diff --git a/block/blk-core.c b/block/blk-core.c
index b5776c1fd52a..a496727df7ef 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -624,10 +624,6 @@ blk_alloc_request(struct request_queue *q, int rw, int priv, gfp_t gfp_mask)
 
 	blk_rq_init(q, rq);
 
-	/*
-	 * first three bits are identical in rq->cmd_flags and bio->bi_rw,
-	 * see bio.h and blkdev.h
-	 */
 	rq->cmd_flags = rw | REQ_ALLOCED;
 
 	if (priv) {
@@ -2012,7 +2008,8 @@ EXPORT_SYMBOL_GPL(blk_end_request_callback);
 void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
 		     struct bio *bio)
 {
-	/* first two bits are identical in rq->cmd_flags and bio->bi_rw */
+	/* Bit 0 (R/W) is identical in rq->cmd_flags and bio->bi_rw, and
+	   we want BIO_RW_AHEAD (bit 1) to imply REQ_FAILFAST (bit 1). */
 	rq->cmd_flags |= (bio->bi_rw & 3);
 
 	rq->nr_phys_segments = bio_phys_segments(q, bio);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index dbeb66f813ab..17f1fbdb31bf 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -150,8 +150,8 @@ struct bio {
  * bit 3 -- fail fast, don't want low level driver retries
  * bit 4 -- synchronous I/O hint: the block layer will unplug immediately
  */
-#define BIO_RW		0
-#define BIO_RW_AHEAD	1
+#define BIO_RW		0	/* Must match RW in req flags (blkdev.h) */
+#define BIO_RW_AHEAD	1	/* Must match FAILFAST in req flags */
 #define BIO_RW_BARRIER	2
 #define BIO_RW_FAILFAST	3
 #define BIO_RW_SYNC	4
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 53ea933cf60b..e0ba018f5e88 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -84,7 +84,7 @@ enum {
 };
 
 /*
- * request type modified bits. first three bits match BIO_RW* bits, important
+ * request type modified bits. first two bits match BIO_RW* bits, important
  */
 enum rq_flag_bits {
 	__REQ_RW,		/* not set, read. set, write */