dm: eliminate copying of dm_io fields in dm_io_dec_pending

There is no need for dm_io_dec_pending() to copy dm_io fields
anymore now that DM provides its own pending_io counters again.

The race documented in commit d208b894 ("dm: fix mempool NULL
pointer race when completing IO") no longer exists now that block
core's in_flight counters aren't used to signal all dm_io is
complete.

Also, rename {start,end}_io_acct to dm_{start,end}_io_acct.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -518,15 +518,14 @@ static void dm_io_acct(bool end, struct mapped_device *md, struct bio *bio,
 		bio->bi_iter.bi_size = bi_size;
 }
 
-static void start_io_acct(struct dm_io *io)
+static void dm_start_io_acct(struct dm_io *io)
 {
 	dm_io_acct(false, io->md, io->orig_bio, io->start_time, &io->stats_aux);
 }
 
-static void end_io_acct(struct mapped_device *md, struct bio *bio,
-			unsigned long start_time, struct dm_stats_aux *stats_aux)
+static void dm_end_io_acct(struct dm_io *io)
 {
-	dm_io_acct(true, md, bio, start_time, stats_aux);
+	dm_io_acct(true, io->md, io->orig_bio, io->start_time, &io->stats_aux);
 }
 
 static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
@@ -808,8 +807,6 @@ void dm_io_dec_pending(struct dm_io *io, blk_status_t error)
 	blk_status_t io_error;
 	struct bio *bio;
 	struct mapped_device *md = io->md;
-	unsigned long start_time = 0;
-	struct dm_stats_aux stats_aux;
 
 	/* Push-back supersedes any I/O errors */
 	if (unlikely(error)) {
@@ -841,10 +838,8 @@ void dm_io_dec_pending(struct dm_io *io, blk_status_t error)
 		}
 
 		io_error = io->status;
-		start_time = io->start_time;
-		stats_aux = io->stats_aux;
+		dm_end_io_acct(io);
 		free_io(md, io);
-		end_io_acct(md, bio, start_time, &stats_aux);
 		smp_wmb();
 		this_cpu_dec(*md->pending_io);
 
@@ -1444,8 +1439,8 @@ static void __split_and_process_bio(struct mapped_device *md,
 			 * so that it gets handled *after* bios already submitted
 			 * have been completely processed.
 			 * We take a clone of the original to store in
-			 * ci.io->orig_bio to be used by end_io_acct() and
-			 * for dec_pending to use for completion handling.
+			 * ci.io->orig_bio to be used by dm_end_io_acct() and
+			 * for dm_io_dec_pending() to use for completion handling.
 			 */
 			struct bio *b = bio_split(bio, bio_sectors(bio) - ci.sector_count,
 						  GFP_NOIO, &md->queue->bio_split);
@@ -1456,7 +1451,7 @@ static void __split_and_process_bio(struct mapped_device *md,
 			submit_bio_noacct(bio);
 		}
 	}
-	start_io_acct(ci.io);
+	dm_start_io_acct(ci.io);
 
 	/* drop the extra reference count */
 	dm_io_dec_pending(ci.io, errno_to_blk_status(error));