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

From: Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	Andrey Vagin <avagin@virtuozzo.com>,
	Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
	Konstantin Khorenko <khorenko@virtuozzo.com>,
	Vasiliy Averin <vvs@virtuozzo.com>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/2] overlayfs: C/R enhancements
Date: Thu, 4 Jun 2020 21:34:07 +0000	[thread overview]
Message-ID: <AM6PR08MB36394A00DC129791CC89296AE8890@AM6PR08MB3639.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <CAOQ4uxhGswjxZjc3mN7K99pPrDgMV9_194U46b2MgszZnq1SDw@mail.gmail.com>

Hello,

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

Will it be problematic if we simply ignore "lowerdir_mnt_id" and "upperdir_mnt_id" options in ovl_parse_opt()?

>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?

Problem is that we need to know on C/R "dump stage" which mounts are used on lower layers and upper layer. Most likely I don't understand something but I can't catch how "mount-time" options will help us.

>I believe C/R is using a similar method to export inotify/fanotify
marks.

Yes, we are using fhandles to C/R inotify/fanotify. On "dump" we get fhandle from /proc/<pid>/fdinfo and on "restore" we open fhandle through open_by_handle_at() syscall.

Regards, Alex

________________________________________
From: Amir Goldstein <amir73il@gmail.com>
Sent: Thursday, June 4, 2020 21:04
To: Alexander Mikhalitsyn
Cc: Miklos Szeredi; Andrey Vagin; Pavel Tikhomirov; Konstantin Khorenko; Vasiliy Averin; Kirill Tkhai; overlayfs; linux-kernel
Subject: Re: [PATCH 0/2] overlayfs: C/R enhancements

On Thu, Jun 4, 2020 at 7:13 PM Alexander Mikhalitsyn
<alexander.mikhalitsyn@virtuozzo.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.