[PATCH v5] Return bytes transferred for partial direct I/O

[Goldwyn Rodrigues <rgoldwyn@xxxxxxx>]

From: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>

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.

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. Fixed.

Changes since v3:
 - [hch] initialize transferred with dio->size and use transferred instead
   of dio->size.

Changes since v4:
 - Refreshed to v4.14

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
---
 fs/block_dev.c |  2 +-
 fs/direct-io.c |  4 +---
 fs/iomap.c     | 20 ++++++++++----------
 3 files changed, 12 insertions(+), 14 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 789f55e851ae..4d3e4603f687 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 b53e66d9abd7..a8d2710f4ee9 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -262,8 +262,6 @@ static ssize_t dio_complete(struct dio *dio, ssize_t ret, unsigned int flags)
 		ret = dio->page_errors;
 	if (ret == 0)
 		ret = dio->io_error;
-	if (ret == 0)
-		ret = transferred;
 
 	if (dio->end_io) {
 		// XXX: ki_pos??
@@ -310,7 +308,7 @@ static ssize_t dio_complete(struct dio *dio, ssize_t ret, unsigned int flags)
 	}
 
 	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 d4801f8dd4fd..6e37acba578d 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -715,23 +715,23 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
 	struct kiocb *iocb = dio->iocb;
 	struct inode *inode = file_inode(iocb->ki_filp);
 	loff_t offset = iocb->ki_pos;
-	ssize_t ret;
+	ssize_t err;
+	ssize_t transferred = dio->size;
 
 	if (dio->end_io) {
-		ret = dio->end_io(iocb,
-				dio->error ? dio->error : dio->size,
+		err = dio->end_io(iocb,
+				transferred ? transferred : dio->error,
 				dio->flags);
 	} else {
-		ret = dio->error;
+		err = dio->error;
 	}
 
-	if (likely(!ret)) {
-		ret = dio->size;
+	if (likely(transferred)) {
 		/* check for short read */
-		if (offset + ret > dio->i_size &&
+		if (offset + transferred > dio->i_size &&
 		    !(dio->flags & IOMAP_DIO_WRITE))
-			ret = dio->i_size - offset;
-		iocb->ki_pos += ret;
+			transferred = dio->i_size - offset;
+		iocb->ki_pos += transferred;
 	}
 
 	/*
@@ -758,7 +758,7 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
 	inode_dio_end(file_inode(iocb->ki_filp));
 	kfree(dio);
 
-	return ret;
+	return transferred ? transferred : err;
 }
 
 static void iomap_dio_complete_work(struct work_struct *work)
-- 
2.14.2