linux-ext4 - Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps

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Date:   Wed, 7 Oct 2020 20:57:12 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with
 overlapped bitmaps

On Oct 7, 2020, at 2:14 PM, Josh Triplett <josh@...htriplett.org> wrote:
> If those aren't the right way to express that, I could potentially
> adapt. I had a similar such conversation on linux-ext4 already (about
> inline data with 128-bit inodes), which led to me choosing to abandon
> 128-byte inodes rather than try to get ext4 to support what I wanted
> with them, because I didn't want to be disruptive to ext4 for a niche
> use case. In the particular case that motivated this thread, what I was
> doing already worked in previous kernels, and it seemed reasonable to
> ask for it to continue to work in new kernels, while preserving the
> newly added checks in the new kernels.

This was discussed in the "Inline data with 128-byte inodes?" thread
back in May.  While Jan was not necessarily in favour of this, I was
actually OK with improving the ext4 code to handle this case better,
since it would (at minimum) clean up ext4 to make a clear separation
of how it is detecting data in the i_block[] array and the system.data
xattr, and I don't think it added any complexity to the code.

I even posted a WIP patch to that effect, but didn't get a response back:
https://marc.info/?l=linux-ext4&m=158863275019187

I *do* think that inline_data is an under-appreciated feature that I
would be happy to see some improvements with.  I don't think that small
files are a niche use case, and if we can clean up the inline_data code
to work with 128-byte inodes I'm not against that, even though I'm not
going to use that combination of features myself.

Cheers, Andreas

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