linux-ext4 - Re: [PATCH -RFC] ext4: add feature file to advertise that ext4 supports idmapped mounts

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Date:   Thu, 15 Apr 2021 09:49:21 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Theodore Ts'o <tytso@....edu>, Eryu Guan <guan@...u.me>,
        Christian Brauner <brauner@...nel.org>,
        fstests@...r.kernel.org, linux-ext4@...r.kernel.org,
        "Darrick J . Wong" <djwong@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [PATCH -RFC] ext4: add feature file to advertise that ext4
 supports idmapped mounts

On Thu, Apr 15, 2021 at 07:54:08AM +0200, Christoph Hellwig wrote:
> On Wed, Apr 14, 2021 at 04:47:10PM -0400, Theodore Ts'o wrote:
> > On Mon, Apr 12, 2021 at 06:41:20PM -0400, Theodore Ts'o wrote:
> > > In the ideal world, if the kernel wasn't compiled with the necessary
> > > CONFIG options enabled, it's desirable of the test can detect that
> > > fact and skip running the test instead failing and forcing the person
> > > running the test to try to figure out whether this is a legitmate file
> > > system bug or a just a test setup bug.
> > 
> > So it would make it easier for me to manage running xfstests on ext4
> > if I had added something like this to ext4 and sent it to Linus before
> > v5.12 is released.  What do folks think?
> 
> Completely stupid.  This is a VFS-level feature and we need to have
> proper VFS-level testing (which we have through creating a mount
> with the option), not fs-specific band aids.

Harsh words. :)
Christoph's right though I think for the xfstests we don't need it and
we're covered with what we have in the version I sent out last Sunday.

However, as a general way of advertising to users that ext4 supports
idmapped mounts I don't necessarily see a problem with that. It doesn't
have implications for other filesystems and ext4 already advertises
other features in a similar way.

Christian