[PATCH] arm: mm: Fix definition of pmd_mknotpresent

[Kirill A. Shutemov]

On Wed, Mar 02, 2016 at 01:03:00AM +0000, Steve Capper wrote:
> Currently pmd_mknotpresent will use a zero entry to respresent an
> invalidated pmd.
> 
> Unfortunately this definition clashes with pmd_none, thus it is
> possible for a race condition to occur if zap_pmd_range sees pmd_none
> whilst __split_huge_pmd_locked is running too with pmdp_invalidate
> just called.
> 
> This patch fixes the race condition by modifying pmd_mknotpresent to
> create non-zero faulting entries (as is done in other architectures),
> removing the ambiguity with pmd_none.
> 
> Fixes: 8d9625070073 ("ARM: mm: Transparent huge page support for LPAE systems.")
> Reported-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
> Signed-off-by: Steve Capper <steve.capper at arm.com>
> Acked-by: Will Deacon <will.deacon at arm.com>
> 
> ---
> 
> This patch addresses an issue discussed at:
> http://lkml.iu.edu/hypermail/linux/kernel/1602.3/01846.html
> 
> ---
>  arch/arm/include/asm/pgtable-3level.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/include/asm/pgtable-3level.h b/arch/arm/include/asm/pgtable-3level.h
> index dc46398..451b6e7 100644
> --- a/arch/arm/include/asm/pgtable-3level.h
> +++ b/arch/arm/include/asm/pgtable-3level.h
> @@ -249,10 +249,10 @@ PMD_BIT_FUNC(mkyoung,   |= PMD_SECT_AF);
>  #define pfn_pmd(pfn,prot)	(__pmd(((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot)))
>  #define mk_pmd(page,prot)	pfn_pmd(page_to_pfn(page),prot)
>  
> -/* represent a notpresent pmd by zero, this is used by pmdp_invalidate */
> +/* represent a notpresent pmd by faulting entry, this is used by pmdp_invalidate */
>  static inline pmd_t pmd_mknotpresent(pmd_t pmd)
>  {
> -	return __pmd(0);
> +	return __pmd(pmd_val(pmd) & ~PMD_TYPE_MASK);

I'm confused. Wouldn't it still be present?

arch/arm/include/asm/pgtable.h:#define pmd_present(pmd) (pmd_val(pmd))

Shouldn't pmd_present() be re-defined as (pmd_val(pmd) & PMD_TYPE_MASK) or
something?

-- 
 Kirill A. Shutemov