fs/jbd2, locking/mutex, sched/wait: Use mutex_lock_io() for journal->j_checkpoint_mutex

When an ext4 fs is bogged down by a lot of metadata IOs (in the
reported case, it was deletion of millions of files, but any massive
amount of journal writes would do), after the journal is filled up,
tasks which try to access the filesystem and aren't currently
performing the journal writes end up waiting in
__jbd2_log_wait_for_space() for journal->j_checkpoint_mutex.

Because those mutex sleeps aren't marked as iowait, this condition can
lead to misleadingly low iowait and /proc/stat:procs_blocked.  While
iowait propagation is far from strict, this condition can be triggered
fairly easily and annotating these sleeps correctly helps initial
diagnosis quite a bit.

Use the new mutex_lock_io() for journal->j_checkpoint_mutex so that
these sleeps are properly marked as iowait.

Reported-by: Mingbo Wan <mingbo@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Kara <jack@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-team@fb.com
Link: http://lkml.kernel.org/r/1477673892-28940-5-git-send-email-tj@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

fs/jbd2/commit.c

@@ -393,7 +393,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	/* Do we need to erase the effects of a prior jbd2_journal_flush? */
 	if (journal->j_flags & JBD2_FLUSHED) {
 		jbd_debug(3, "super block updated\n");
-		mutex_lock(&journal->j_checkpoint_mutex);
+		mutex_lock_io(&journal->j_checkpoint_mutex);
 		/*
 		 * We hold j_checkpoint_mutex so tail cannot change under us.
 		 * We don't need any special data guarantees for writing sb

fs/jbd2/journal.c

@@ -944,7 +944,7 @@ out:
  */
 void jbd2_update_log_tail(journal_t *journal, tid_t tid, unsigned long block)
 {
-	mutex_lock(&journal->j_checkpoint_mutex);
+	mutex_lock_io(&journal->j_checkpoint_mutex);
 	if (tid_gt(tid, journal->j_tail_sequence))
 		__jbd2_update_log_tail(journal, tid, block);
 	mutex_unlock(&journal->j_checkpoint_mutex);
@@ -1304,7 +1304,7 @@ static int journal_reset(journal_t *journal)
 		journal->j_flags |= JBD2_FLUSHED;
 	} else {
 		/* Lock here to make assertions happy... */
-		mutex_lock(&journal->j_checkpoint_mutex);
+		mutex_lock_io(&journal->j_checkpoint_mutex);
 		/*
 		 * Update log tail information. We use REQ_FUA since new
 		 * transaction will start reusing journal space and so we
@@ -1691,7 +1691,7 @@ int jbd2_journal_destroy(journal_t *journal)
 	spin_lock(&journal->j_list_lock);
 	while (journal->j_checkpoint_transactions != NULL) {
 		spin_unlock(&journal->j_list_lock);
-		mutex_lock(&journal->j_checkpoint_mutex);
+		mutex_lock_io(&journal->j_checkpoint_mutex);
 		err = jbd2_log_do_checkpoint(journal);
 		mutex_unlock(&journal->j_checkpoint_mutex);
 		/*
@@ -1713,7 +1713,7 @@ int jbd2_journal_destroy(journal_t *journal)
 
 	if (journal->j_sb_buffer) {
 		if (!is_journal_aborted(journal)) {
-			mutex_lock(&journal->j_checkpoint_mutex);
+			mutex_lock_io(&journal->j_checkpoint_mutex);
 
 			write_lock(&journal->j_state_lock);
 			journal->j_tail_sequence =
@@ -1955,7 +1955,7 @@ int jbd2_journal_flush(journal_t *journal)
 	spin_lock(&journal->j_list_lock);
 	while (!err && journal->j_checkpoint_transactions != NULL) {
 		spin_unlock(&journal->j_list_lock);
-		mutex_lock(&journal->j_checkpoint_mutex);
+		mutex_lock_io(&journal->j_checkpoint_mutex);
 		err = jbd2_log_do_checkpoint(journal);
 		mutex_unlock(&journal->j_checkpoint_mutex);
 		spin_lock(&journal->j_list_lock);
@@ -1965,7 +1965,7 @@ int jbd2_journal_flush(journal_t *journal)
 	if (is_journal_aborted(journal))
 		return -EIO;
 
-	mutex_lock(&journal->j_checkpoint_mutex);
+	mutex_lock_io(&journal->j_checkpoint_mutex);
 	if (!err) {
 		err = jbd2_cleanup_journal_tail(journal);
 		if (err < 0) {