aufs: ramfs branch

Basically ramfs is limited for its size, and is not suitable for aufs RW
branch. But people sometimes use it as RW branch without knowing it or
with knowing it.
This configuration is for those who knows what he is doing.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

fs/aufs/Kconfig

@@ -138,6 +138,19 @@ config AUFS_SHWH
 	sounds like philosophy or something, but in technically it
 	simply shows the name of whiteout with keeping its behaviour.
 
+config AUFS_BR_RAMFS
+	bool "Ramfs (initramfs/rootfs) as an aufs branch"
+	help
+	If you want to use ramfs as an aufs branch fs, then enable this
+	option. Generally tmpfs is recommended.
+	Aufs prohibited them to be a branch fs by default, because
+	initramfs becomes unusable after switch_root or something
+	generally. If you sets initramfs as an aufs branch and boot your
+	system by switch_root, you will meet a problem easily since the
+	files in initramfs may be inaccessible.
+	Unless you are going to use ramfs as an aufs branch fs without
+	switch_root or something, leave it N.
+
 config AUFS_BR_HFSPLUS
 	bool "Hfsplus as an aufs branch"
 	depends on HFSPLUS_FS

fs/aufs/fstype.h

@@ -232,7 +232,9 @@ static inline int au_test_hfsplus(struct super_block *sb __maybe_unused)
 static inline int au_test_fs_unsuppoted(struct super_block *sb)
 {
 	return
+#ifndef CONFIG_AUFS_BR_RAMFS
 		au_test_ramfs(sb) ||
+#endif
 		au_test_procfs(sb)
 		|| au_test_sysfs(sb)
 		|| au_test_configfs(sb)
@@ -247,6 +249,9 @@ static inline int au_test_fs_unsuppoted(struct super_block *sb)
 static inline int au_test_fs_remote(struct super_block *sb)
 {
 	return !au_test_tmpfs(sb)
+#ifdef CONFIG_AUFS_BR_RAMFS
+		&& !au_test_ramfs(sb)
+#endif
 		&& !(sb->s_type->fs_flags & FS_REQUIRES_DEV);
 }
 
@@ -286,6 +291,9 @@ static inline int au_test_fs_bad_iattr(struct super_block *sb)
 static inline int au_test_fs_no_limit_nlink(struct super_block *sb)
 {
 	return au_test_tmpfs(sb)
+#ifdef CONFIG_AUFS_BR_RAMFS
+		|| au_test_ramfs(sb)
+#endif
 		|| au_test_ubifs(sb)
 		|| au_test_hfsplus(sb);
 }