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

Ryan Roberts
2024-02-13T16:48:55Z

On 13/02/2024 16:43, Mark Rutland wrote:
> 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:

I found it a little odd at first, but its avoiding the ptep and addr increments
the last time through the loop. Its the preferred pattern for these functions in
core-mm. See default set_ptes(), wrprotect_ptes(), clear_full_ptes() in
include/linux/pgtable.h.

So I'd prefer to leave it as is so that we match them. What do you think?

>
> 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
>>