Re: [PATCH V3 1/3] arm64/mm: Drop __HAVE_ARCH_HUGE_PTEP_GET

[Mike Kravetz <mike.kravetz@oracle.com>]

On 5/7/20 8:07 PM, Anshuman Khandual wrote:
> Platform specific huge_ptep_get() is required only when fetching the huge
> PTE involves more than just dereferencing the page table pointer. This is
> not the case on arm64 platform. Hence huge_ptep_pte() can be dropped along
> with it's __HAVE_ARCH_HUGE_PTEP_GET subscription. Before that, it updates
> the generic huge_ptep_get() with READ_ONCE() which will prevent known page
> table issues with THP on arm64.
> 
> https://lore.kernel.org/r/1506527369-19535-1-git-send-email-will.deacon@arm.com/
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  arch/arm64/include/asm/hugetlb.h | 6 ------
>  include/asm-generic/hugetlb.h    | 2 +-
>  2 files changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h
> index 2eb6c234d594..b88878ddc88b 100644
> --- a/arch/arm64/include/asm/hugetlb.h
> +++ b/arch/arm64/include/asm/hugetlb.h
> @@ -17,12 +17,6 @@
>  extern bool arch_hugetlb_migration_supported(struct hstate *h);
>  #endif
>  
> -#define __HAVE_ARCH_HUGE_PTEP_GET
> -static inline pte_t huge_ptep_get(pte_t *ptep)
> -{
> -	return READ_ONCE(*ptep);
> -}
> -
>  static inline int is_hugepage_only_range(struct mm_struct *mm,
>  					 unsigned long addr, unsigned long len)
>  {
> diff --git a/include/asm-generic/hugetlb.h b/include/asm-generic/hugetlb.h
> index 822f433ac95c..40f85decc2ee 100644
> --- a/include/asm-generic/hugetlb.h
> +++ b/include/asm-generic/hugetlb.h
> @@ -122,7 +122,7 @@ static inline int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>  #ifndef __HAVE_ARCH_HUGE_PTEP_GET
>  static inline pte_t huge_ptep_get(pte_t *ptep)
>  {
> -	return *ptep;
> +	return READ_ONCE(*ptep);
>  }
>  #endif

I know you made this change in response to Will's comment.  And, since
changes were made to consistently use READ_ONCE in arm64 code, it makes
sense for that architecture.

However, with this change to generic code, you introduce READ_ONCE to
other architectures where it was not used before.  Could this possibly
introduce inconsistencies in their use of READ_ONCE?  To be honest, I
am not very good at identifying any possible issues this could cause.
However, it does seem possible.

Will was nervous about dropping this from arm64.  I'm just a little nervous
about adding it to other architectures.
-- 
Mike Kravetz