block: support a full bio worth of IO for simplified bdev direct-io

Just alloc the bio_vec array if we exceed the inline limit.
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/block_dev.c

@@ -193,7 +193,7 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
 	struct file *file = iocb->ki_filp;
 	struct block_device *bdev = I_BDEV(bdev_file_inode(file));
 	unsigned blkbits = blksize_bits(bdev_logical_block_size(bdev));
-	struct bio_vec inline_vecs[DIO_INLINE_BIO_VECS], *bvec;
+	struct bio_vec inline_vecs[DIO_INLINE_BIO_VECS], *vecs, *bvec;
 	loff_t pos = iocb->ki_pos;
 	bool should_dirty = false;
 	struct bio bio;
@@ -204,9 +204,17 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
 	if ((pos | iov_iter_alignment(iter)) & ((1 << blkbits) - 1))
 		return -EINVAL;
 
+	if (nr_pages <= DIO_INLINE_BIO_VECS)
+		vecs = inline_vecs;
+	else {
+		vecs = kmalloc(nr_pages * sizeof(struct bio_vec), GFP_KERNEL);
+		if (!vecs)
+			return -ENOMEM;
+	}
+
 	bio_init(&bio);
 	bio.bi_max_vecs = nr_pages;
-	bio.bi_io_vec = inline_vecs;
+	bio.bi_io_vec = vecs;
 	bio.bi_bdev = bdev;
 	bio.bi_iter.bi_sector = pos >> blkbits;
 	bio.bi_private = current;
@@ -243,6 +251,9 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
 		put_page(bvec->bv_page);
 	}
 
+	if (vecs != inline_vecs)
+		kfree(vecs);
+
 	if (unlikely(bio.bi_error))
 		return bio.bi_error;
 	iocb->ki_pos += ret;
@@ -256,10 +267,10 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	struct inode *inode = bdev_file_inode(file);
 	int nr_pages;
 
-	nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
+	nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
 	if (!nr_pages)
 		return 0;
-	if (is_sync_kiocb(iocb) && nr_pages <= DIO_INLINE_BIO_VECS)
+	if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
 		return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
 	return __blockdev_direct_IO(iocb, inode, I_BDEV(inode), iter,
 				    blkdev_get_block, NULL, NULL,