linux-kernel - Re: [PATCH 3/3] ovl: redirect on rename-dir

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Date:   Sun, 6 Nov 2016 22:14:47 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Raphael Hertzog <hertzog@...ian.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        Guillem Jover <guillem@...ian.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] ovl: redirect on rename-dir

On Wed, Oct 26, 2016 at 2:12 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Tue, Oct 25, 2016 at 1:57 PM, Raphael Hertzog <hertzog@...ian.org> wrote:
>
>> Do you plan to make it the default in the future when it has been
>> available for a while?
>>
>> Barring any regression introduced by your patch, it seems that the feature
>> is best available by default since it allows legitimate operations to
>> succeed that are otherwise refused. I understand that it makes it
>> impossible to mount the overlay filesystem with an older kernel but is
>> that problem more widespread than the one we're fixing here?  On my side,
>> overlayfs is only used in scenarios where the kernel is always the same
>> (or newer compared to what created the initial filesystem).
>
> I think it would be safe to make it the default if upperdir is empty.
> Nonempty implies that it was created with old kernel (or it was
> crafted by hand).   But there should be a way to explicitly turn it
> off;  either because of the need for backward compatibility or because
> the old format is simply easier to work with for humans.
>
> How about:
>
> - If upper is nonempty, then leave redirect feature alone except when
> mount option "-oredirect=on" is used to force enabling it.
> - If upper is empty, then enable redirect feature except when mount
> option "-oredirect=off" is used to force disabling it.

I don't like this empty-nonempty upper logic.

I think this feature should be off by default and be enabled
explicitly in mount option.
Available features could be listed in sysfs /sys/fs/overlay/..., like ext4 does.

Overlayfs mounting anyway is complicated operation.
User must know a lot about it and provide persistent state for each mount:
list layers in correct order, work and uppder directory on the same disk, etc.
Enabled features is a part of this state.

Probably this could be solved in userspace tool "mount.overlay" - it could load
features and layers from config file or xattr and set required mount
options automatically.

--
Konstantin