On Wed, Jan 20, 2021 at 7:57 AM Amir Goldstein <amir73il@xxxxxxxxx> 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
