blk-merge: fix blk_recount_segments

QUEUE_FLAG_NO_SG_MERGE is set at default for blk-mq devices,
so bio->bi_phys_segment computed may be bigger than
queue_max_segments(q) for blk-mq devices, then drivers will
fail to handle the case, for example, BUG_ON() in
virtio_queue_rq() can be triggerd for virtio-blk:

	https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1359146

This patch fixes the issue by ignoring the QUEUE_FLAG_NO_SG_MERGE
flag if the computed bio->bi_phys_segment is bigger than
queue_max_segments(q), and the regression is caused by commit
05f1dd53152173(block: add queue flag for disabling SG merging).

Reported-by: Kick In <pierre-andre.morey@canonical.com>
Tested-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-merge.c

@@ -10,10 +10,11 @@
 #include "blk.h"
 
 static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
-					     struct bio *bio)
+					     struct bio *bio,
+					     bool no_sg_merge)
 {
 	struct bio_vec bv, bvprv = { NULL };
-	int cluster, high, highprv = 1, no_sg_merge;
+	int cluster, high, highprv = 1;
 	unsigned int seg_size, nr_phys_segs;
 	struct bio *fbio, *bbio;
 	struct bvec_iter iter;
@@ -35,7 +36,6 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
 	cluster = blk_queue_cluster(q);
 	seg_size = 0;
 	nr_phys_segs = 0;
-	no_sg_merge = test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags);
 	high = 0;
 	for_each_bio(bio) {
 		bio_for_each_segment(bv, bio, iter) {
@@ -88,18 +88,23 @@ new_segment:
 
 void blk_recalc_rq_segments(struct request *rq)
 {
-	rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio);
+	bool no_sg_merge = !!test_bit(QUEUE_FLAG_NO_SG_MERGE,
+			&rq->q->queue_flags);
+
+	rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio,
+			no_sg_merge);
 }
 
 void blk_recount_segments(struct request_queue *q, struct bio *bio)
 {
-	if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags))
+	if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags) &&
+			bio->bi_vcnt < queue_max_segments(q))
 		bio->bi_phys_segments = bio->bi_vcnt;
 	else {
 		struct bio *nxt = bio->bi_next;
 
 		bio->bi_next = NULL;
-		bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio);
+		bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, false);
 		bio->bi_next = nxt;
 	}