linux-ext4 - A new special orphan inode 12 in ext4 only?

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Date:   Wed, 22 Mar 2023 17:09:28 +0800
From:   Qu Wenruo <quwenruo.btrfs@....com>
To:     Ext4 <linux-ext4@...r.kernel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>
Subject: A new special orphan inode 12 in ext4 only?

Hi,

Recently I observed newer mkfs.ext4 seems to create a new orphan inode 
12, with some file extents.

Which seems to have no direct parent directory, thus tools like 
btrfs-convert would also follow the ext4 inodes by creating an orphan 
inode too.

On the other hand, if I go mkfs.ext3, the mysterious inode seems to be gone.

Is this inode 12 a known special inode?
If so, how can we avoid such special inode?
(s_special_ino is still 11, thus checking against that value doesn't 
seem to help).

Some details of btrfs-convert:

It goes with ext2fs_open_inode_scan() to iterate all inodes of an ext4.

And if we hit an directory inode, we iterate the directory by using 
ext2fs_dir_iterate2() to insert the dir entries between parent and child 
inodes.

So if we hit an inode without any parent dir, an equivalent btrfs inode 
would still be created, but btrfs-check would complain about such orphan 
inode.

Thanks,
Qu