[PATCH V2 1/3] block: respect queue limit of max discard segment

[Ming Lei <ming.lei@redhat.com>]

When queue_max_discard_segments(q) is 1, blk_discard_mergable() will
return false for discard request, then normal request merge is applied.
However, only queue_max_segments() is checked, so max discard segment
limit isn't respected.

Check max discard segment limit in the request merge code for fixing
the issue.

Discard request failure of virtio_blk is fixed.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Fixes: 69840466086d ("block: fix the DISCARD request merge")
Cc: Christoph Hellwig <hch@lst.de>
Cc: Stefano Garzarella <sgarzare@redhat.com>
---
 block/blk-merge.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5196dc145270..d18fb88ca8bd 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -533,10 +533,16 @@ int __blk_rq_map_sg(struct request_queue *q, struct request *rq,
 }
 EXPORT_SYMBOL(__blk_rq_map_sg);
 
+static inline unsigned int blk_rq_get_max_segments(struct request *rq)
+{
+	return req_op(rq) == REQ_OP_DISCARD ?
+		queue_max_discard_segments(rq->q) : queue_max_segments(rq->q);
+}
+
 static inline int ll_new_hw_segment(struct request *req, struct bio *bio,
 		unsigned int nr_phys_segs)
 {
-	if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(req->q))
+	if (req->nr_phys_segments + nr_phys_segs > blk_rq_get_max_segments(req))
 		goto no_merge;
 
 	if (blk_integrity_merge_bio(req->q, req, bio) == false)
@@ -624,7 +630,7 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 		return 0;
 
 	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
-	if (total_phys_segments > queue_max_segments(q))
+	if (total_phys_segments > blk_rq_get_max_segments(req))
 		return 0;
 
 	if (blk_integrity_merge_rq(q, req, next) == false)
-- 
2.25.2