scsi: target: core: add missing file_{start,end}_write()

The callers of vfs_iter_write() are required to hold file_start_write().
file_start_write() is a no-op for the S_ISBLK() case, but it is really
needed when the backing file is a regular file.

We are going to move file_{start,end}_write() into vfs_iter_write(), but
we need to fix this first, so that the fix could be backported to stable
kernels.

Suggested-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]/
Cc:  <[email protected]>
Signed-off-by: Amir Goldstein <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Acked-by: Martin K. Petersen <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: amir73il

drivers/target/target_core_file.c

@@ -332,11 +332,13 @@ static int fd_do_rw(struct se_cmd *cmd, struct file *fd,
 	}
 
 	iov_iter_bvec(&iter, is_write, bvec, sgl_nents, len);
-	if (is_write)
+	if (is_write) {
+		file_start_write(fd);
 		ret = vfs_iter_write(fd, &iter, &pos, 0);
-	else
+		file_end_write(fd);
+	} else {
 		ret = vfs_iter_read(fd, &iter, &pos, 0);
-
+	}
 	if (is_write) {
 		if (ret < 0 || ret != data_length) {
 			pr_err("%s() write returned %d\n", __func__, ret);
@@ -467,7 +469,9 @@ fd_execute_write_same(struct se_cmd *cmd)
 	}
 
 	iov_iter_bvec(&iter, ITER_SOURCE, bvec, nolb, len);
+	file_start_write(fd_dev->fd_file);
 	ret = vfs_iter_write(fd_dev->fd_file, &iter, &pos, 0);
+	file_end_write(fd_dev->fd_file);
 
 	kfree(bvec);
 	if (ret < 0 || ret != len) {