[PATCH] direct_io: Allow partial writes

[Dmitry Monakhov]

Current direct io allocation behavior is inconvenient. Partial writes
are not supported. If we try to issue 10Mb chunk, but only 5Mb is
available then we will allocate thees 5Mb until ENOSPC, and then
drop such space and return ENOSPC.
But in fact partial writes are possible in case of holes.
Seems that there is no enough reason to deny partial writes.

Signed-off-by: Dmitry Monakhov <dmonakhov@xxxxxxxxxx>
---
 fs/direct-io.c     |    4 ++++
 fs/ext4/inode.c    |   11 +++++++----
 include/linux/fs.h |    2 ++
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/direct-io.c b/fs/direct-io.c
index e82adc2..250a041 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -229,6 +229,10 @@ static int dio_complete(struct dio *dio, loff_t offset, int ret)
 		ret = 0;
 
 	if (dio->result) {
+		/* Ignore error if we have written some data */
+		if (dio->flags & DIO_PARTIAL_WRITE)
+			ret = 0;
+
 		transferred = dio->result;
 
 		/* Check for short read case */
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 218ea0b..8c00127 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3447,10 +3447,13 @@ retry:
 				 offset, nr_segs,
 				 ext4_get_block, NULL);
 	else
-		ret = blockdev_direct_IO(rw, iocb, inode,
-				 inode->i_sb->s_bdev, iov,
-				 offset, nr_segs,
-				 ext4_get_block, NULL);
+		ret = __blockdev_direct_IO(rw, iocb, inode,
+					inode->i_sb->s_bdev, iov,
+					offset, nr_segs,
+					ext4_get_block, NULL,
+					DIO_LOCKING | DIO_SKIP_HOLES |
+					DIO_PARTIAL_WRITE);
+
 	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
 		goto retry;
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 9147ca8..d887685 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2259,6 +2259,8 @@ enum {
 
 	/* filesystem does not support filling holes */
 	DIO_SKIP_HOLES	= 0x02,
+	/* allow partial writes */
+	DIO_PARTIAL_WRITE = 0x04,
 };
 
 static inline ssize_t blockdev_direct_IO(int rw, struct kiocb *iocb,
-- 
1.6.6


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