diff --git a/block/blk-core.c b/block/blk-core.c
index 5cc29299f6ac..33eded00c5b1 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1866,8 +1866,7 @@ int blk_rq_check_limits(struct request_queue *q, struct request *rq)
 	if (!rq_mergeable(rq))
 		return 0;
 
-	if (blk_rq_sectors(rq) > queue_max_sectors(q) ||
-	    blk_rq_bytes(rq) > queue_max_hw_sectors(q) << 9) {
+	if (blk_rq_sectors(rq) > blk_queue_get_max_sectors(q, rq->cmd_flags)) {
 		printk(KERN_ERR "%s: over max size limit.\n", __func__);
 		return -EIO;
 	}
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 86710ca408b8..642b862608a1 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -275,14 +275,8 @@ static inline int ll_new_hw_segment(struct request_queue *q,
 int ll_back_merge_fn(struct request_queue *q, struct request *req,
 		     struct bio *bio)
 {
-	unsigned short max_sectors;
-
-	if (unlikely(req->cmd_type == REQ_TYPE_BLOCK_PC))
-		max_sectors = queue_max_hw_sectors(q);
-	else
-		max_sectors = queue_max_sectors(q);
-
-	if (blk_rq_sectors(req) + bio_sectors(bio) > max_sectors) {
+	if (blk_rq_sectors(req) + bio_sectors(bio) >
+	    blk_rq_get_max_sectors(req)) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
@@ -299,15 +293,8 @@ int ll_back_merge_fn(struct request_queue *q, struct request *req,
 int ll_front_merge_fn(struct request_queue *q, struct request *req,
 		      struct bio *bio)
 {
-	unsigned short max_sectors;
-
-	if (unlikely(req->cmd_type == REQ_TYPE_BLOCK_PC))
-		max_sectors = queue_max_hw_sectors(q);
-	else
-		max_sectors = queue_max_sectors(q);
-
-
-	if (blk_rq_sectors(req) + bio_sectors(bio) > max_sectors) {
+	if (blk_rq_sectors(req) + bio_sectors(bio) >
+	    blk_rq_get_max_sectors(req)) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
@@ -338,7 +325,8 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 	/*
 	 * Will it become too large?
 	 */
-	if ((blk_rq_sectors(req) + blk_rq_sectors(next)) > queue_max_sectors(q))
+	if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
+	    blk_rq_get_max_sectors(req))
 		return 0;
 
 	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
@@ -417,6 +405,9 @@ static int attempt_merge(struct request_queue *q, struct request *req,
 	if (!rq_mergeable(req) || !rq_mergeable(next))
 		return 0;
 
+	if (!blk_check_merge_flags(req->cmd_flags, next->cmd_flags))
+		return 0;
+
 	/*
 	 * not contiguous
 	 */
@@ -512,6 +503,9 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
 	if (!rq_mergeable(rq) || !bio_mergeable(bio))
 		return false;
 
+	if (!blk_check_merge_flags(rq->cmd_flags, bio->bi_rw))
+		return false;
+
 	/* different data direction or already started, don't merge */
 	if (bio_data_dir(bio) != rq_data_dir(rq))
 		return false;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 3a6fea7460f1..90f7abe8f183 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -605,6 +605,18 @@ static inline bool rq_mergeable(struct request *rq)
 	return true;
 }
 
+static inline bool blk_check_merge_flags(unsigned int flags1,
+					 unsigned int flags2)
+{
+	if ((flags1 & REQ_DISCARD) != (flags2 & REQ_DISCARD))
+		return false;
+
+	if ((flags1 & REQ_SECURE) != (flags2 & REQ_SECURE))
+		return false;
+
+	return true;
+}
+
 /*
  * q->prep_rq_fn return values
  */
@@ -800,6 +812,25 @@ static inline unsigned int blk_rq_cur_sectors(const struct request *rq)
 	return blk_rq_cur_bytes(rq) >> 9;
 }
 
+static inline unsigned int blk_queue_get_max_sectors(struct request_queue *q,
+						     unsigned int cmd_flags)
+{
+	if (unlikely(cmd_flags & REQ_DISCARD))
+		return q->limits.max_discard_sectors;
+
+	return q->limits.max_sectors;
+}
+
+static inline unsigned int blk_rq_get_max_sectors(struct request *rq)
+{
+	struct request_queue *q = rq->q;
+
+	if (unlikely(rq->cmd_type == REQ_TYPE_BLOCK_PC))
+		return q->limits.max_hw_sectors;
+
+	return blk_queue_get_max_sectors(q, rq->cmd_flags);
+}
+
 /*
  * Request issue related functions.
  */