[PATCH 1/2] blk-mq: don't call callbacks for requests that bypassed the scheduler

[Niklas Cassel <Niklas.Cassel@wdc.com>]

From: Niklas Cassel <niklas.cassel@wdc.com>

Currently, __blk_mq_alloc_request() (via blk_mq_rq_ctx_init()) calls the
I/O scheduler callback e->type->ops.prepare_request(), which will set
RQF_ELVPRIV, even though passthrough (and flush) requests will later
bypass the I/O scheduler in blk_mq_submit_bio().

Later, blk_mq_free_request() checks if the RQF_ELVPRIV flag is set,
if it is, the e->type->ops.finish_request() I/O scheduler callback
will be called.

i.e., the prepare_request and finish_request I/O scheduler callbacks
will be called for requests which were never inserted to the I/O
scheduler.

Fix this by not calling e->type->ops.prepare_request(), nor setting
the RQF_ELVPRIV flag for passthrough requests.
Since the RQF_ELVPRIV flag will not get set for passthrough requests,
e->type->ops.prepare_request() will no longer get called for
passthrough requests which were never inserted to the I/O scheduler.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
---
 block/blk-mq.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 65d3a63aecc6..0816af125059 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -328,7 +328,12 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 	data->ctx->rq_dispatched[op_is_sync(data->cmd_flags)]++;
 	refcount_set(&rq->ref, 1);
 
-	if (!op_is_flush(data->cmd_flags)) {
+	/*
+	 * Flush/passthrough requests are special and go directly to the
+	 * dispatch list, bypassing the scheduler.
+	 */
+	if (!op_is_flush(data->cmd_flags) &&
+	    !blk_op_is_passthrough(data->cmd_flags)) {
 		struct elevator_queue *e = data->q->elevator;
 
 		rq->elv.icq = NULL;
-- 
2.31.1