[PATCH v3] Return bytes transferred for partial direct I/O

[Goldwyn Rodrigues <rgoldwyn@xxxxxxx>]

In case direct I/O encounters an error midway, it returns the error.
Instead it should be returning the number of bytes transferred so far.

Test case for filesystems (with ENOSPC):
1. Create an almost full filesystem
2. Create a file, say /mnt/lastfile, until the filesystem is full.
3. Direct write() with count > sizeof /mnt/lastfile.

Result: write() returns -ENOSPC. However, file content has data written
in step 3.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>

Changes since v1:
 - incorporated iomap and block devices

Changes since v2:
 - realized that file size was not increasing when performing a (partial)
   direct I/O because end_io function was receiving the error instead of
   size in iomap's dio end io. Fixed.

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 93d088ffc05c..1c6640ffb929 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -421,7 +421,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
 
 	if (!ret)
 		ret = blk_status_to_errno(dio->bio.bi_status);
-	if (likely(!ret))
+	if (likely(dio->size))
 		ret = dio->size;
 
 	bio_put(&dio->bio);
diff --git a/fs/direct-io.c b/fs/direct-io.c
index 5fa2211e49ae..0fc1789498ae 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -255,8 +255,6 @@ static ssize_t dio_complete(struct dio *dio, ssize_t ret, bool is_async)
 		ret = dio->page_errors;
 	if (ret == 0)
 		ret = dio->io_error;
-	if (ret == 0)
-		ret = transferred;
 
 	if (dio->end_io) {
 		int err;
@@ -284,7 +282,7 @@ static ssize_t dio_complete(struct dio *dio, ssize_t ret, bool is_async)
 	}
 
 	kmem_cache_free(dio_cache, dio);
-	return ret;
+	return transferred ? transferred : ret;
 }
 
 static void dio_aio_complete_work(struct work_struct *work)
diff --git a/fs/iomap.c b/fs/iomap.c
index 269b24a01f32..019c4d993300 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -714,28 +714,29 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
 {
 	struct kiocb *iocb = dio->iocb;
 	ssize_t ret;
+	ssize_t transferred = 0;
 
 	if (dio->end_io) {
 		ret = dio->end_io(iocb,
-				dio->error ? dio->error : dio->size,
+				dio->size ? dio->size : dio->error,
 				dio->flags);
 	} else {
 		ret = dio->error;
 	}
 
-	if (likely(!ret)) {
-		ret = dio->size;
+	if (likely(dio->size)) {
+		transferred = dio->size;
 		/* check for short read */
-		if (iocb->ki_pos + ret > dio->i_size &&
+		if (iocb->ki_pos + transferred > dio->i_size &&
 		    !(dio->flags & IOMAP_DIO_WRITE))
-			ret = dio->i_size - iocb->ki_pos;
-		iocb->ki_pos += ret;
+			transferred = dio->i_size - iocb->ki_pos;
+		iocb->ki_pos += transferred;
 	}
 
 	inode_dio_end(file_inode(iocb->ki_filp));
 	kfree(dio);
 
-	return ret;
+	return transferred ? transferred : ret;
 }
 
 static void iomap_dio_complete_work(struct work_struct *work)