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From:   Martin Steigerwald <martin@lichtvoll.de>
To:     Miklos Szeredi <miklos@szeredi.hu>, NeilBrown <neilb@suse.de>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>, linux-fsdevel@vger.kernel.org,
        Linux NFS list <linux-nfs@vger.kernel.org>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: A Third perspective on BTRFS nfsd subvol dev/inode number issues.
Date:   Mon, 02 Aug 2021 09:15:09 +0200
Message-ID: <3318968.VgehHcluNF@ananda>
In-Reply-To: <162787790940.32159.14588617595952736785@noble.neil.brown.name>
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Hi Neil!

Wow, this is a bit overwhelming for me. However, I got a very specific 
question for userspace developers in order to probably provide valuable 
input to the KDE Baloo desktop search developers:

NeilBrown - 02.08.21, 06:18:29 CEST:
> The "obvious" choice for a replacement is the file handle provided by
> name_to_handle_at() (falling back to st_ino if name_to_handle_at isn't
> supported by the filesystem).  This returns an extensible opaque
> byte-array.  It is *already* more reliable than st_ino.  Comparing
> st_ino is only a reliable way to check if two files are the same if
> you have both of them open.  If you don't, then one of the files
> might have been deleted and the inode number reused for the other.  A
> filehandle contains a generation number which protects against this.
> 
> So I think we need to strongly encourage user-space to start using
> name_to_handle_at() whenever there is a need to test if two things are
> the same.

How could that work for Baloo's use case to see whether a file it 
encounters is already in its database or whether it is a new file.

Would Baloo compare the whole file handle or just certain fields or make a 
hash of the filehandle or what ever? Could you, in pseudo code or 
something, describe the approach you'd suggest. I'd then share it on:

Bug 438434 - Baloo appears to be indexing twice the number of files than 
are actually in my home directory

https://bugs.kde.org/438434

Best,
-- 
Martin