direct-io: allow direct writes to empty inodes

On a DIO_SKIP_HOLES filesystem, the ->get_block() method is currently
not allowed to create blocks for an empty inode.  This confusion comes
from trying to bit shift a negative number, so check the size of the
inode first.

The problem is most visible for hfsplus, because the fallback to
buffered I/O doesn't happen and the write fails with EIO.  This is in
part the fault of the module, because it gives a wrong return value on
->get_block(); that will be fixed in a separate patch.
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/direct-io.c

@@ -679,6 +679,7 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
 	unsigned long fs_count;	/* Number of filesystem-sized blocks */
 	int create;
 	unsigned int i_blkbits = sdio->blkbits + sdio->blkfactor;
+	loff_t i_size;
 
 	/*
 	 * If there was a memory error and we've overwritten all the
@@ -708,8 +709,8 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
 		 */
 		create = dio->op == REQ_OP_WRITE;
 		if (dio->flags & DIO_SKIP_HOLES) {
-			if (fs_startblk <= ((i_size_read(dio->inode) - 1) >>
-							i_blkbits))
+			i_size = i_size_read(dio->inode);
+			if (i_size && fs_startblk <= (i_size - 1) >> i_blkbits)
 				create = 0;
 		}