[dm-devel] blk-mq request allocation stalls

[Keith Busch]

On Mon, 12 Jan 2015, Keith Busch wrote:
> Oh, let's look at "__blk_rq_prep_clone". dm calls that after
> blk_get_request() for the blk-mq based multipath types and overrides the
> destinations cmd_flags with the source's even though the source was not
> allocated from a blk-mq based queue, much less a shared tag.

Untested patch. This will also preserve the failfast cmd_flag dm-mpath
set after allocating.

---
diff --git a/block/blk-core.c b/block/blk-core.c
index 7e78931..6201090 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2895,7 +2895,10 @@ EXPORT_SYMBOL_GPL(blk_rq_unprep_clone);
  static void __blk_rq_prep_clone(struct request *dst, struct request *src)
  {
  	dst->cpu = src->cpu;
-	dst->cmd_flags = (src->cmd_flags & REQ_CLONE_MASK) | REQ_NOMERGE;
+	if (dst->q->mq_ops)
+		dst->cmd_flags |= (src->cmd_flags & REQ_CLONE_MASK) | REQ_NOMERGE;
+	else
+		dst->cmd_flags = (src->cmd_flags & REQ_CLONE_MASK) | REQ_NOMERGE;
  	dst->cmd_type = src->cmd_type;
  	dst->__sector = blk_rq_pos(src);
  	dst->__data_len = blk_rq_bytes(src);
--