dm: support turning off block-core's io stats accounting

Commit bc58ba9 ("block: add sysfs file for controlling io stats
accounting") allowed users to turn off disk stat accounting completely
by checking if queue flag QUEUE_FLAG_IO_STAT is set. In dm, this flag
is neither set nor checked: so block-core's io stats are continuously
counted and cannot be turned off.

Add support for turning off block-core's io stats accounting for dm.
Set QUEUE_FLAG_IO_STAT for dm's request_queue. If QUEUE_FLAG_IO_STAT
is set when an io starts, record the need for block core's io stats by
setting the DM_IO_BLK_STAT dm_io flag to avoid io stats being disabled
in the middle of the io.

DM statistics (dm-stats) is independent of block-core's io stats and
remains unchanged.

Signed-off-by: Li Nan <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>

Author: Li Nan

drivers/md/dm-core.h

@@ -306,7 +306,8 @@ struct dm_io {
  */
 enum {
 	DM_IO_ACCOUNTED,
-	DM_IO_WAS_SPLIT
+	DM_IO_WAS_SPLIT,
+	DM_IO_BLK_STAT
 };
 
 static inline bool dm_io_flagged(struct dm_io *io, unsigned int bit)

drivers/md/dm.c

@@ -511,11 +511,14 @@ static void dm_io_acct(struct dm_io *io, bool end)
 	else
 		sectors = io->sectors;
 
-	if (!end)
-		bdev_start_io_acct(bio->bi_bdev, bio_op(bio), start_time);
-	else
-		bdev_end_io_acct(bio->bi_bdev, bio_op(bio), sectors,
-				 start_time);
+	if (dm_io_flagged(io, DM_IO_BLK_STAT)) {
+		if (!end)
+			bdev_start_io_acct(bio->bi_bdev, bio_op(bio),
+					   start_time);
+		else
+			bdev_end_io_acct(bio->bi_bdev, bio_op(bio),
+					 sectors, start_time);
+	}
 
 	if (static_branch_unlikely(&stats_enabled) &&
 	    unlikely(dm_stats_used(&md->stats))) {
@@ -592,6 +595,8 @@ static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
 	spin_lock_init(&io->lock);
 	io->start_time = jiffies;
 	io->flags = 0;
+	if (blk_queue_io_stat(md->queue))
+		dm_io_set_flag(io, DM_IO_BLK_STAT);
 
 	if (static_branch_unlikely(&stats_enabled))
 		dm_stats_record_start(&md->stats, &io->stats_aux);
@@ -2341,6 +2346,7 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t)
 		break;
 	case DM_TYPE_BIO_BASED:
 	case DM_TYPE_DAX_BIO_BASED:
+		blk_queue_flag_set(QUEUE_FLAG_IO_STAT, md->queue);
 		break;
 	case DM_TYPE_NONE:
 		WARN_ON_ONCE(true);