linux-kernel - Re: [PATCHv5 28/37] x86/vdso: Enable static branches for the timens vdso

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Date:   Wed, 31 Jul 2019 22:21:33 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        criu@...nvz.org, Linux API <linux-api@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCHv5 28/37] x86/vdso: Enable static branches for the timens vdso

On Mon, Jul 29, 2019 at 2:58 PM Dmitry Safonov <dima@...sta.com> wrote:
>
> From: Andrei Vagin <avagin@...il.com>
>
> As it has been discussed on timens RFC, adding a new conditional branch
> `if (inside_time_ns)` on VDSO for all processes is undesirable.
>
> Addressing those problems, there are two versions of VDSO's .so:
> for host tasks (without any penalty) and for processes inside of time
> namespace with clk_to_ns() that subtracts offsets from host's time.
>
> The timens code in vdso looks like this:
>
> if (timens_static_branch_unlikely()) {
>        clk_to_ns(clk, ts);
> }

I'm confused.  Now we effectively have *three* versions: the vDSO
without timens, and vDSO with timens but with it switched off, and the
vDSO with timens on.  This seems like too much.

What you need is, IMO, a static-branch-ish thing that is per mm.  This
has a fundamental problem that the vDSO can be modified using
FOLL_FORCE.  Perhaps any CoW of the vDSO should implicitly switch the
static branch on, which at least gives some degree of correctness even
if it's a bit surprising.

--Andy