Ok, I looked into this a bit more, and while I'm still fine with the
patch I think it's only half of what we should do here.  There really
is no point in doing the first non-blocking path in blk_mq_map_request
either as bt_get itself already does the non-blocking pass, and also
runs the queue when scheduling in the later loop as well.  So to get
towards what I had in my tree we also need something like this:

>From c69bf02929d9c37d193b004a4c3c85c1142fa996 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <h...@lst.de>
Date: Thu, 22 Sep 2016 11:38:23 -0700
Subject: blk-mq: remove non-blocking pass in blk_mq_map_request

bt_get already does a non-blocking pass as well as running the queue
when scheduling internally, no need to duplicate it.

Signed-off-by: Christoph Hellwig <h...@lst.de>
 block/blk-mq.c | 14 +-------------
 1 file changed, 1 insertion(+), 13 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index eae2f12..e9ebe98 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1210,20 +1210,8 @@ static struct request *blk_mq_map_request(struct 
request_queue *q,
                op_flags |= REQ_SYNC;
        trace_block_getrq(q, bio, op);
-       blk_mq_set_alloc_data(&alloc_data, q, BLK_MQ_REQ_NOWAIT, ctx, hctx);
+       blk_mq_set_alloc_data(&alloc_data, q, 0, ctx, hctx);
        rq = __blk_mq_alloc_request(&alloc_data, op, op_flags);
-       if (unlikely(!rq)) {
-               blk_mq_run_hw_queue(hctx, false);
-               blk_mq_put_ctx(ctx);
-               trace_block_sleeprq(q, bio, op);
-               ctx = blk_mq_get_ctx(q);
-               hctx = q->mq_ops->map_queue(q, ctx->cpu);
-               blk_mq_set_alloc_data(&alloc_data, q, 0, ctx, hctx);
-               rq = __blk_mq_alloc_request(&alloc_data, op, op_flags);
-               ctx = alloc_data.ctx;
-               hctx = alloc_data.hctx;
-       }
        data->hctx = hctx;

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