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 10:59:28 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Christoph Hellwig <hch@....de>, 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:
> 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.

Sorry, I had missed your v13 patch set and that it had included tests
for the existence of idmapped.  I had sent out the RFC patch because I
was under the impression that we hadn't made forward progress on
testing for the support idmapped mounts.

If we have a way of doing this, and you're comfortable that it is
reliable (e.g., that a bug might cause the test to be skipped because
it thinks the file system doesn't support idmapped mount, when really
it was caused by a regression), and I'm happy to just rely on the
method you've used in the v13 fstests patch set.

That being said, I still think it would be helpful to have a
VFS-standard way for userspace to be able to test for the presence of
a particular kernel feature/capability without having to do a test
mount, possibly requiring setting up a test user_ns, etc., etc.  But
that isn't as urgent as making sure we can easily the feature without
needing manual customization of the test suite as file systems add
support for the feature, or as the feature gets backported to
enterprise distro kernels.

Cheers,

					- Ted