linux-kernel - Re: [PATCH 4/4] x86: use pte_none() to test for empty PTE

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Date:	Wed, 13 Jul 2016 17:18:20 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	bp@...en8.de, ak@...ux.intel.com, dave.hansen@...el.com,
	dave.hansen@...ux.intel.com, Julia Lawall <Julia.Lawall@...6.fr>
Subject: Re: [PATCH 4/4] x86: use pte_none() to test for empty PTE

[CCing Julia]

On Thu 07-07-16 17:19:15, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> The page table manipulation code seems to have grown a couple of
> sites that are looking for empty PTEs.  Just in case one of these
> entries got a stray bit set, use pte_none() instead of checking
> for a zero pte_val().

This looks like something that coccinelle could help with and automate.
Especially when the patch seems interesting for applying to older kernel
code streams.

Julia would it be hard to generate a metapatch which would check the
{pte,pmd}_val() usage in conditions and replace them with {pte,pmd}_none
equivalents?

> The use pte_same() makes me a bit nervous.  If we were doing a
> pte_same() check against two cleared entries and one of them had
> a stray bit set, it might fail the pte_same() check.  But, I
> don't think we ever _do_ pte_same() for cleared entries.  It is
> almost entirely used for checking for races in fault-in paths.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>

Other than that looks good to me. Feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
> 
>  b/arch/x86/mm/init_64.c    |   12 ++++++------
>  b/arch/x86/mm/pageattr.c   |    2 +-
>  b/arch/x86/mm/pgtable_32.c |    2 +-
>  3 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff -puN arch/x86/mm/init_64.c~knl-strays-50-pte_val-cleanups arch/x86/mm/init_64.c
> --- a/arch/x86/mm/init_64.c~knl-strays-50-pte_val-cleanups	2016-07-07 17:17:44.942808493 -0700
> +++ b/arch/x86/mm/init_64.c	2016-07-07 17:17:44.949808807 -0700
> @@ -354,7 +354,7 @@ phys_pte_init(pte_t *pte_page, unsigned
>  		 * pagetable pages as RO. So assume someone who pre-setup
>  		 * these mappings are more intelligent.
>  		 */
> -		if (pte_val(*pte)) {
> +		if (!pte_none(*pte)) {
>  			if (!after_bootmem)
>  				pages++;
>  			continue;
> @@ -396,7 +396,7 @@ phys_pmd_init(pmd_t *pmd_page, unsigned
>  			continue;
>  		}
>  
> -		if (pmd_val(*pmd)) {
> +		if (!pmd_none(*pmd)) {
>  			if (!pmd_large(*pmd)) {
>  				spin_lock(&init_mm.page_table_lock);
>  				pte = (pte_t *)pmd_page_vaddr(*pmd);
> @@ -470,7 +470,7 @@ phys_pud_init(pud_t *pud_page, unsigned
>  			continue;
>  		}
>  
> -		if (pud_val(*pud)) {
> +		if (!pud_none(*pud)) {
>  			if (!pud_large(*pud)) {
>  				pmd = pmd_offset(pud, 0);
>  				last_map_addr = phys_pmd_init(pmd, addr, end,
> @@ -673,7 +673,7 @@ static void __meminit free_pte_table(pte
>  
>  	for (i = 0; i < PTRS_PER_PTE; i++) {
>  		pte = pte_start + i;
> -		if (pte_val(*pte))
> +		if (!pte_none(*pte))
>  			return;
>  	}
>  
> @@ -691,7 +691,7 @@ static void __meminit free_pmd_table(pmd
>  
>  	for (i = 0; i < PTRS_PER_PMD; i++) {
>  		pmd = pmd_start + i;
> -		if (pmd_val(*pmd))
> +		if (!pmd_none(*pmd))
>  			return;
>  	}
>  
> @@ -710,7 +710,7 @@ static bool __meminit free_pud_table(pud
>  
>  	for (i = 0; i < PTRS_PER_PUD; i++) {
>  		pud = pud_start + i;
> -		if (pud_val(*pud))
> +		if (!pud_none(*pud))
>  			return false;
>  	}
>  
> diff -puN arch/x86/mm/pageattr.c~knl-strays-50-pte_val-cleanups arch/x86/mm/pageattr.c
> --- a/arch/x86/mm/pageattr.c~knl-strays-50-pte_val-cleanups	2016-07-07 17:17:44.944808582 -0700
> +++ b/arch/x86/mm/pageattr.c	2016-07-07 17:17:44.950808852 -0700
> @@ -1185,7 +1185,7 @@ repeat:
>  		return __cpa_process_fault(cpa, address, primary);
>  
>  	old_pte = *kpte;
> -	if (!pte_val(old_pte))
> +	if (pte_none(old_pte))
>  		return __cpa_process_fault(cpa, address, primary);
>  
>  	if (level == PG_LEVEL_4K) {
> diff -puN arch/x86/mm/pgtable_32.c~knl-strays-50-pte_val-cleanups arch/x86/mm/pgtable_32.c
> --- a/arch/x86/mm/pgtable_32.c~knl-strays-50-pte_val-cleanups	2016-07-07 17:17:44.946808672 -0700
> +++ b/arch/x86/mm/pgtable_32.c	2016-07-07 17:17:44.950808852 -0700
> @@ -47,7 +47,7 @@ void set_pte_vaddr(unsigned long vaddr,
>  		return;
>  	}
>  	pte = pte_offset_kernel(pmd, vaddr);
> -	if (pte_val(pteval))
> +	if (!pte_none(pteval))
>  		set_pte_at(&init_mm, vaddr, pte, pteval);
>  	else
>  		pte_clear(&init_mm, vaddr, pte);
> _

-- 
Michal Hocko
SUSE Labs