Re: [PATCH v2 2/2] arm64/hugetlb: Implement arm64 specific huge_ptep_get()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 5/13/22 09:07, Baolin Wang wrote:
> Now we use huge_ptep_get() to get the pte value of a hugetlb page,
> however it will only return one specific pte value for the CONT-PTE
> or CONT-PMD size hugetlb on ARM64 system, which can contain seravel

A small nit.

s/seravel/several

> continuous pte or pmd entries with same page table attributes. And it
> will not take into account the subpages' dirty or young bits of a
> CONT-PTE/PMD size hugetlb page.
> 
> So the huge_ptep_get() is inconsistent with huge_ptep_get_and_clear(),
> which already takes account the dirty or young bits for any subpages
> in this CONT-PTE/PMD size hugetlb [1]. Meanwhile we can miss dirty or
> young flags statistics for hugetlb pages with current huge_ptep_get(),
> such as the gather_hugetlb_stats() function, and CONT-PTE/PMD hugetlb
> monitoring with DAMON.
> 
> Thus define an ARM64 specific  huge_ptep_get() implementation, that will
> take into account any subpages' dirty or young bits for CONT-PTE/PMD size
> hugetlb page, for those functions that want to check the dirty and young
> flags of a hugetlb page.
> 
> [1] https://lore.kernel.org/linux-mm/85bd80b4-b4fd-0d3f-a2e5-149559f2f387@oracle.com/

Might be worth mentioning that arm64 now enables __HAVE_ARCH_HUGE_PTEP_GET.

> 
> Suggested-by: Muchun Song <songmuchun@bytedance.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  arch/arm64/include/asm/hugetlb.h |  2 ++
>  arch/arm64/mm/hugetlbpage.c      | 24 ++++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h
> index 616b2ca..1fd2846 100644
> --- a/arch/arm64/include/asm/hugetlb.h
> +++ b/arch/arm64/include/asm/hugetlb.h
> @@ -44,6 +44,8 @@ extern pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
>  #define __HAVE_ARCH_HUGE_PTE_CLEAR
>  extern void huge_pte_clear(struct mm_struct *mm, unsigned long addr,
>  			   pte_t *ptep, unsigned long sz);
> +#define __HAVE_ARCH_HUGE_PTEP_GET
> +extern pte_t huge_ptep_get(pte_t *ptep);
>  extern void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr,
>  				 pte_t *ptep, pte_t pte, unsigned long sz);
>  #define set_huge_swap_pte_at set_huge_swap_pte_at
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 9553851..9a3f7f1 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -158,6 +158,30 @@ static inline int num_contig_ptes(unsigned long size, size_t *pgsize)
>  	return contig_ptes;
>  }
>  
> +pte_t huge_ptep_get(pte_t *ptep)
> +{
> +	int ncontig, i;
> +	size_t pgsize;
> +	pte_t orig_pte = ptep_get(ptep);
> +
> +	if (!pte_present(orig_pte) || !pte_cont(orig_pte))
> +		return orig_pte;
> +
> +	ncontig = num_contig_ptes(page_size(pte_page(orig_pte)), &pgsize);

Hmm, I guess there is no better way of deriving page size here.

Please drop the extra line here.

> +
> +	for (i = 0; i < ncontig; i++, ptep++) {
> +		pte_t pte = ptep_get(ptep);
> +
> +		if (pte_dirty(pte))
> +			orig_pte = pte_mkdirty(orig_pte);
> +
> +		if (pte_young(pte))
> +			orig_pte = pte_mkyoung(orig_pte);
> +	}

Please drop the extra line here.

> +
> +	return orig_pte;
> +}
> +
>  /*
>   * Changing some bits of contiguous entries requires us to follow a
>   * Break-Before-Make approach, breaking the whole contiguous set
Otherwise LGTM.

With those small changes accommodated,

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>