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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with
 overlapped bitmaps
Date:   Thu, 8 Oct 2020 13:25:57 -0600
In-Reply-To: <20201008191231.GA44285@localhost>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Jan Kara <jack@suse.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>
To:     Josh Triplett <josh@joshtriplett.org>
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On Oct 8, 2020, at 1:12 PM, Josh Triplett <josh@joshtriplett.org> wrote:
> 
> On Wed, Oct 07, 2020 at 08:57:12PM -0600, Andreas Dilger wrote:
>> On Oct 7, 2020, at 2:14 PM, Josh Triplett <josh@joshtriplett.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
> 
> My apologies, I thought I responded to that. It looks promising to me,
> though I wouldn't have the bandwidth to take it to completion anytime
> soon.

NP, I don't have bandwidth to work on it right now either.

>> 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.
> 
> I'd love to see that happen. At the time, it seemed like too large of a
> change to block on, which is why I ended up deciding to switch to
> 256-byte inodes.

Does that mean you are using inline_data with 256-byte inodes?  That would
also be good to know, since there haven't been any well-known users of
this feature so far (AFAIK).  Since you are using this in a read-only
manner, you won't hit the one know issue when an inline_data inode is
extended to use an external block that may temporarily leave the inode
in an inconsistent state.

Cheers, Andreas


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