[RFC PATCH] rootfs: force mounting rootfs as tmpfs

[Mimi Zohar]

Commit 16203a7a9422 ("initmpfs: make rootfs use tmpfs when CONFIG_TMPFS
enabled") introduced using tmpfs as the rootfs filesystem.  The use of
tmpfs is limited to systems that do not specify "root=" on the boot
command line.

Without the check "!saved_root_name[0]", rootfs uses tmpfs.  As there
must be a valid reason for this check, this patch introduces a new boot
command line option named "noramfs" to force rootfs to use tmpfs.

Signed-off-by: Mimi Zohar <zo...@linux.vnet.ibm.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  2 ++
 init/do_mounts.c                                | 15 +++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 6571fbfdb2a1..fd82df2ff150 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2733,6 +2733,8 @@
 
        nopcid          [X86-64] Disable the PCID cpu feature.
 
+       noramfs         Don't use ramfs for rootfs, use tmpfs.
+
        norandmaps      Don't use address space randomization.  Equivalent to
                        echo 0 > /proc/sys/kernel/randomize_va_space
 
diff --git a/init/do_mounts.c b/init/do_mounts.c
index 7cf4f6dafd5f..74d8bfcd1294 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -315,6 +315,16 @@ static int __init root_data_setup(char *str)
        return 1;
 }
 
+static bool force_tmpfs;
+static int __init force_tmpfs_setup(char *str)
+{
+       if (*str)
+               return 0;
+       force_tmpfs = true;
+       return 1;
+
+}
+
 static char * __initdata root_fs_names;
 static int __init fs_names_setup(char *str)
 {
@@ -332,6 +342,7 @@ static int __init root_delay_setup(char *str)
 __setup("rootflags=", root_data_setup);
 __setup("rootfstype=", fs_names_setup);
 __setup("rootdelay=", root_delay_setup);
+__setup("noramfs", force_tmpfs_setup);
 
 static void __init get_fs_names(char *page)
 {
@@ -632,8 +643,8 @@ int __init init_rootfs(void)
        if (err)
                return err;
 
-       if (IS_ENABLED(CONFIG_TMPFS) && !saved_root_name[0] &&
-               (!root_fs_names || strstr(root_fs_names, "tmpfs"))) {
+       if (IS_ENABLED(CONFIG_TMPFS) && (force_tmpfs || (!saved_root_name[0] &&
+               (!root_fs_names || strstr(root_fs_names, "tmpfs"))))) {
                err = shmem_init();
                is_tmpfs = true;
        } else {
-- 
2.7.5