block: do not reassig ->bi_bdev when partition remapping

There is no good reason to reassign ->bi_bdev when remapping the
partition-relative block number to the device wide one, as all the
information required by the drivers comes from the gendisk anyway.

Keeping the original ->bi_bdev alive will allow to greatly simplify
the partition-away I/O accounting.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -752,7 +752,7 @@ static int blk_partition_remap(struct bio *bio)
 				      bio->bi_iter.bi_sector -
 				      p->bd_start_sect);
 	}
-	bio->bi_bdev = bdev_whole(p);
+	bio_set_flag(bio, BIO_REMAPPED);
 	return 0;
 }
 
@@ -817,7 +817,8 @@ static noinline_for_stack bool submit_bio_checks(struct bio *bio)
 		goto end_io;
 	if (unlikely(bio_check_eod(bio)))
 		goto end_io;
-	if (bio->bi_bdev->bd_partno && unlikely(blk_partition_remap(bio)))
+	if (bio->bi_bdev->bd_partno && !bio_flagged(bio, BIO_REMAPPED) &&
+	    unlikely(blk_partition_remap(bio)))
 		goto end_io;
 
 	/*

include/linux/bio.h

@@ -485,6 +485,7 @@ extern const char *bio_devname(struct bio *bio, char *buffer);
 
 #define bio_set_dev(bio, bdev) 				\
 do {							\
+	bio_clear_flag(bio, BIO_REMAPPED);		\
 	if ((bio)->bi_bdev != (bdev))			\
 		bio_clear_flag(bio, BIO_THROTTLED);	\
 	(bio)->bi_bdev = (bdev);			\
@@ -493,6 +494,7 @@ do {							\
 
 #define bio_copy_dev(dst, src)			\
 do {						\
+	bio_clear_flag(dst, BIO_REMAPPED);		\
 	(dst)->bi_bdev = (src)->bi_bdev;	\
 	bio_clone_blkg_association(dst, src);	\
 } while (0)

include/linux/blk_types.h

@@ -303,6 +303,7 @@ enum {
 				 * of this bio. */
 	BIO_CGROUP_ACCT,	/* has been accounted to a cgroup */
 	BIO_TRACKED,		/* set if bio goes through the rq_qos path */
+	BIO_REMAPPED,
 	BIO_FLAG_LAST
 };