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 09:55:37 +0200
From:   Christoph Hellwig <hch@....de>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Christoph Hellwig <hch@....de>, 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 09:49:21AM +0200, Christian Brauner wrote:
> 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.

I still think that is a very bad idea.  idmapped mounts are foremost
a VFS feature, with just a tiny bit of glue in every file system that
does not affect the on-disk format.  So any discovery for it needs
to be using VFS infrastructure.  IMHO just trying to mount is the
proper way to do it, but if people want some other form of discovery
it needs to be something in generic code.