iomap: Support partial direct I/O on user copy failures

In iomap_dio_rw, when iomap_apply returns an -EFAULT error and the
IOMAP_DIO_PARTIAL flag is set, complete the request synchronously and
return a partial result.  This allows the caller to deal with the page
fault and retry the remainder of the request.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

fs/iomap/direct-io.c

@@ -581,6 +581,12 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 	if (iov_iter_rw(iter) == READ && iomi.pos >= dio->i_size)
 		iov_iter_revert(iter, iomi.pos - dio->i_size);
 
+	if (ret == -EFAULT && dio->size && (dio_flags & IOMAP_DIO_PARTIAL)) {
+		if (!(iocb->ki_flags & IOCB_NOWAIT))
+			wait_for_completion = true;
+		ret = 0;
+	}
+
 	/* magic error code to fall back to buffered I/O */
 	if (ret == -ENOTBLK) {
 		wait_for_completion = true;

include/linux/iomap.h

@@ -330,6 +330,13 @@ struct iomap_dio_ops {
   */
 #define IOMAP_DIO_OVERWRITE_ONLY	(1 << 1)
 
+/*
+ * When a page fault occurs, return a partial synchronous result and allow
+ * the caller to retry the rest of the operation after dealing with the page
+ * fault.
+ */
+#define IOMAP_DIO_PARTIAL		(1 << 2)
+
 ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 		const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
 		unsigned int dio_flags);