Support .read/write_iter callbacks in lieu of .aio_read/write

The aio_read and aio_write callbacks are no longer used by newer
kernels which now uses iter based readers and writers.

We can avoid implementing plain .read and .write as an iter will
be generated when needed for us automatically.

We add a new data_wait_check_iter() function accordingly.

With these methods removed from the kernel, the el8 kernel no
longer uses the extended ops wrapper struct and is much closer now
to upstream. As a result, a lot of methods are moving around from
inode_dir_operations to and from inode_file_operations etc, and
perhaps things will look a bit more structured as a result.

As a result, we need a slightly different data_wait_check() that
accounts for the iter and offset properly.

Signed-off-by: Auke Kok <auke.kok@versity.com>

kmod/src/data.c

@@ -1807,6 +1807,37 @@ int scoutfs_data_wait_check_iov(struct inode *inode, const struct iovec *iov,
 	return ret;
 }
 
+int scoutfs_data_wait_check_iter(struct inode *inode, loff_t pos, struct iov_iter *iter,
+				 u8 sef, u8 op, struct scoutfs_data_wait *dw,
+				 struct scoutfs_lock *lock)
+{
+	size_t count = iov_iter_count(iter);
+	size_t off = iter->iov_offset;
+	const struct iovec *iov;
+	size_t len;
+	int ret = 0;
+
+	for (iov = iter->iov; count > 0; iov++) {
+		len = iov->iov_len - off;
+		if (len == 0)
+			continue;
+
+		/* aren't we waiting on too much data here ? */
+		ret = scoutfs_data_wait_check(inode, pos, len,
+					      sef, op, dw, lock);
+
+		if (ret != 0)
+			break;
+
+
+		pos += len;
+		count -= len;
+		off = 0;
+	}
+
+	return ret;
+}
+
 int scoutfs_data_wait(struct inode *inode, struct scoutfs_data_wait *dw)
 {
 	DECLARE_DATA_WAIT_ROOT(inode->i_sb, rt);
@@ -1909,10 +1940,15 @@ const struct address_space_operations scoutfs_file_aops = {
 };
 
 const struct file_operations scoutfs_file_fops = {
+#ifdef KC_LINUX_HAVE_FOP_AIO_READ
 	.read		= do_sync_read,
 	.write		= do_sync_write,
 	.aio_read	= scoutfs_file_aio_read,
 	.aio_write	= scoutfs_file_aio_write,
+#else
+	.read_iter	= scoutfs_file_read_iter,
+	.write_iter	= scoutfs_file_write_iter,
+#endif
 	.unlocked_ioctl	= scoutfs_ioctl,
 	.fsync		= scoutfs_file_fsync,
 	.llseek		= scoutfs_file_llseek,