linux-kernel - Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu

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Message-ID: <CA+55aFwFUeNA6_0a1MOU-7VGzCw5T99cfVCPiyTpZQJdd6z=bw@mail.gmail.com>
Date:   Thu, 12 Apr 2018 13:07:15 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7)

On Thu, Apr 12, 2018 at 12:59 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> What are your concerns about page pinning ?

Pretty much everything.

It's the most complex part by far, and the vmalloc space is a limited
resource on 32-bit architectures.

> Do you have an alternative approach in mind ?

Do everything in user space.

And even if you absolutely want cpu_opv at all, why not do it in the
user space *mapping* without the aliasing into kernel space?

The cpu_opv approach isn't even fast. It's *really* slow if it has to
do VM crap.

The whole rseq thing was billed as "faster than atomics". I
*guarantee* that the cpu_opv's aren't faster than atomics.

                 Linus