On Wed, Jan 20, 2021 at 7:57 AM Amir Goldstein [off-list ref] wrote:

And as long as I am ranting, I'd like to point out that it is a shame
that whiteout
was not implemented as a special (constant) inode whose nlink is irrelevant
(or a special dirent with d_ino 0 and d_type DT_WHT for that matter).
It would have been a rather small RO_COMPAT on-disk change for ext4.
It could also be implemented in slightly more backward compat manner by
maintaining a valid nlink and postpone setting the RO_COMPAT flag until
EXT4_LINK_MAX is reached.

As things stand now, overlayfs makes an effort to maintain a singleton
hardlinked whiteout inode, without being able to use it with RENAME_WHITEOUT
and filesystems have to take special care to journal the metadata of all
individual whiteout inodes, without any added value to the only user
(overlayfs).

But I guess that train has left the station long ago...

Not so, I believe.  Kernel internal interfaces are easy to change, and
adding support for DT_WHT to overlayfs would mostly be a trivial
undertaking.

The big issue (as always) is userspace API's and not introducing
DT_WHT there was a very deliberate choice.  Adding a translation layer
from an internal whiteout representation to the userspace API also
does not seem to be a very complex problem, but I haven't looked into
that deeply.

So AFAICS there's really nothing preventing the addition of whiteout
objects to filesystems, other than developer dedication.

Thanks,
Miklos