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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
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jann Horn <jannh@google.com>
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Cc:     the arch/x86 maintainers <x86@kernel.org>,
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From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <f5a36e32-7c5f-91fe-9e98-fb44867fda11@linux.intel.com>
Date:   Fri, 31 Aug 2018 07:47:29 -0700
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On 08/31/2018 07:33 AM, Yu-cheng Yu wrote:
> Please use the form:
> 
> 	pte_t new_pte, pte = READ_ONCE(*ptep);
> 	do {
> 		new_pte = /* ... */;
> 	} while (!try_cmpxchg(ptep, &pte, new_pte);

It's probably also worth doing some testing to see if you can detect the
cost of the cmpxchg.  It's definitely more than the old code.

A loop that does mprotect(PROT_READ) followed by
mprotect(PROT_READ|PROT_WRITE) should do it.