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

From: Amir Goldstein <amir73il@gmail.com>
To: Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.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: Fri, 5 Jun 2020 05:35:33 +0300	[thread overview]
Message-ID: <CAOQ4uxisdLt-0eT1R=V1ihagMoNfjiTrUdcdF2yDgD4O94Zjcw@mail.gmail.com> (raw)
In-Reply-To: <AM6PR08MB36394A00DC129791CC89296AE8890@AM6PR08MB3639.eurprd08.prod.outlook.com>

On Fri, Jun 5, 2020 at 12:34 AM Alexander Mikhalitsyn
<alexander.mikhalitsyn@virtuozzo.com> wrote:
>
> 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()?
>

That would solve this small 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?
>
> 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.

As you already know from inotify/fanotify C/R fhandle is timeless, so
there would be no distinction between mount time and dump time.
About mnt_id, your patches will cause the original mount-time mounts to be busy.
That is a problem as well.

I think you should describe the use case is more details.
Is your goal to C/R any overlayfs mount that the process has open
files on? visible to process?
For NFS export, we use the persistent descriptor {uuid;fhandle}
(a.k.a. struct ovl_fh) to encode
an underlying layer object.

CRIU can look for an existing mount to a filesystem with uuid as restore stage
(or even mount this filesystem) and use open_by_handle_at() to open a
path to layer.
After mounting overlay, that mount to underlying fs can even be discarded.

And if this works for you, you don't have to export the layers ovl_fh in
/proc/mounts, you can export them in numerous other ways.
One way from the top of my head, getxattr on overlay root dir.
"trusted.overlay" xattr is anyway a reserved prefix, so "trusted.overlay.layers"
for example could work.

Thanks,
Amir.