linux-kernel - Re: [PATCH v5 1/2] move_mount: allow to add a mount into an existing group

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Date:   Sat, 17 Jul 2021 20:33:06 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        linux-fsdevel@...r.kernel.org,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Mattias Nissler <mnissler@...omium.org>,
        Aleksa Sarai <cyphar@...har.com>, linux-api@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/2] move_mount: allow to add a mount into an existing
 group

On Thu, Jul 15, 2021 at 01:07:13PM +0300, Pavel Tikhomirov wrote:
> Previously a sharing group (shared and master ids pair) can be only
> inherited when mount is created via bindmount. This patch adds an
> ability to add an existing private mount into an existing sharing group.
> 
> With this functionality one can first create the desired mount tree from
> only private mounts (without the need to care about undesired mount
> propagation or mount creation order implied by sharing group
> dependencies), and next then setup any desired mount sharing between
> those mounts in tree as needed.
> 
> This allows CRIU to restore any set of mount namespaces, mount trees and
> sharing group trees for a container.
> 
> We have many issues with restoring mounts in CRIU related to sharing
> groups and propagation:
> - reverse sharing groups vs mount tree order requires complex mounts
>   reordering which mostly implies also using some temporary mounts
> (please see https://lkml.org/lkml/2021/3/23/569 for more info)
> 
> - mount() syscall creates tons of mounts due to propagation
> - mount re-parenting due to propagation
> - "Mount Trap" due to propagation
> - "Non Uniform" propagation, meaning that with different tricks with
>   mount order and temporary children-"lock" mounts one can create mount
>   trees which can't be restored without those tricks
> (see https://www.linuxplumbersconf.org/event/7/contributions/640/)
> 
> With this new functionality we can resolve all the problems with
> propagation at once.
> 
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Christian Brauner <christian.brauner@...ntu.com>
> Cc: Mattias Nissler <mnissler@...omium.org>
> Cc: Aleksa Sarai <cyphar@...har.com>
> Cc: Andrei Vagin <avagin@...il.com>

Looks good to me.

Co-developed-by: Andrei Vagin <avagin@...il.com>
Signed-off-by: Andrei Vagin <avagin@...il.com>

> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-api@...r.kernel.org
> Cc: lkml <linux-kernel@...r.kernel.org>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
>

Thanks,
Andrei