ext4: handle errors on ext4_commit_super

When remounting ext4 from ro to rw, currently it allows its transition,
even if ext4_commit_super() returns EIO. Even worse thing is, after that,
fs/buffer complains buffer dirty bits like:

 Call trace:
 [<ffffff9750c259dc>] mark_buffer_dirty+0x184/0x1a4
 [<ffffff9750cb398c>] __ext4_handle_dirty_super+0x4c/0xfc
 [<ffffff9750c7a9fc>] ext4_file_open+0x154/0x1c0
 [<ffffff9750bea51c>] do_dentry_open+0x114/0x2d0
 [<ffffff9750bea75c>] vfs_open+0x5c/0x94
 [<ffffff9750bf879c>] path_openat+0x668/0xfe8
 [<ffffff9750bf8088>] do_filp_open+0x74/0x120
 [<ffffff9750beac98>] do_sys_open+0x148/0x254
 [<ffffff9750beade0>] SyS_openat+0x10/0x18
 [<ffffff9750a83ab0>] el0_svc_naked+0x24/0x28
 EXT4-fs (dm-1): previous I/O error to superblock detected
 Buffer I/O error on dev dm-1, logical block 0, lost sync page write
 EXT4-fs (dm-1): re-mounted. Opts: (null)
 Buffer I/O error on dev dm-1, logical block 80, lost async page write

Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -2146,12 +2146,12 @@ static int ext4_setup_super(struct super_block *sb, struct ext4_super_block *es,
 			    int read_only)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
-	int res = 0;
+	int err = 0;
 
 	if (le32_to_cpu(es->s_rev_level) > EXT4_MAX_SUPP_REV) {
 		ext4_msg(sb, KERN_ERR, "revision level too high, "
 			 "forcing read-only mode");
-		res = SB_RDONLY;
+		err = -EROFS;
 	}
 	if (read_only)
 		goto done;
@@ -2184,7 +2184,7 @@ static int ext4_setup_super(struct super_block *sb, struct ext4_super_block *es,
 	if (sbi->s_journal)
 		ext4_set_feature_journal_needs_recovery(sb);
 
-	ext4_commit_super(sb, 1);
+	err = ext4_commit_super(sb, 1);
 done:
 	if (test_opt(sb, DEBUG))
 		printk(KERN_INFO "[EXT4 FS bs=%lu, gc=%u, "
@@ -2196,7 +2196,7 @@ done:
 			sbi->s_mount_opt, sbi->s_mount_opt2);
 
 	cleancache_init_fs(sb);
-	return res;
+	return err;
 }
 
 int ext4_alloc_flex_bg_array(struct super_block *sb, ext4_group_t ngroup)
@@ -4254,8 +4254,12 @@ no_journal:
 		goto failed_mount4;
 	}
 
-	if (ext4_setup_super(sb, es, sb_rdonly(sb)))
+	ret = ext4_setup_super(sb, es, sb_rdonly(sb));
+	if (ret == -EROFS) {
 		sb->s_flags |= SB_RDONLY;
+		ret = 0;
+	} else if (ret)
+		goto failed_mount4a;
 
 	/* determine the minimum size of new large inodes, if present */
 	if (sbi->s_inode_size > EXT4_GOOD_OLD_INODE_SIZE &&
@@ -4790,11 +4794,7 @@ static int ext4_commit_super(struct super_block *sb, int sync)
 		unlock_buffer(sbh);
 		error = __sync_dirty_buffer(sbh,
 			REQ_SYNC | (test_opt(sb, BARRIER) ? REQ_FUA : 0));
-		if (error)
-			return error;
-
-		error = buffer_write_io_error(sbh);
-		if (error) {
+		if (buffer_write_io_error(sbh)) {
 			ext4_msg(sb, KERN_ERR, "I/O error while writing "
 			       "superblock");
 			clear_buffer_write_io_error(sbh);
@@ -5195,8 +5195,12 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 			if (sbi->s_journal)
 				ext4_clear_journal_err(sb, es);
 			sbi->s_mount_state = le16_to_cpu(es->s_state);
-			if (!ext4_setup_super(sb, es, 0))
-				sb->s_flags &= ~SB_RDONLY;
+
+			err = ext4_setup_super(sb, es, 0);
+			if (err)
+				goto restore_opts;
+
+			sb->s_flags &= ~SB_RDONLY;
 			if (ext4_has_feature_mmp(sb))
 				if (ext4_multi_mount_protect(sb,
 						le64_to_cpu(es->s_mmp_block))) {
@@ -5220,8 +5224,11 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 	}
 
 	ext4_setup_system_zone(sb);
-	if (sbi->s_journal == NULL && !(old_sb_flags & SB_RDONLY))
-		ext4_commit_super(sb, 1);
+	if (sbi->s_journal == NULL && !(old_sb_flags & SB_RDONLY)) {
+		err = ext4_commit_super(sb, 1);
+		if (err)
+			goto restore_opts;
+	}
 
 #ifdef CONFIG_QUOTA
 	/* Release old quota file names */