ext4: add sysfs attr /sys/fs/ext4/<disk>/journal_task

This is useful for moving journal thread into cgroup or
for tracing it with ftrace/perf/blktrace.

For now the only way is `pgrep jbd2/$DISK` but this is not reliable:
name may be longer than "comm" limit and any task could mock it.

Attribute shows pid in current pid-namespace or 0 if task is unreachable.

Signed-off-by: Konstantin Khlebnikov <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>

Documentation/ABI/testing/sysfs-fs-ext4

···
 		write operation (since a 4k random write might turn
 		into a much larger write due to the zeroout
 		operation).
+
+What:		/sys/fs/ext4/<disk>/journal_task
+Date:		February 2019
+Contact:	"Theodore Ts'o" <[e-mail]>
+Description:
+		This file is read-only and shows the pid of journal thread in
+		current pid-namespace or 0 if task is unreachable.

fs/ext4/sysfs.c

···
 	attr_feature,
 	attr_pointer_ui,
 	attr_pointer_atomic,
+	attr_journal_task,
 } attr_id_t;
 
 typedef enum {
···
 	return count;
 }
 
+static ssize_t journal_task_show(struct ext4_sb_info *sbi, char *buf)
+{
+	if (!sbi->s_journal)
+		return snprintf(buf, PAGE_SIZE, "<none>\n");
+	return snprintf(buf, PAGE_SIZE, "%d\n",
+			task_pid_vnr(sbi->s_journal->j_task));
+}
+
 #define EXT4_ATTR(_name,_mode,_id)					\
 static struct ext4_attr ext4_attr_##_name = {				\
 	.attr = {.name = __stringify(_name), .mode = _mode },		\
···
 EXT4_RO_ATTR_ES_UI(errors_count, s_error_count);
 EXT4_ATTR(first_error_time, 0444, first_error_time);
 EXT4_ATTR(last_error_time, 0444, last_error_time);
+EXT4_ATTR(journal_task, 0444, journal_task);
 
 static unsigned int old_bump_val = 128;
 EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
···
 	ATTR_LIST(errors_count),
 	ATTR_LIST(first_error_time),
 	ATTR_LIST(last_error_time),
+	ATTR_LIST(journal_task),
 	NULL,
 };
 
···
 		return print_tstamp(buf, sbi->s_es, s_first_error_time);
 	case attr_last_error_time:
 		return print_tstamp(buf, sbi->s_es, s_last_error_time);
+	case attr_journal_task:
+		return journal_task_show(sbi, buf);
 	}
 
 	return 0;