linux-kernel - Re: [PATCH 0/2] overlayfs: C/R enhancements

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Date:   Thu, 4 Jun 2020 21:04:58 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Alexander Mikhalitsyn <alexander.mikhalitsyn@...tuozzo.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Andrei Vagin <avagin@...nvz.org>, ptikhomirov@...tuozzo.com,
        khorenko@...tuozzo.com, vvs@...tuozzo.com, ktkhai@...tuozzo.com,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] overlayfs: C/R enhancements

On Thu, Jun 4, 2020 at 7:13 PM Alexander Mikhalitsyn
<alexander.mikhalitsyn@...tuozzo.com> wrote:
>
> This patchset aimed to make C/R of overlayfs mounts with CRIU possible.
> We introduce two new overlayfs module options -- dyn_path_opts and
> mnt_id_path_opts. If enabled this options allows to see real *full* paths
> in lowerdir, workdir, upperdir options, and also mnt_ids for corresponding
> paths.
>
> This changes should not break anything because for showing mnt_ids we simply
> introduce new show-time mount options. And for paths we simply *always*
> provide *full paths* instead of relative path on mountinfo.
>
> BEFORE
>
> overlay on /var/lib/docker/overlay2/XYZ/merged type overlay (rw,relatime,
> lowerdir=/var/lib/docker/overlay2/XYZ-init/diff:/var/lib/docker/overlay2/
> ABC/diff,upperdir=/var/lib/docker/overlay2/XYZ/diff,workdir=/var/lib/docker
> /overlay2/XYZ/work)
> none on /sys type sysfs (rw,relatime)
>
> AFTER
>
> overlay on /var/lib/docker/overlay2/XYZ/merged type overlay (rw,relatime,
> lowerdir=/var/lib/docker/overlay2/XYZ-init/diff:/var/lib/docker/overlay2/
> ABC/diff,upperdir=/var/lib/docker/overlay2/XYZ/diff,workdir=/var/lib/docker
> /overlay2/XYZ/work,lowerdir_mnt_id=175:175,upperdir_mnt_id=175)
> none on /sys type sysfs (rw,relatime)
>

But overlayfs won't accept these "output only" options as input args,
which is a problem.

Wouldn't it be better for C/R to implement mount options
that overlayfs can parse and pass it mntid and fhandle instead
of paths?
I believe C/R is using a similar method to export inotify/fanotify
marks.

FWIW overlayfs already has utilities that encode filehandle to
text and back, see  ovl_get_index_name().

Thanks,
Amir.