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 <firstname.lastname@example.org>
Signed-off-by: Theodore Ts'o <email@example.com>