[overlay] what's the purpose for setting RENAME_EXCHANGE when dest is WHITEOUT dev

[王旭 <xuw2015@xxxxxxxxx>]

Hi,
  I read the source code of overlay fs, it's amazing, but a few
confusion for the rename2 function.

  Here is the code:
799     if (overwrite) {

800         if (old_opaque) {
801             if (new->d_inode || !new_opaque) {
802                 /* Whiteout source */
803                 flags |= RENAME_WHITEOUT;
804             } else {
805                 /* Switch whiteouts */
806                 flags |= RENAME_EXCHANGE;
807             }
808         } else if (is_dir && !new->d_inode && new_opaque) {
809             flags |= RENAME_EXCHANGE;
                  ^^^^^^^^^^Is necessary to use RENAME_EXCHANGE? why
not let vfs_rename to remove the new(dest) entry?
810             cleanup_whiteout = true;
811         }
812     }

  Thanks,
  George.Wang
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