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: <20180831095300.GF24124@hirez.programming.kicks-ass.net>
Date:   Fri, 31 Aug 2018 11:53:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     yu-cheng.yu@...el.com, Dave Hansen <dave.hansen@...ux.intel.com>,
        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 Thu, Aug 30, 2018 at 11:47:16PM +0200, Jann Horn wrote:
>         do {
>                 pte = pte_wrprotect(pte);
>                 /* note: relies on _PAGE_DIRTY_HW < _PAGE_DIRTY_SW */
>                 /* dirty direct bit-twiddling; you can probably write
> this in a nicer way */
>                 pte.pte |= (pte.pte & _PAGE_DIRTY_HW) >>
> _PAGE_BIT_DIRTY_HW << _PAGE_BIT_DIRTY_SW;
>                 pte.pte &= ~_PAGE_DIRTY_HW;
>                 pte = cmpxchg(ptep, pte, new_pte);
>         } while (pte != new_pte);

Please use the form:

	pte_t new_pte, pte = READ_ONCE(*ptep);
	do {
		new_pte = /* ... */;
	} while (!try_cmpxchg(ptep, &pte, new_pte);

Also, this will fail to build on i386-PAE, but I suspect this code will
be under some CONFIG option specific to x86_64 anyway.