nvmet: don't split large I/Os unconditionally

If we know that the I/O size exceeds our inline bio vec, no
point using it and split the rest to begin with. We could
in theory reuse the inline bio and only allocate the bio_vec,
but its really not worth optimizing for.
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/io-cmd-bdev.c

@@ -58,7 +58,7 @@ static void nvmet_bio_done(struct bio *bio)
 static void nvmet_bdev_execute_rw(struct nvmet_req *req)
 {
 	int sg_cnt = req->sg_cnt;
-	struct bio *bio = &req->b.inline_bio;
+	struct bio *bio;
 	struct scatterlist *sg;
 	sector_t sector;
 	blk_qc_t cookie;
@@ -81,7 +81,12 @@ static void nvmet_bdev_execute_rw(struct nvmet_req *req)
 	sector = le64_to_cpu(req->cmd->rw.slba);
 	sector <<= (req->ns->blksize_shift - 9);
 
+	if (req->data_len <= NVMET_MAX_INLINE_DATA_LEN) {
+		bio = &req->b.inline_bio;
 		bio_init(bio, req->inline_bvec, ARRAY_SIZE(req->inline_bvec));
+	} else {
+		bio = bio_alloc(GFP_KERNEL, min(sg_cnt, BIO_MAX_PAGES));
+	}
 	bio_set_dev(bio, req->ns->bdev);
 	bio->bi_iter.bi_sector = sector;
 	bio->bi_private = req;

drivers/nvme/target/nvmet.h

@@ -264,6 +264,7 @@ struct nvmet_fabrics_ops {
 };
 
 #define NVMET_MAX_INLINE_BIOVEC	8
+#define NVMET_MAX_INLINE_DATA_LEN NVMET_MAX_INLINE_BIOVEC * PAGE_SIZE
 
 struct nvmet_req {
 	struct nvme_command	*cmd;