[PATCH 3/3] blk-merge: warn if figured out segment number is bigger than nr_phys_segments

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

We had seen lots of reports of this kind issue, so add one
warnning in blk-merge, then it can be triggered easily and
avoid to depend on warning/bug from drivers.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
 block/blk-merge.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 50793cd..41a55ba 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -428,6 +428,12 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 	if (sg)
 		sg_mark_end(sg);
 
+	/*
+	 * Something must have been wrong if the figured number of
+	 * segment is bigger than number of req's physical segments
+	 */
+	WARN_ON(nsegs > rq->nr_phys_segments);
+
 	return nsegs;
 }
 EXPORT_SYMBOL(blk_rq_map_sg);
-- 
1.9.1