aufs: native/real readonly branch

Some filesystems are natively readonly. And aufs can make a few
optimization for them. This new attribute tells aufs.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

fs/aufs/fstype.h

@@ -14,6 +14,7 @@
 
 #include <linux/fs.h>
 #include <linux/magic.h>
+#include <linux/romfs_fs.h>
 
 static inline int au_test_aufs(struct super_block *sb)
 {
@@ -25,6 +26,32 @@ static inline const char *au_sbtype(struct super_block *sb)
 	return sb->s_type->name;
 }
 
+static inline int au_test_iso9660(struct super_block *sb __maybe_unused)
+{
+#if IS_ENABLED(CONFIG_ISO9660_FS)
+	return sb->s_magic == ISOFS_SUPER_MAGIC;
+#else
+	return 0;
+#endif
+}
+
+static inline int au_test_romfs(struct super_block *sb __maybe_unused)
+{
+#if IS_ENABLED(CONFIG_ROMFS_FS)
+	return sb->s_magic == ROMFS_MAGIC;
+#else
+	return 0;
+#endif
+}
+
+static inline int au_test_cramfs(struct super_block *sb __maybe_unused)
+{
+#if IS_ENABLED(CONFIG_CRAMFS)
+	return sb->s_magic == CRAMFS_MAGIC;
+#endif
+	return 0;
+}
+
 static inline int au_test_nfs(struct super_block *sb __maybe_unused)
 {
 #if IS_ENABLED(CONFIG_NFS_FS)
@@ -124,6 +151,15 @@ static inline int au_test_securityfs(struct super_block *sb __maybe_unused)
 #endif
 }
 
+static inline int au_test_squashfs(struct super_block *sb __maybe_unused)
+{
+#if IS_ENABLED(CONFIG_SQUASHFS)
+	return sb->s_magic == SQUASHFS_MAGIC;
+#else
+	return 0;
+#endif
+}
+
 static inline int au_test_btrfs(struct super_block *sb __maybe_unused)
 {
 #if IS_ENABLED(CONFIG_BTRFS_FS)
@@ -233,5 +269,16 @@ static inline int au_test_fs_trunc_xino(struct super_block *sb)
 		|| au_test_ramfs(sb);
 }
 
+/*
+ * test if the @sb is real-readonly.
+ */
+static inline int au_test_fs_rr(struct super_block *sb)
+{
+	return au_test_squashfs(sb)
+		|| au_test_iso9660(sb)
+		|| au_test_cramfs(sb)
+		|| au_test_romfs(sb);
+}
+
 #endif /* __KERNEL__ */
 #endif /* __AUFS_FSTYPE_H__ */

fs/aufs/opts.c

@@ -90,8 +90,8 @@ out:
 
 static match_table_t brperm = {
 	{AuBrPerm_RO, AUFS_BRPERM_RO},
+	{AuBrPerm_RR, AUFS_BRPERM_RR},
 	{AuBrPerm_RW, AUFS_BRPERM_RW},
-	/* add more later */
 	{0, NULL}
 };
 
@@ -188,6 +188,7 @@ static int noinline_for_stack br_perm_val(char *perm)
 	bad = 0;
 	switch (val & AuBrPerm_Mask) {
 	case AuBrPerm_RO:
+	case AuBrPerm_RR:
 		bad = val & AuBrWAttr_Mask;
 		val &= ~AuBrWAttr_Mask;
 		break;
@@ -333,7 +334,9 @@ static int opt_add(struct au_opt *opt, char *opt_str, unsigned long sb_flags,
 	if (!err) {
 		if (!p) {
 			add->perm = AuBrPerm_RO;
-			if (!bindex && !(sb_flags & SB_RDONLY))
+			if (au_test_fs_rr(add->path.dentry->d_sb))
+				add->perm = AuBrPerm_RR;
+			else if (!bindex && !(sb_flags & SB_RDONLY))
 				add->perm = AuBrPerm_RW;
 		}
 		opt->type = Opt_add;

include/uapi/linux/aufs_type.h

@@ -96,12 +96,14 @@ typedef int16_t aufs_bindex_t;
 /* branch permissions and attributes */
 #define AUFS_BRPERM_RW		"rw"
 #define AUFS_BRPERM_RO		"ro"
+#define AUFS_BRPERM_RR		"rr"
 #define AUFS_BRRATTR_WH		"wh"
 #define AUFS_BRWATTR_NLWH	"nolwh"
 
 #define AuBrPerm_RW		1		/* writable, hardlinkable wh */
 #define AuBrPerm_RO		(1 << 1)	/* readonly */
-#define AuBrPerm_Mask		(AuBrPerm_RW | AuBrPerm_RO)
+#define AuBrPerm_RR		(1 << 2)	/* natively readonly */
+#define AuBrPerm_Mask		(AuBrPerm_RW | AuBrPerm_RO | AuBrPerm_RR)
 
 #define AuBrRAttr_WH		(1 << 7)	/* whiteout-able */
 #define AuBrRAttr_Mask		AuBrRAttr_WH