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Subject: Re: [PATCH 2/3] x86/mm: introduce __PAGE_KERNEL_GLOBAL
To:     Nadav Amit <nadav.amit@gmail.com>
References: <20180215132053.6C9B48C8@viggo.jf.intel.com>
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Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org, luto@kernel.org,
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From:   Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <562aaaf0-fb8e-1cc8-61eb-1d74b5922714@linux.intel.com>
Date:   Fri, 16 Feb 2018 15:19:00 -0800
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On 02/16/2018 10:25 AM, Nadav Amit wrote:
>> --- a/arch/x86/mm/pageattr.c~kpti-no-global-for-kernel-mappings	2018-02-13 15:17:56.148210060 -0800
>> +++ b/arch/x86/mm/pageattr.c	2018-02-13 15:17:56.153210060 -0800
>> @@ -593,7 +593,8 @@ try_preserve_large_page(pte_t *kpte, uns
>> 	 * different bit positions in the two formats.
>> 	 */
>> 	req_prot = pgprot_4k_2_large(req_prot);
>> -	req_prot = pgprot_set_on_present(req_prot, _PAGE_GLOBAL | _PAGE_PSE);
>> +	req_prot = pgprot_set_on_present(req_prot,
>> +			__PAGE_KERNEL_GLOBAL | _PAGE_PSE);
>> 	req_prot = canon_pgprot(req_prot);
> From these chunks, it seems to me as req_prot will not have the global bit
> on when “nopti” parameter is provided. What am I missing?

BTW, this code is broken.  It's trying to unconditionally set
_PAGE_GLOBAL whenever set do change_page_attr() and friends.  It gets
fixed up by canon_pgprot(), but it's wrong to do in the first place.
I've got a better fix for this coming.