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Date:   Mon, 5 Oct 2020 23:18:34 -0400
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-ext4@vger.kernel.org
Subject: Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with
 overlapped bitmaps
Message-ID: <20201006031834.GA5797@mit.edu>
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On Mon, Oct 05, 2020 at 07:51:10PM -0700, Darrick J. Wong wrote:
> > > Could /somebody/ please document the ondisk format changes that are
> > > associated with this feature?
> > 
> > I pretty much had to sort it out by looking at a combination of
> > e2fsprogs and the kernel, and a lot of experimentation, until I ended up
> > with something that the kernel was completely happy with without a
> > single complaint.
> > 
> > I'd be happy to write up a summary of the format.
> 
> Seems like a good idea, particularly since you're asking for a format
> change that requires kernel support and the ondisk format documentation
> lives under Documentation/.  That said...

> > If you set up the rest of the metadata consistently with it (for
> > instance, 0 free blocks and 0 free inodes), you'll only get a single
> > complaint, from the e2fsck equivalent of block_validity. See
> > https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=956509 for details on
> > that;
> 
> ...Ted shot down this whole thing six months ago.
> 
> The Debian bug database is /not/ the designated forum to discuss changes
> to the ondisk format; linux-ext4 is.

What Josh is proposing I'm pretty sure would also break "e2fsck -E
unshare_blocks", so that's another reason not to accept this as a
valid format change.

As far as I'm concerned, contrib/e2fsdroid is the canonical definition
of how to create valid file systems with shared_blocks.

					- Ted