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

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Message-ID: <20210415081318.lmvxgztvlynbc4sq@wittgenstein>
Date:   Thu, 15 Apr 2021 10:13:18 +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 09:55:37AM +0200, Christoph Hellwig wrote:
> 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.

I agree with you there.
I think fsinfo() (Arguably a slimmed-down version of it.) would've been
the best place for this.