mm: call filemap_fdatawrite_range_kick() after IOCB_DONTCACHE issue

When a buffered write submitted with IOCB_DONTCACHE has been successfully
submitted, call filemap_fdatawrite_range_kick() to kick off the IO.  File
systems call generic_write_sync() for any successful buffered write
submission, hence add the logic here rather than needing to modify the
file system.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
Cc: Brian Foster <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: axboe

include/linux/fs.h

@@ -2912,6 +2912,11 @@ static inline ssize_t generic_write_sync(struct kiocb *iocb, ssize_t count)
 				(iocb->ki_flags & IOCB_SYNC) ? 0 : 1);
 		if (ret)
 			return ret;
+	} else if (iocb->ki_flags & IOCB_DONTCACHE) {
+		struct address_space *mapping = iocb->ki_filp->f_mapping;
+
+		filemap_fdatawrite_range_kick(mapping, iocb->ki_pos,
+					      iocb->ki_pos + count);
 	}
 
 	return count;