Re: [PATCH 10/7] riscv: Implement the new page table range API

Yin, Fengwei
2023-02-16T13:28:05Z

Hi Alex,

On 2/16/2023 4:14 PM, Alexandre Ghiti wrote:
> Hi Yin,
>
> On 2/15/23 09:38, Yin, Fengwei wrote:
>> On Wed, 2023-02-15 at 00:04 +0000, Matthew Wilcox (Oracle) wrote:
>>> Add set_ptes(), update_mmu_cache_range() and flush_dcache_folio().
>>>
>>> The PG_dcache_clear flag changes from being a per-page bit to being a
>>> per-folio bit.
>>>
>>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>>> ---
>>>   arch/riscv/include/asm/cacheflush.h | 19 +++++++++----------
>>>   arch/riscv/include/asm/pgtable.h    | 25 ++++++++++++++++++-------
>>>   arch/riscv/mm/cacheflush.c          | 11 ++---------
>>>   3 files changed, 29 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/arch/riscv/include/asm/cacheflush.h
>>> b/arch/riscv/include/asm/cacheflush.h
>>> index 03e3b95ae6da..10e5e96f09b5 100644
>>> --- a/arch/riscv/include/asm/cacheflush.h
>>> +++ b/arch/riscv/include/asm/cacheflush.h
>>> @@ -15,20 +15,19 @@ static inline void local_flush_icache_all(void)
>>>     #define PG_dcache_clean PG_arch_1
>>>   -static inline void flush_dcache_page(struct page *page)
>>> +static inline void flush_dcache_folio(struct folio *folio)
>>>   {
>>> -       /*
>>> -        * HugeTLB pages are always fully mapped and only head page
>>> will be
>>> -        * set PG_dcache_clean (see comments in flush_icache_pte()).
>>> -        */
>>> -       if (PageHuge(page))
>>> -               page = compound_head(page);
>>> -
>>> -       if (test_bit(PG_dcache_clean, &page->flags))
>>> -               clear_bit(PG_dcache_clean, &page->flags);
>>> +       if (test_bit(PG_dcache_clean, &folio->flags))
>>> +               clear_bit(PG_dcache_clean, &folio->flags);
>>>   }
>>> +#define flush_dcache_folio flush_dcache_folio
>>>   #define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 1
>>>   +static inline void flush_dcache_page(struct page *page)
>>> +{
>>> +       flush_dcache_folio(page_folio(page));
>>> +}
>>> +
>>>   /*
>>>    * RISC-V doesn't have an instruction to flush parts of the
>>> instruction cache,
>>>    * so instead we just flush the whole thing.
>>> diff --git a/arch/riscv/include/asm/pgtable.h
>>> b/arch/riscv/include/asm/pgtable.h
>>> index 13222fd5c4b4..03706c833e70 100644
>>> --- a/arch/riscv/include/asm/pgtable.h
>>> +++ b/arch/riscv/include/asm/pgtable.h
>>> @@ -405,8 +405,8 @@ static inline pte_t pte_modify(pte_t pte,
>>> pgprot_t newprot)
>>>       /* Commit new configuration to MMU hardware */
>>> -static inline void update_mmu_cache(struct vm_area_struct *vma,
>>> -       unsigned long address, pte_t *ptep)
>>> +static inline void update_mmu_cache_range(struct vm_area_struct
>>> *vma,
>>> +               unsigned long address, pte_t *ptep, unsigned int nr)
>>>   {
>>>          /*
>>>           * The kernel assumes that TLBs don't cache invalid entries,
>>> but
>>> @@ -415,8 +415,10 @@ static inline void update_mmu_cache(struct
>>> vm_area_struct *vma,
>>>           * Relying on flush_tlb_fix_spurious_fault would suffice, but
>>>           * the extra traps reduce performance.  So, eagerly
>>> SFENCE.VMA.
>>>           */
>>> -       flush_tlb_page(vma, address);
>>> +       flush_tlb_range(vma, address, address + nr * PAGE_SIZE);
>> The flush_tlb_range() of riscv is a little bit strange to me. It gives
>> __sbi_tlb_flush_range() stride PAGE_SIZE. That means if (end - start)
>> is larger than stride, it will trigger flush_tlb_all().
>>
>> So this change could trigger flush_tlb_all() while original
>> flush_tlb_page() just trigger flush_tlb_page().
>
>
> Maybe I'm missing something but update_mmu_cache behaviour is not changed here, it will always call flush_tlb_page as nr == 1, right?
Yes. This is my understanding too. Thanks.

Regards
Yin, Fengwei

>
> update_mmu_cache_range though will likely call flush_tlb_all: I have to admit that I'm wondering why we don't only flush the range of pages instead of flushing everything, I'll look into that.
>
> Alex
>
>
>>
>> My understanding is flush_tlb_page() should be better because
>> flush_pmd_tlb_range() has PMD_SIZE as stride to avoid flush_tlb_all().
>> I must miss something here.
>>
>> Regards
>> Yin, Fengwei
>>
>>>   }
>>> +#define update_mmu_cache(vma, addr, ptep) \
>>> +       update_mmu_cache_range(vma, addr, ptep, 1)
>>>     #define __HAVE_ARCH_UPDATE_MMU_TLB
>>>   #define update_mmu_tlb update_mmu_cache
>>> @@ -456,12 +458,21 @@ static inline void __set_pte_at(struct
>>> mm_struct *mm,
>>>          set_pte(ptep, pteval);
>>>   }
>>>   -static inline void set_pte_at(struct mm_struct *mm,
>>> -       unsigned long addr, pte_t *ptep, pte_t pteval)
>>> +static inline void set_ptes(struct mm_struct *mm, unsigned long
>>> addr,
>>> +               pte_t *ptep, pte_t pteval, unsigned int nr)
>>>   {
>>> -       page_table_check_ptes_set(mm, addr, ptep, pteval, 1);
>>> -       __set_pte_at(mm, addr, ptep, pteval);
>>> +       page_table_check_ptes_set(mm, addr, ptep, pteval, nr);
>>> +
>>> +       for (;;) {
>>> +               __set_pte_at(mm, addr, ptep, pteval);
>>> +               if (--nr == 0)
>>> +                       break;
>>> +               ptep++;
>>> +               addr += PAGE_SIZE;
>>> +               pte_val(pteval) += 1 << _PAGE_PFN_SHIFT;
>>> +       }
>>>   }
>>> +#define set_pte_at(mm, addr, ptep, pte) set_ptes(mm, addr, ptep,
>>> pte, 1)
>>>     static inline void pte_clear(struct mm_struct *mm,
>>>          unsigned long addr, pte_t *ptep)
>>> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
>>> index 3cc07ed45aeb..b725c3f6f57f 100644
>>> --- a/arch/riscv/mm/cacheflush.c
>>> +++ b/arch/riscv/mm/cacheflush.c
>>> @@ -81,16 +81,9 @@ void flush_icache_mm(struct mm_struct *mm, bool
>>> local)
>>>   #ifdef CONFIG_MMU
>>>   void flush_icache_pte(pte_t pte)
>>>   {
>>> -       struct page *page = pte_page(pte);
>>> +       struct folio *folio = page_folio(pte_page(pte));
>>>   -       /*
>>> -        * HugeTLB pages are always fully mapped, so only setting
>>> head page's
>>> -        * PG_dcache_clean flag is enough.
>>> -        */
>>> -       if (PageHuge(page))
>>> -               page = compound_head(page);
>>> -
>>> -       if (!test_and_set_bit(PG_dcache_clean, &page->flags))
>>> +       if (!test_and_set_bit(PG_dcache_clean, &folio->flags))
>>>                  flush_icache_all();
>>>   }
>>>   #endif /* CONFIG_MMU */