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Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>, Jann Horn <jannh@google.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
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        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Florian Weimer <fweimer@redhat.com>, hjl.tools@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, keescook@chromiun.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com
Date:   Fri, 31 Aug 2018 07:33:52 -0700
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On Fri, 2018-08-31 at 11:53 +0200, Peter Zijlstra wrote:
> 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.

Thanks!  I will work on it.

Yu-cheng