nilfs2: accept future revisions

Compatibility of nilfs partitions is now managed with three feature
sets.  This changes old compatibility check with revision number so
that it can accept future revisions.

Note that we can stop support of experimental versions of nilfs that
doesn't know the feature sets by incrementing NILFS_CURRENT_REV.  We
don't have to do it soon, but it would be a possible option whenever
the need arises.
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/the_nilfs.c

@@ -468,8 +468,8 @@ static unsigned long long nilfs_max_size(unsigned int blkbits)
 static int nilfs_store_disk_layout(struct the_nilfs *nilfs,
 				   struct nilfs_super_block *sbp)
 {
-	if (le32_to_cpu(sbp->s_rev_level) != NILFS_CURRENT_REV) {
-		printk(KERN_ERR "NILFS: revision mismatch "
+	if (le32_to_cpu(sbp->s_rev_level) < NILFS_MIN_SUPP_REV) {
+		printk(KERN_ERR "NILFS: unsupported revision "
 		       "(superblock rev.=%d.%d, current rev.=%d.%d). "
 		       "Please check the version of mkfs.nilfs.\n",
 		       le32_to_cpu(sbp->s_rev_level),

include/linux/nilfs2_fs.h

@@ -229,6 +229,7 @@ struct nilfs_super_block {
  */
 #define NILFS_CURRENT_REV	2	/* current major revision */
 #define NILFS_MINOR_REV		0	/* minor revision */
+#define NILFS_MIN_SUPP_REV	2	/* minimum supported revision */
 
 /*
  * Feature set definitions