Use generic io accounting functions to manage io stats. There was an
attempt to do this earlier in commit 18c0b223cf990172 ("md: use generic
io stats accounting functions to simplify io stat accounting"), but it
did not include a call to generic_end_io_acct() and caused issues with
tracking in-flight IOs, so it was later removed in commit
74672d069b298b03 ("md: fix md io stats accounting broken").

This patch attempts to fix this by using both generic_start_io_acct()
and generic_end_io_acct(). To make it possible, in md_make_request() a
bio is cloned with additional data - struct md_io, which includes the io
start_time. A new bioset is introduced for this purpose. We call
generic_start_io_acct() and pass the clone instead of the original to
md_handle_request(). When it completes, we call generic_end_io_acct()
and complete the original bio.

This adds correct statistics about in-flight IOs and IO processing time,
interpreted e.g. in iostat as await, svctm, aqu-sz and %util.

It also fixes a situation where too many IOs where reported if a bio was
re-submitted to the mddev, because io accounting is now performed only
on newly arriving bios.

Signed-off-by: Artur Paszkiewicz <redacted>
---
  drivers/md/md.c | 65 +++++++++++++++++++++++++++++++++++++++----------
  drivers/md/md.h |  1 +
  2 files changed, 53 insertions(+), 13 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index f567f536b529..5a9f167ef5b9 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c

@@ -463,12 +463,32 @@ void md_handle_request(struct mddev *mddev, struct bio *bio)
  }
  EXPORT_SYMBOL(md_handle_request);
  
+struct md_io {
+	struct mddev *mddev;
+	struct bio *orig_bio;
+	unsigned long start_time;
+	struct bio orig_bio_clone;
+};
+
+static void md_end_request(struct bio *bio)
+{
+	struct md_io *md_io = bio->bi_private;
+	struct mddev *mddev = md_io->mddev;
+	struct bio *orig_bio = md_io->orig_bio;
+
+	orig_bio->bi_status = bio->bi_status;
+
+	generic_end_io_acct(mddev->queue, bio_op(orig_bio),
+			    &mddev->gendisk->part0, md_io->start_time);