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Date:   Thu, 23 Mar 2023 16:54:25 +0800
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Subject: Re: A new special orphan inode 12 in ext4 only?
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To:     Andrei Borzenkov <arvidjaar@gmail.com>,
        Qu Wenruo <quwenruo.btrfs@gmx.com>,
        Ext4 <linux-ext4@vger.kernel.org>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
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 <6cde43c8-3300-9269-7a8a-8ff6e8b1e287@gmail.com>
From:   Qu Wenruo <wqu@suse.com>
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On 2023/3/23 01:27, Andrei Borzenkov wrote:
> On 22.03.2023 12:09, Qu Wenruo wrote:
>> 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?
> 
> This is orphan file. It is normal file; mke2fs creates first normal 
> inode for lost+found (11) and if enabled creates orphan file next which 
> gets next inode number (12). Inode number is recorded in superblock as 
> s_orphan_file_num.
> 
> /*27c*/ __le16  s_encoding;             /* Filename charset encoding */
>          __le16  s_encoding_flags;       /* Filename charset encoding 
> flags */
>          __le32  s_orphan_file_inum;     /* Inode for tracking orphan 
> inodes */

Thanks for the info.

Now btrfs-convert can skip that orphan file if the COMPAT_ORPHAN_FILE 
feature is enabled.

Thanks,
Qu
> 
> 
>> 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
>