ext4: add debugging /proc file showing file system options

Now that /proc/mounts is consistently showing only those mount options
which need to be specified in /etc/fstab or on the mount command line,
it is useful to have file which shows exactly which file system
options are enabled.  This can be useful when debugging a user
problem.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -847,6 +847,7 @@ static void ext4_put_super(struct super_block *sb)
 		ext4_commit_super(sb, 1);
 	}
 	if (sbi->s_proc) {
+		remove_proc_entry("options", sbi->s_proc);
 		remove_proc_entry(sb->s_id, ext4_proc_root);
 	}
 	kobject_del(&sbi->s_kobj);
@@ -1683,16 +1684,17 @@ static const char *token2str(int token)
  *  - it's set to a non-default value OR
  *  - if the per-sb default is different from the global default
  */
-static int ext4_show_options(struct seq_file *seq, struct dentry *root)
+static int _ext4_show_options(struct seq_file *seq, struct super_block *sb,
+			      int nodefs)
 {
-	int def_errors;
-	struct super_block *sb = root->d_sb;
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_super_block *es = sbi->s_es;
+	int def_errors, def_mount_opt = nodefs ? 0 : sbi->s_def_mount_opt;
 	const struct mount_opts *m;
+	char sep = nodefs ? '\n' : ',';
 
-#define SEQ_OPTS_PUTS(str) seq_printf(seq, "," str)
-#define SEQ_OPTS_PRINT(str, arg) seq_printf(seq, "," str, arg)
+#define SEQ_OPTS_PUTS(str) seq_printf(seq, "%c" str, sep)
+#define SEQ_OPTS_PRINT(str, arg) seq_printf(seq, "%c" str, sep, arg)
 
 	if (sbi->s_sb_block != 1)
 		SEQ_OPTS_PRINT("sb=%llu", sbi->s_sb_block);
@@ -1702,7 +1704,7 @@ static int ext4_show_options(struct seq_file *seq, struct dentry *root)
 		if (((m->flags & (MOPT_SET|MOPT_CLEAR)) == 0) ||
 		    (m->flags & MOPT_CLEAR_ERR))
 			continue;
-		if (!(m->mount_opt & (sbi->s_mount_opt ^ sbi->s_def_mount_opt)))
+		if (!(m->mount_opt & (sbi->s_mount_opt ^ def_mount_opt)))
 			continue; /* skip if same as the default */
 		if ((want_set &&
 		     (sbi->s_mount_opt & m->mount_opt) != m->mount_opt) ||
@@ -1711,30 +1713,30 @@ static int ext4_show_options(struct seq_file *seq, struct dentry *root)
 		SEQ_OPTS_PRINT("%s", token2str(m->token));
 	}
 
-	if (sbi->s_resuid != EXT4_DEF_RESUID ||
+	if (nodefs || sbi->s_resuid != EXT4_DEF_RESUID ||
 	    le16_to_cpu(es->s_def_resuid) != EXT4_DEF_RESUID)
 		SEQ_OPTS_PRINT("resuid=%u", sbi->s_resuid);
-	if (sbi->s_resgid != EXT4_DEF_RESGID ||
+	if (nodefs || sbi->s_resgid != EXT4_DEF_RESGID ||
 	    le16_to_cpu(es->s_def_resgid) != EXT4_DEF_RESGID)
 		SEQ_OPTS_PRINT("resgid=%u", sbi->s_resgid);
-	def_errors = le16_to_cpu(es->s_errors);
+	def_errors = nodefs ? -1 : le16_to_cpu(es->s_errors);
 	if (test_opt(sb, ERRORS_RO) && def_errors != EXT4_ERRORS_RO)
 		SEQ_OPTS_PUTS("errors=remount-ro");
 	if (test_opt(sb, ERRORS_CONT) && def_errors != EXT4_ERRORS_CONTINUE)
 		SEQ_OPTS_PUTS("errors=continue");
 	if (test_opt(sb, ERRORS_PANIC) && def_errors != EXT4_ERRORS_PANIC)
 		SEQ_OPTS_PUTS("errors=panic");
-	if (sbi->s_commit_interval != JBD2_DEFAULT_MAX_COMMIT_AGE*HZ)
+	if (nodefs || sbi->s_commit_interval != JBD2_DEFAULT_MAX_COMMIT_AGE*HZ)
 		SEQ_OPTS_PRINT("commit=%lu", sbi->s_commit_interval / HZ);
-	if (sbi->s_min_batch_time != EXT4_DEF_MIN_BATCH_TIME)
+	if (nodefs || sbi->s_min_batch_time != EXT4_DEF_MIN_BATCH_TIME)
 		SEQ_OPTS_PRINT("min_batch_time=%u", sbi->s_min_batch_time);
-	if (sbi->s_max_batch_time != EXT4_DEF_MAX_BATCH_TIME)
+	if (nodefs || sbi->s_max_batch_time != EXT4_DEF_MAX_BATCH_TIME)
 		SEQ_OPTS_PRINT("max_batch_time=%u", sbi->s_max_batch_time);
 	if (sb->s_flags & MS_I_VERSION)
 		SEQ_OPTS_PUTS("i_version");
-	if (sbi->s_stripe)
+	if (nodefs || sbi->s_stripe)
 		SEQ_OPTS_PRINT("stripe=%lu", sbi->s_stripe);
-	if (EXT4_MOUNT_DATA_FLAGS & (sbi->s_mount_opt ^ sbi->s_def_mount_opt)) {
+	if (EXT4_MOUNT_DATA_FLAGS & (sbi->s_mount_opt ^ def_mount_opt)) {
 		if (test_opt(sb, DATA_FLAGS) == EXT4_MOUNT_JOURNAL_DATA)
 			SEQ_OPTS_PUTS("data=journal");
 		else if (test_opt(sb, DATA_FLAGS) == EXT4_MOUNT_ORDERED_DATA)
@@ -1742,18 +1744,48 @@ static int ext4_show_options(struct seq_file *seq, struct dentry *root)
 		else if (test_opt(sb, DATA_FLAGS) == EXT4_MOUNT_WRITEBACK_DATA)
 			SEQ_OPTS_PUTS("data=writeback");
 	}
-	if (sbi->s_inode_readahead_blks != EXT4_DEF_INODE_READAHEAD_BLKS)
+	if (nodefs ||
+	    sbi->s_inode_readahead_blks != EXT4_DEF_INODE_READAHEAD_BLKS)
 		SEQ_OPTS_PRINT("inode_readahead_blks=%u",
 			       sbi->s_inode_readahead_blks);
 
-	if (test_opt(sb, INIT_INODE_TABLE) &&
-	    (sbi->s_li_wait_mult != EXT4_DEF_LI_WAIT_MULT))
+	if (nodefs || (test_opt(sb, INIT_INODE_TABLE) &&
+		       (sbi->s_li_wait_mult != EXT4_DEF_LI_WAIT_MULT)))
 		SEQ_OPTS_PRINT("init_itable=%u", sbi->s_li_wait_mult);
 
 	ext4_show_quota_options(seq, sb);
 	return 0;
 }
 
+static int ext4_show_options(struct seq_file *seq, struct dentry *root)
+{
+	return _ext4_show_options(seq, root->d_sb, 0);
+}
+
+static int options_seq_show(struct seq_file *seq, void *offset)
+{
+	struct super_block *sb = seq->private;
+	int rc;
+
+	seq_puts(seq, (sb->s_flags & MS_RDONLY) ? "ro" : "rw");
+	rc = _ext4_show_options(seq, sb, 1);
+	seq_puts(seq, "\n");
+	return rc;
+}
+
+static int options_open_fs(struct inode *inode, struct file *file)
+{
+	return single_open(file, options_seq_show, PDE(inode)->data);
+}
+
+static const struct file_operations ext4_seq_options_fops = {
+	.owner = THIS_MODULE,
+	.open = options_open_fs,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
 static int ext4_setup_super(struct super_block *sb, struct ext4_super_block *es,
 			    int read_only)
 {
@@ -3350,6 +3382,10 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	if (ext4_proc_root)
 		sbi->s_proc = proc_mkdir(sb->s_id, ext4_proc_root);
 
+	if (sbi->s_proc)
+		proc_create_data("options", S_IRUGO, sbi->s_proc,
+				 &ext4_seq_options_fops, sb);
+
 	bgl_lock_init(sbi->s_blockgroup_lock);
 
 	for (i = 0; i < db_count; i++) {
@@ -3674,6 +3710,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	ext4_kvfree(sbi->s_group_desc);
 failed_mount:
 	if (sbi->s_proc) {
+		remove_proc_entry("options", sbi->s_proc);
 		remove_proc_entry(sb->s_id, ext4_proc_root);
 	}
 #ifdef CONFIG_QUOTA