linux-kernel - Re: Time Namespaces: CLONE

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Date:   Mon, 17 Feb 2020 20:37:55 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Andrei Vagin <avagin@...nvz.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers <containers@...ts.linux-foundation.org>,
        criu@...nvz.org, Linux API <linux-api@...r.kernel.org>,
        x86@...nel.org, Andrei Vagin <avagin@...il.com>
Subject: Re: Time Namespaces: CLONE_NEWTIME and clone3()?

Thomas Gleixner <tglx@...utronix.de> writes:

> Christian Brauner <christian.brauner@...ntu.com> writes:
>> On Mon, Feb 17, 2020 at 10:47:53PM +0100, Michael Kerrisk (man-pages) wrote:
>>> Actually, I think the alternative you propose just here is better. I
>>> imagine there are times when one will want to create multiple
>>> namespaces with a single call to clone3(), including a time namespace.
>>> I think this should be allowed by the API. And, otherwise, clone3()
>>> becomes something of a second-class citizen for creating namespaces.
>>> (I don't really get the "less invasive" argument. Implementing this is
>>> just a piece of kernel to code to make user-space's life a bit simpler
>>> and more consistent.)
>>
>> I don't particularly mind either way. If there's actual users that need
>> to set it at clone3() time then we can extend it. So I'd like to hear
>> what Adrian, Dmitry, and Thomas think since they are well-versed how
>> this will be used in the wild. I'm weary of exposing a whole new uapi
>> struct and extending clone3() without any real use-case but I'm happy to
>> if there is!
>
> I really have no clue. I merily helped getting this in shape without
> creating havoc for timekeeping and VDSO. I have to punt to the container
> wizards.

Short version.  If you are going to do migration of a container with
CRIU you want the time namespace in your container.  Possibly you can
avoid creating the time namespace until restore, but I don't think so.

Without the time namespace you get all kinds of applications that use
monotonic timers that will see their timers be ill behaved (probably
going backwards) over a checkpoint-restart event.

Eric