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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 20 Jan 2021 09:42:16 +0100
Message-ID: <CAJfpegsVYF2wCiMKfRUzS_MpH9UKPh8g7ucG6w9uOcQodAzRAQ@mail.gmail.com>
Subject: Re: [PATCH v3] ext4: fix bug for rename with RENAME_WHITEOUT
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     "Theodore Ts'o" <tytso@mit.edu>,
        harshad shirwadkar <harshadshirwadkar@gmail.com>,
        yangerkun <yangerkun@huawei.com>,
        Ext4 <linux-ext4@vger.kernel.org>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.cz>, "zhangyi (F)" <yi.zhang@huawei.com>,
        lihaotian <lihaotian9@huawei.com>, lutianxiong@huawei.com,
        linfeilong <linfeilong@huawei.com>,
        fstests <fstests@vger.kernel.org>,
        Vijaychidambaram Velayudhan Pillai <vijay@cs.utexas.edu>
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On Wed, Jan 20, 2021 at 7:57 AM Amir Goldstein <amir73il@gmail.com> 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