blk-flush: reuse rq queuelist in flush state machine

Since we don't need to maintain inflight flush_data requests list
anymore, we can reuse rq->queuelist for flush pending list.

Note in mq_flush_data_end_io(), we need to re-initialize rq->queuelist
before reusing it in the state machine when end, since the rq->rq_next
also reuse it, may have corrupted rq->queuelist by the driver.

This patch decrease the size of struct request by 16 bytes.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230717040058.3993930-5-chengming.zhou@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -183,14 +183,13 @@ static void blk_flush_complete_seq(struct request *rq,
 		/* queue for flush */
 		if (list_empty(pending))
 			fq->flush_pending_since = jiffies;
-		list_move_tail(&rq->flush.list, pending);
+		list_move_tail(&rq->queuelist, pending);
 		break;
 
 	case REQ_FSEQ_DATA:
-		list_del_init(&rq->flush.list);
 		fq->flush_data_in_flight++;
 		spin_lock(&q->requeue_lock);
-		list_add(&rq->queuelist, &q->requeue_list);
+		list_move(&rq->queuelist, &q->requeue_list);
 		spin_unlock(&q->requeue_lock);
 		blk_mq_kick_requeue_list(q);
 		break;
@@ -202,7 +201,7 @@ static void blk_flush_complete_seq(struct request *rq,
 		 * flush data request completion path.  Restore @rq for
 		 * normal completion and end it.
 		 */
-		list_del_init(&rq->flush.list);
+		list_del_init(&rq->queuelist);
 		blk_flush_restore_request(rq);
 		blk_mq_end_request(rq, error);
 		break;
@@ -258,7 +257,7 @@ static enum rq_end_io_ret flush_end_io(struct request *flush_rq,
 	fq->flush_running_idx ^= 1;
 
 	/* and push the waiting requests to the next stage */
-	list_for_each_entry_safe(rq, n, running, flush.list) {
+	list_for_each_entry_safe(rq, n, running, queuelist) {
 		unsigned int seq = blk_flush_cur_seq(rq);
 
 		BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
@@ -292,7 +291,7 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
 {
 	struct list_head *pending = &fq->flush_queue[fq->flush_pending_idx];
 	struct request *first_rq =
-		list_first_entry(pending, struct request, flush.list);
+		list_first_entry(pending, struct request, queuelist);
 	struct request *flush_rq = fq->flush_rq;
 
 	/* C1 described at the top of this file */
@@ -376,6 +375,11 @@ static enum rq_end_io_ret mq_flush_data_end_io(struct request *rq,
 	 */
 	spin_lock_irqsave(&fq->mq_flush_lock, flags);
 	fq->flush_data_in_flight--;
+	/*
+	 * May have been corrupted by rq->rq_next reuse, we need to
+	 * re-initialize rq->queuelist before reusing it here.
+	 */
+	INIT_LIST_HEAD(&rq->queuelist);
 	blk_flush_complete_seq(rq, fq, REQ_FSEQ_DATA, error);
 	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
 
@@ -386,7 +390,6 @@ static enum rq_end_io_ret mq_flush_data_end_io(struct request *rq,
 static void blk_rq_init_flush(struct request *rq)
 {
 	rq->flush.seq = 0;
-	INIT_LIST_HEAD(&rq->flush.list);
 	rq->rq_flags |= RQF_FLUSH_SEQ;
 	rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
 	rq->end_io = mq_flush_data_end_io;

include/linux/blk-mq.h

@@ -178,7 +178,6 @@ struct request {
 
 	struct {
 		unsigned int		seq;
-		struct list_head	list;
 		rq_end_io_fn		*saved_end_io;
 	} flush;