[PATCH v4 6/8] iomap: Support restarting direct I/O requests after user copy failures

[Andreas Gruenbacher <agruenba@redhat.com>]

In iomap_dio_rw, when iomap_apply returns an -EFAULT error, complete the
request synchronously.  Either return a partial result (when the
IOMAP_DIO_FAULT_RETRY flag is set and the caller is thus prepared to handle
partial results), or reset the iterator to the start to allow a restart.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/iomap/direct-io.c  | 13 +++++++++++++
 include/linux/iomap.h |  7 +++++++
 2 files changed, 20 insertions(+)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 51831ce93f6e..1ba825bab08b 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -565,6 +565,19 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 		ret = iomap_apply(inode, pos, count, iomap_flags, ops, dio,
 				iomap_dio_actor);
 		if (ret <= 0) {
+			if (ret == -EFAULT) {
+				/*
+				 * Finish synchronously and revert the iterator
+				 * when failing the request to allow a retry.
+				 */
+				wait_for_completion = true;
+				if (dio->size &&
+				    (dio_flags & IOMAP_DIO_FAULT_RETRY))
+					ret = 0;
+				else
+					iov_iter_revert(iter, dio->size);
+			}
+
 			/* magic error code to fall back to buffered I/O */
 			if (ret == -ENOTBLK) {
 				wait_for_completion = true;
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 8aea35f1a003..9dbba9f7c945 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -267,6 +267,13 @@ 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_FAULT_RETRY		(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, size_t done_before);
-- 
2.26.3