f2fs: don't need to wait for node writes for atomic write

We have a node chain to serialize node block writes, so if any IOs for
node block writes are reordered, we'll get broken node chain. IOWs,
roll-forward recovery will see all or none node blocks given fsync
mark.

E.g.,
Node chain consists of:
 N1 -> N2 -> N3 -> NFSYNC -> N1' -> N2' -> N'FSYNC

Reordered to:
1) N1 -> N2 -> N3 -> N2' -> NFSYNC -> N'FSYNC -> power-cut
2) N1 -> N2 -> N3 -> N1' -> NFSYNC -> power-cut
3) N1 -> N2 -> NFSYNC -> N1' -> N'FSYNC -> N3 -> power-cut
4) N1 -> NFSYNC -> N1' -> N2' -> N'FSYNC -> N3 -> power-cut

Roll-forward recovery can proceed to:
1) N1 -> N2 -> N3 -> NFSYNC -> X
2) N1 -> N2 -> N3 -> NFSYNC -> N1' -> X
3) N1 -> N2 -> N3 -> FSYNC -> N1' -> X
4) N1 -> X
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -274,9 +274,19 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
 		goto sync_nodes;
 	}
 
+	/*
+	 * If it's atomic_write, it's just fine to keep write ordering. So
+	 * here we don't need to wait for node write completion, since we use
+	 * node chain which serializes node blocks. If one of node writes are
+	 * reordered, we can see simply broken chain, resulting in stopping
+	 * roll-forward recovery. It means we'll recover all or none node blocks
+	 * given fsync mark.
+	 */
+	if (!atomic) {
 		ret = wait_on_node_pages_writeback(sbi, ino);
 		if (ret)
 			goto out;
+	}
 
 	/* once recovery info is written, don't need to tack this */
 	remove_ino_entry(sbi, ino, APPEND_INO);