block: provide plug based way of signaling forced no-wait semantics

Provide a way for the caller to specify that IO should be marked
with REQ_NOWAIT to avoid blocking on allocation.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -958,6 +958,7 @@ generic_make_request_checks(struct bio *bio)
 	struct request_queue *q;
 	int nr_sectors = bio_sectors(bio);
 	blk_status_t status = BLK_STS_IOERR;
+	struct blk_plug *plug;
 	char b[BDEVNAME_SIZE];
 
 	might_sleep();
@@ -971,6 +972,10 @@ generic_make_request_checks(struct bio *bio)
 		goto end_io;
 	}
 
+	plug = blk_mq_plug(q, bio);
+	if (plug && plug->nowait)
+		bio->bi_opf |= REQ_NOWAIT;
+
 	/*
 	 * For a REQ_NOWAIT based request, return -EOPNOTSUPP
 	 * if queue is not a request based queue.
@@ -1800,6 +1805,7 @@ void blk_start_plug(struct blk_plug *plug)
 	INIT_LIST_HEAD(&plug->cb_list);
 	plug->rq_count = 0;
 	plug->multiple_queues = false;
+	plug->nowait = false;
 
 	/*
 	 * Store ordering should not be needed here, since a potential

include/linux/blkdev.h

@@ -1189,6 +1189,7 @@ struct blk_plug {
 	struct list_head cb_list; /* md requires an unplug callback */
 	unsigned short rq_count;
 	bool multiple_queues;
+	bool nowait;
 };
 #define BLK_MAX_REQUEST_COUNT 16
 #define BLK_PLUG_FLUSH_SIZE (128 * 1024)