linux-kernel - Re: Read-only `slaves` with shared subtrees?

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Message-ID: <CAOEu9U5zUc2K_BE5xUW42vgVQuSOyDT74sW66Ki_APhVr2pyBA@mail.gmail.com>
Date:   Tue, 19 Sep 2017 16:18:02 -0700
From:   Dawid Ciezarkiewicz <dawid.ciezarkiewicz@...rik.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ram Pai <linuxram@...ibm.com>,
        Dawid Ciezarkiewicz <dawid.ciezarkiewicz@...rik.com>
Subject: Re: Read-only `slaves` with shared subtrees?

On Mon, Sep 18, 2017 at 1:47 PM, Ram Pai <linuxram@...ibm.com> wrote:
> It is possible to make a slave mount readonly, by  remounting it with
> 'ro' flags.
>
> something like
>
> mount -o bind,remount,ro <slave-mount-dir>
>
> Any mount-propagation events reaching a read-only-slave does
> inherit the slave attribute. However it does not inherit the
> read-only attribute.

I did try manually remounting, and it worked for me. If this could be
done atomically
 (which I assume can't be, in the userspace) it could even be a workaround.

> Should it inherit? or should it not? -- that has not been thought
> off AFAICT. it think we should let it inherit.

It makes sense, and it would work in my use-case. I wonder
if that would break any existing expectations though.

I could at least test such a patch, it seems like a tiny change.
Should I give it a try and submit a patch? If you could PM me any pointers
it could help a lot since I'm not familiar with FS internals. So far I got here:

http://elixir.free-electrons.com/linux/latest/source/fs/pnode.c#L294

Regards,
Dawid Ciezarkiewicz