Use generic io accounting functions to manage io stats. There was an
attempt to do this earlier in commit 18c0b223cf99 ("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 74672d069b29
("md: fix md io stats accounting broken").

This patch attempts to fix this by using both bio_start_io_acct() and
bio_end_io_acct(). To make it possible, a struct md_io is allocated for
every new md bio, which includes the io start_time. A new mempool is
introduced for this purpose. We override bio->bi_end_io with our own
callback and call bio_start_io_acct() before passing the bio to
md_handle_request(). When it completes, we call bio_end_io_acct() and
the original bi_end_io callback.

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>
---
v2:
- Just override the bi_end_io without having to clone the original bio.
- Rebased onto latest md-next.

  drivers/md/md.c | 56 ++++++++++++++++++++++++++++++++++++++-----------
  drivers/md/md.h |  1 +
  2 files changed, 45 insertions(+), 12 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 8bb69c61afe0..25dd3f4116c3 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c

@@ -463,12 +463,33 @@ void md_handle_request(struct mddev *mddev, struct bio *bio)
  }
  EXPORT_SYMBOL(md_handle_request);
  
+struct md_io {
+	struct mddev *mddev;
+	bio_end_io_t *orig_bi_end_io;
+	void *orig_bi_private;
+	unsigned long start_time;
+};
+
+static void md_end_io(struct bio *bio)
+{
+	struct md_io *md_io = bio->bi_private;
+	struct mddev *mddev = md_io->mddev;
+
+	bio_end_io_acct(bio, md_io->start_time);
+
+	bio->bi_end_io = md_io->orig_bi_end_io;
+	bio->bi_private = md_io->orig_bi_private;
+
+	mempool_free(md_io, &mddev->md_io_pool);
+
+	if (bio->bi_end_io)
+		bio->bi_end_io(bio);
+}
+
  static blk_qc_t md_submit_bio(struct bio *bio)
  {
  	const int rw = bio_data_dir(bio);
-	const int sgrp = op_stat_group(bio_op(bio));
  	struct mddev *mddev = bio->bi_disk->private_data;
-	unsigned int sectors;
  
  	if (unlikely(test_bit(MD_BROKEN, &mddev->flags)) && (rw == WRITE)) {
  		bio_io_error(bio);

@@ -488,21 +509,26 @@ static blk_qc_t md_submit_bio(struct bio *bio)
  		return BLK_QC_T_NONE;
  	}
  
-	/*
-	 * save the sectors now since our bio can
-	 * go away inside make_request
-	 */
-	sectors = bio_sectors(bio);
+	if (bio->bi_end_io != md_end_io) {
+		struct md_io *md_io;
+
+		md_io = mempool_alloc(&mddev->md_io_pool, GFP_NOIO);
+		md_io->mddev = mddev;
+		md_io->start_time = jiffies;
+		md_io->orig_bi_end_io = bio->bi_end_io;
+		md_io->orig_bi_private = bio->bi_private;
+
+		bio->bi_end_io = md_end_io;
+		bio->bi_private = md_io;
+
+		bio_start_io_acct(bio);