block: support large requests in blk_rq_map_user_iov

This patch adds support for larger requests in blk_rq_map_user_iov by
allowing it to build multiple bios for a request.  This functionality
used to exist for the non-vectored blk_rq_map_user in the past, and
this patch reuses the existing functionality for it on the unmap side,
which stuck around.  Thanks to the iov_iter API supporting multiple
bios is fairly trivial, as we can just iterate the iov until we've
consumed the whole iov_iter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Jeff Lien <Jeff.Lien@hgst.com>
Tested-by: Jeff Lien <Jeff.Lien@hgst.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-map.c

@@ -57,6 +57,49 @@ static int __blk_rq_unmap_user(struct bio *bio)
 	return ret;
 }
 
+static int __blk_rq_map_user_iov(struct request *rq,
+		struct rq_map_data *map_data, struct iov_iter *iter,
+		gfp_t gfp_mask, bool copy)
+{
+	struct request_queue *q = rq->q;
+	struct bio *bio, *orig_bio;
+	int ret;
+
+	if (copy)
+		bio = bio_copy_user_iov(q, map_data, iter, gfp_mask);
+	else
+		bio = bio_map_user_iov(q, iter, gfp_mask);
+
+	if (IS_ERR(bio))
+		return PTR_ERR(bio);
+
+	if (map_data && map_data->null_mapped)
+		bio_set_flag(bio, BIO_NULL_MAPPED);
+
+	iov_iter_advance(iter, bio->bi_iter.bi_size);
+	if (map_data)
+		map_data->offset += bio->bi_iter.bi_size;
+
+	orig_bio = bio;
+	blk_queue_bounce(q, &bio);
+
+	/*
+	 * We link the bounce buffer in and could have to traverse it
+	 * later so we have to get a ref to prevent it from being freed
+	 */
+	bio_get(bio);
+
+	ret = blk_rq_append_bio(q, rq, bio);
+	if (ret) {
+		bio_endio(bio);
+		__blk_rq_unmap_user(orig_bio);
+		bio_put(bio);
+		return ret;
+	}
+
+	return 0;
+}
+
 /**
  * blk_rq_map_user_iov - map user data to a request, for REQ_TYPE_BLOCK_PC usage
  * @q:		request queue where request should be inserted
@@ -82,10 +125,11 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
 			struct rq_map_data *map_data,
 			const struct iov_iter *iter, gfp_t gfp_mask)
 {
-	struct bio *bio;
-	int unaligned = 0;
-	struct iov_iter i;
 	struct iovec iov, prv = {.iov_base = NULL, .iov_len = 0};
+	bool copy = (q->dma_pad_mask & iter->count) || map_data;
+	struct bio *bio = NULL;
+	struct iov_iter i;
+	int ret;
 
 	if (!iter || !iter->count)
 		return -EINVAL;
@@ -101,42 +145,29 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
 		 */
 		if ((uaddr & queue_dma_alignment(q)) ||
 		    iovec_gap_to_prv(q, &prv, &iov))
-			unaligned = 1;
+			copy = true;
 
 		prv.iov_base = iov.iov_base;
 		prv.iov_len = iov.iov_len;
 	}
 
-	if (unaligned || (q->dma_pad_mask & iter->count) || map_data)
-		bio = bio_copy_user_iov(q, map_data, iter, gfp_mask);
-	else
-		bio = bio_map_user_iov(q, iter, gfp_mask);
-
-	if (IS_ERR(bio))
-		return PTR_ERR(bio);
-
-	if (map_data && map_data->null_mapped)
-		bio_set_flag(bio, BIO_NULL_MAPPED);
-
-	if (bio->bi_iter.bi_size != iter->count) {
-		/*
-		 * Grab an extra reference to this bio, as bio_unmap_user()
-		 * expects to be able to drop it twice as it happens on the
-		 * normal IO completion path
-		 */
-		bio_get(bio);
-		bio_endio(bio);
-		__blk_rq_unmap_user(bio);
-		return -EINVAL;
-	}
+	i = *iter;
+	do {
+		ret =__blk_rq_map_user_iov(rq, map_data, &i, gfp_mask, copy);
+		if (ret)
+			goto unmap_rq;
+		if (!bio)
+			bio = rq->bio;
+	} while (iov_iter_count(&i));
 
 	if (!bio_flagged(bio, BIO_USER_MAPPED))
 		rq->cmd_flags |= REQ_COPY_USER;
-
-	blk_queue_bounce(q, &bio);
-	bio_get(bio);
-	blk_rq_bio_prep(q, rq, bio);
 	return 0;
+
+unmap_rq:
+	__blk_rq_unmap_user(bio);
+	rq->bio = NULL;
+	return -EINVAL;
 }
 EXPORT_SYMBOL(blk_rq_map_user_iov);