linux-kernel - Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY

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Message-ID: <6d31bd30-6d5b-bbde-1e97-1d8255eff76d@linux.intel.com>
Date:   Fri, 31 Aug 2018 08:58:39 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com,
        Jonathan Corbet <corbet@....net>, keescook@...omiun.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW

On 08/31/2018 08:48 AM, Yu-cheng Yu wrote:
> To trigger a race in ptep_set_wrprotect(), we need to fork from one of
> three pthread siblings.
> 
> Or do we measure only how much this affects fork?
> If there is no racing, the effect should be minimal.

We don't need a race.

I think the cmpxchg will be slower, even without a race, than the code
that was there before.  The cmpxchg is a simple, straightforward
solution, but we're putting it in place of a plain memory write, which
is suboptimal.

But, before I nitpick the performance, I wanted to see if we could even
detect a delta.