block: move direct_IO into our own read_iter handler

Don't call into generic_file_read_iter() if we know it's O_DIRECT, just
set it up ourselves and call our own handler. This avoids an indirect call
for O_DIRECT.

Fall back to filemap_read() if we fail.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/fops.c

@@ -566,21 +566,48 @@ static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
 	struct block_device *bdev = iocb->ki_filp->private_data;
 	loff_t size = bdev_nr_bytes(bdev);
+	size_t count = iov_iter_count(to);
 	loff_t pos = iocb->ki_pos;
 	size_t shorted = 0;
-	ssize_t ret;
+	ssize_t ret = 0;
 
-	if (unlikely(pos + iov_iter_count(to) > size)) {
+	if (unlikely(pos + count > size)) {
 		if (pos >= size)
 			return 0;
 		size -= pos;
-		if (iov_iter_count(to) > size) {
-			shorted = iov_iter_count(to) - size;
+		if (count > size) {
+			shorted = count - size;
 			iov_iter_truncate(to, size);
 		}
 	}
 
-	ret = generic_file_read_iter(iocb, to);
+	if (iocb->ki_flags & IOCB_DIRECT) {
+		struct address_space *mapping = iocb->ki_filp->f_mapping;
+
+		if (iocb->ki_flags & IOCB_NOWAIT) {
+			if (filemap_range_needs_writeback(mapping, iocb->ki_pos,
+						iocb->ki_pos + count - 1))
+				return -EAGAIN;
+		} else {
+			ret = filemap_write_and_wait_range(mapping,
+						iocb->ki_pos,
+					        iocb->ki_pos + count - 1);
+			if (ret < 0)
+				return ret;
+		}
+
+		file_accessed(iocb->ki_filp);
+
+		ret = blkdev_direct_IO(iocb, to);
+		if (ret >= 0) {
+			iocb->ki_pos += ret;
+			count -= ret;
+		}
+		if (ret < 0 || !count)
+			return ret;
+	}
+
+	ret = filemap_read(iocb, to, ret);
 
 	if (unlikely(shorted))
 		iov_iter_reexpand(to, iov_iter_count(to) + shorted);