587,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			      reboot_cpu is s[mp]#### with #### being the processor
 					to be used for rebooting.
 
+	recovery= 	[KNL]
+			Recovery root filesystem. This partition is attempted as
+			root in case default root filesystem does not mount.
+
 	relax_domain_level=
 			[KNL, SMP] Set scheduler's default relax_domain_level.
 			See Documentation/cgroup-v1/cpusets.txt.
diff --git a/init/do_mounts.c b/init/do_mounts.c
index dea5de9..c746dce 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -39,8 +39,11 @@ int __initdata rd_doload;	/* 1 = load RAM disk, 0 = don't load */
 
 int root_mountflags = MS_RDONLY | MS_SILENT;
 static char * __initdata root_device_name;
+static char * __initdata recovery_device_name;
 static char __initdata saved_root_name[64];
+static char __initdata saved_recovery_name[64];
 static int root_wait;
+static int recovery_attempt;
 
 dev_t ROOT_DEV;
 
@@ -298,6 +301,15 @@ static int __init root_dev_setup(char *line)
 
 __setup("root=", root_dev_setup);
 
+static int __init recovery_setup(char *line)
+{
+	strlcpy(saved_recovery_name, line, sizeof(saved_recovery_name));
+	recovery_attempt = 1;
+	return 1;
+}
+
+__setup("recovery=", recovery_setup);
+
 static int __init rootwait_setup(char *str)
 {
 	if (*str)
@@ -384,6 +396,7 @@ void __init mount_block_root(char *name, int flags)
 					__GFP_NOTRACK_FALSE_POSITIVE);
 	char *fs_names = page_address(page);
 	char *p;
+	int err;
 #ifdef CONFIG_BLOCK
 	char b[BDEVNAME_SIZE];
 #else
@@ -393,7 +406,7 @@ void __init mount_block_root(char *name, int flags)
 	get_fs_names(fs_names);
 retry:
 	for (p = fs_names; *p; p += strlen(p)+1) {
-		int err = do_mount_root(name, p, flags, root_mount_data);
+		err = do_mount_root(name, p, flags, root_mount_data);
 		switch (err) {
 			case 0:
 				goto out;
@@ -401,6 +414,31 @@ retry:
 			case -EINVAL:
 				continue;
 		}
+		if (!(flags & MS_RDONLY)) {
+			pr_warn("Retrying rootfs mount as read-only.\n");
+			flags |= MS_RDONLY;
+			goto retry;
+		}
+		if (recovery_device_name && recovery_attempt) {
+			recovery_attempt = 0;
+
+			ROOT_DEV = name_to_dev_t(recovery_device_name);
+			if (strncmp(recovery_device_name, "/dev/", 5) == 0)
+				recovery_device_name += 5;
+
+			pr_warn("Unable to mount rootfs at %s, error %d\n",
+				root_device_name, err);
+			pr_warn("Attempting %s for recovery as requested.\n",
+				recovery_device_name);
+
+			err = create_dev("/dev/root", ROOT_DEV);
+			if (err < 0)
+				pr_emerg("Failed to create /dev/root: %d\n", err);
+
+			root_device_name = recovery_device_name;
+			goto retry;
+		}
+
 	        /*
 		 * Allow the user to distinguish between failed sys_open
 		 * and bad superblock on root device.
@@ -420,10 +458,6 @@ retry:
 #endif
 		panic("VFS: Unable to mount root fs on %s", b);
 	}
-	if (!(flags & MS_RDONLY)) {
-		flags |= MS_RDONLY;
-		goto retry;
-	}
 
 	printk("List of all partitions:\n");
 	printk_all_partitions();
@@ -567,6 +601,9 @@ void __init prepare_namespace(void)
 
 	md_run_setup();
 
+	if (saved_recovery_name[0])
+		recovery_device_name = saved_recovery_name;
+
 	if (saved_root_name[0]) {
 		root_device_name = saved_root_name;
 		if (!strncmp(root_device_name, "mtd", 3) ||
-- 
1.9.1