blk-mq: enforce op-specific segment limits in blk_insert_cloned_request

The block layer might merge together discard requests up until the
max_discard_segments limit is hit, but blk_insert_cloned_request checks
the segment count against max_segments regardless of the req op. This
can result in errors like the following when discards are issued through
a DM device and max_discard_segments exceeds max_segments for the queue
of the chosen underlying device.

blk_insert_cloned_request: over max segments limit. (256 > 129)

Fix this by looking at the req_op and enforcing the appropriate segment
limit - max_discard_segments for REQ_OP_DISCARDs and max_segments for
everything else.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>

block/blk-merge.c

@@ -588,9 +588,7 @@ EXPORT_SYMBOL(__blk_rq_map_sg);
 
 static inline unsigned int blk_rq_get_max_segments(struct request *rq)
 {
-	if (req_op(rq) == REQ_OP_DISCARD)
-		return queue_max_discard_segments(rq->q);
-	return queue_max_segments(rq->q);
+	return blk_queue_get_max_segments(rq->q, req_op(rq));
 }
 
 static inline unsigned int blk_rq_get_max_sectors(struct request *rq,

block/blk-mq.c

@@ -3000,6 +3000,7 @@ blk_status_t blk_insert_cloned_request(struct request *rq)
 {
 	struct request_queue *q = rq->q;
 	unsigned int max_sectors = blk_queue_get_max_sectors(q, req_op(rq));
+	unsigned int max_segments = blk_queue_get_max_segments(q, req_op(rq));
 	blk_status_t ret;
 
 	if (blk_rq_sectors(rq) > max_sectors) {
@@ -3026,9 +3027,9 @@ blk_status_t blk_insert_cloned_request(struct request *rq)
 	 * original queue.
 	 */
 	rq->nr_phys_segments = blk_recalc_rq_segments(rq);
-	if (rq->nr_phys_segments > queue_max_segments(q)) {
+	if (rq->nr_phys_segments > max_segments) {
 		printk(KERN_ERR "%s: over max segments limit. (%hu > %hu)\n",
-			__func__, rq->nr_phys_segments, queue_max_segments(q));
+			__func__, rq->nr_phys_segments, max_segments);
 		return BLK_STS_IOERR;
 	}
 

block/blk.h

@@ -169,6 +169,14 @@ static inline unsigned int blk_queue_get_max_sectors(struct request_queue *q,
 	return q->limits.max_sectors;
 }
 
+static inline unsigned int blk_queue_get_max_segments(struct request_queue *q,
+						      enum req_op op)
+{
+	if (op == REQ_OP_DISCARD)
+		return queue_max_discard_segments(q);
+	return queue_max_segments(q);
+}
+
 #ifdef CONFIG_BLK_DEV_INTEGRITY
 void blk_flush_integrity(void);
 bool __bio_integrity_endio(struct bio *);