Re: [PATCH v5 21/25] arm64/mm: Implement new [get_and_]clear_full_ptes() batch APIs

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Feb 02, 2024 at 08:07:52AM +0000, Ryan Roberts wrote:
> Optimize the contpte implementation to fix some of the
> exit/munmap/dontneed performance regression introduced by the initial
> contpte commit. Subsequent patches will solve it entirely.
> 
> During exit(), munmap() or madvise(MADV_DONTNEED), mappings must be
> cleared. Previously this was done 1 PTE at a time. But the core-mm
> supports batched clear via the new [get_and_]clear_full_ptes() APIs. So
> let's implement those APIs and for fully covered contpte mappings, we no
> longer need to unfold the contpte. This significantly reduces unfolding
> operations, reducing the number of tlbis that must be issued.
> 
> Tested-by: John Hubbard <jhubbard@nvidia.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  arch/arm64/include/asm/pgtable.h | 67 ++++++++++++++++++++++++++++++++
>  arch/arm64/mm/contpte.c          | 17 ++++++++
>  2 files changed, 84 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index c07f0d563733..ad04adb7b87f 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -965,6 +965,37 @@ static inline pte_t __ptep_get_and_clear(struct mm_struct *mm,
>  	return pte;
>  }
>  
> +static inline void __clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> +				pte_t *ptep, unsigned int nr, int full)
> +{
> +	for (;;) {
> +		__ptep_get_and_clear(mm, addr, ptep);
> +		if (--nr == 0)
> +			break;
> +		ptep++;
> +		addr += PAGE_SIZE;
> +	}
> +}

The loop construct is a bit odd; can't this be:

	while (nr--) {
		__ptep_get_and_clear(mm, addr, ptep);
		ptep++;
		addr += PAGE_SIZE;
	}

... or:

	do {
		__ptep_get_and_clear(mm, addr, ptep);
		ptep++;
		addr += PAGE_SIZE;
	} while (--nr);

... ?

Otherwise, this looks good to me.

Mark.

> +
> +static inline pte_t __get_and_clear_full_ptes(struct mm_struct *mm,
> +				unsigned long addr, pte_t *ptep,
> +				unsigned int nr, int full)
> +{
> +	pte_t pte, tmp_pte;
> +
> +	pte = __ptep_get_and_clear(mm, addr, ptep);
> +	while (--nr) {
> +		ptep++;
> +		addr += PAGE_SIZE;
> +		tmp_pte = __ptep_get_and_clear(mm, addr, ptep);
> +		if (pte_dirty(tmp_pte))
> +			pte = pte_mkdirty(pte);
> +		if (pte_young(tmp_pte))
> +			pte = pte_mkyoung(pte);
> +	}
> +	return pte;
> +}
> +
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  #define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
>  static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
> @@ -1167,6 +1198,11 @@ extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
>  extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
>  extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
>  				pte_t *ptep, pte_t pte, unsigned int nr);
> +extern void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> +				pte_t *ptep, unsigned int nr, int full);
> +extern pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
> +				unsigned long addr, pte_t *ptep,
> +				unsigned int nr, int full);
>  extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
>  				unsigned long addr, pte_t *ptep);
>  extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> @@ -1254,6 +1290,35 @@ static inline void pte_clear(struct mm_struct *mm,
>  	__pte_clear(mm, addr, ptep);
>  }
>  
> +#define clear_full_ptes clear_full_ptes
> +static inline void clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> +				pte_t *ptep, unsigned int nr, int full)
> +{
> +	if (likely(nr == 1)) {
> +		contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> +		__clear_full_ptes(mm, addr, ptep, nr, full);
> +	} else {
> +		contpte_clear_full_ptes(mm, addr, ptep, nr, full);
> +	}
> +}
> +
> +#define get_and_clear_full_ptes get_and_clear_full_ptes
> +static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm,
> +				unsigned long addr, pte_t *ptep,
> +				unsigned int nr, int full)
> +{
> +	pte_t pte;
> +
> +	if (likely(nr == 1)) {
> +		contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> +		pte = __get_and_clear_full_ptes(mm, addr, ptep, nr, full);
> +	} else {
> +		pte = contpte_get_and_clear_full_ptes(mm, addr, ptep, nr, full);
> +	}
> +
> +	return pte;
> +}
> +
>  #define __HAVE_ARCH_PTEP_GET_AND_CLEAR
>  static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>  				unsigned long addr, pte_t *ptep)
> @@ -1338,6 +1403,8 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma,
>  #define set_pte					__set_pte
>  #define set_ptes				__set_ptes
>  #define pte_clear				__pte_clear
> +#define clear_full_ptes				__clear_full_ptes
> +#define get_and_clear_full_ptes			__get_and_clear_full_ptes
>  #define __HAVE_ARCH_PTEP_GET_AND_CLEAR
>  #define ptep_get_and_clear			__ptep_get_and_clear
>  #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> index c85e64baf03b..80346108450b 100644
> --- a/arch/arm64/mm/contpte.c
> +++ b/arch/arm64/mm/contpte.c
> @@ -207,6 +207,23 @@ void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
>  }
>  EXPORT_SYMBOL(contpte_set_ptes);
>  
> +void contpte_clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> +				pte_t *ptep, unsigned int nr, int full)
> +{
> +	contpte_try_unfold_partial(mm, addr, ptep, nr);
> +	__clear_full_ptes(mm, addr, ptep, nr, full);
> +}
> +EXPORT_SYMBOL(contpte_clear_full_ptes);
> +
> +pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
> +				unsigned long addr, pte_t *ptep,
> +				unsigned int nr, int full)
> +{
> +	contpte_try_unfold_partial(mm, addr, ptep, nr);
> +	return __get_and_clear_full_ptes(mm, addr, ptep, nr, full);
> +}
> +EXPORT_SYMBOL(contpte_get_and_clear_full_ptes);
> +
>  int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
>  					unsigned long addr, pte_t *ptep)
>  {
> -- 
> 2.25.1
>