blk-mq: reflow blk_insert_flush

Use a switch statement to decide on the disposition of a flush request
instead of multiple if statements, out of which one does checks that are
more complex than required.  Also warn on a malformed request early
on instead of doing a BUG_ON later.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230519044050.107790-3-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -402,6 +402,9 @@ void blk_insert_flush(struct request *rq)
 	struct blk_flush_queue *fq = blk_get_flush_queue(q, rq->mq_ctx);
 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
 
+	/* FLUSH/FUA request must never be merged */
+	WARN_ON_ONCE(rq->bio != rq->biotail);
+
 	/*
 	 * @policy now records what operations need to be done.  Adjust
 	 * REQ_PREFLUSH and FUA for the driver.
@@ -417,39 +420,35 @@ void blk_insert_flush(struct request *rq)
 	 */
 	rq->cmd_flags |= REQ_SYNC;
 
+	switch (policy) {
+	case 0:
 		/*
 		 * An empty flush handed down from a stacking driver may
 		 * translate into nothing if the underlying device does not
 		 * advertise a write-back cache.  In this case, simply
 		 * complete the request.
 		 */
-	if (!policy) {
 		blk_mq_end_request(rq, 0);
 		return;
-	}
-
-	BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */
-
+	case REQ_FSEQ_DATA:
 		/*
-	 * If there's data but flush is not necessary, the request can be
-	 * processed directly without going through flush machinery.  Queue
-	 * for normal execution.
+		 * If there's data, but no flush is necessary, the request can
+		 * be processed directly without going through flush machinery.
+		 * Queue for normal execution.
 		 */
-	if ((policy & REQ_FSEQ_DATA) &&
-	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
 		blk_mq_request_bypass_insert(rq, 0);
 		blk_mq_run_hw_queue(hctx, false);
 		return;
-	}
-
+	default:
 		/*
-	 * @rq should go through flush machinery.  Mark it part of flush
-	 * sequence and submit for further processing.
+		 * Mark the request as part of a flush sequence and submit it
+		 * for further processing to the flush state machine.
 		 */
 		blk_rq_init_flush(rq);
 		spin_lock_irq(&fq->mq_flush_lock);
 		blk_flush_complete_seq(rq, fq, REQ_FSEQ_ACTIONS & ~policy, 0);
 		spin_unlock_irq(&fq->mq_flush_lock);
+	}
 }
 
 /**