Re: [PATCH v2 4/7] arm64: Add batched version of ptep_modify_prot_start

[Dev Jain <dev.jain@arm.com>]

On 30/04/25 11:44 am, Anshuman Khandual wrote:
> 
> 
> On 4/30/25 11:19, Dev Jain wrote:
>>
>>
>> On 30/04/25 11:13 am, Anshuman Khandual wrote:
>>> On 4/29/25 10:53, Dev Jain wrote:
>>>> Override the generic definition to use get_and_clear_full_ptes(), so that
>>>> we do a TLBI possibly only on the "contpte-edges" of the large PTE block,
>>>> instead of doing it for every contpte block, which happens for ptep_get_and_clear().
>>>
>>> Could you please explain what does "contpte-edges" really signify in the
>>> context of large PTE blocks ? Also how TLBI operation only on these edges
>>> will never run into the risk of missing TLB invalidation of some other
>>> mapped areas ?
>>
>> We are doing a TLBI over the whole range already, in the mprotect code:
>> see tlb_flush_pte_range. What the arm64 internal API does, irrespective of the caller, is to do a TLBI for every contpte block in case of unfolding. We don't need that for the intermediate blocks because the caller does that. We do need a TLBI for the start and end contpte block,
>> because in case the range we are invalidating partially covers them, then the caller will not do a TLBI for the non-overlapped PTEs of the block.
> 
> But is not splitting the TLBI flush responsibility between the callers
> (intermediate blocks) and the platform API (contpte-edges) - some what
> problematic from a semantics perspective, and will be more susceptible
> for missing TLB flushes etc ?

I seem to agree that this is a semantic problem. Although, we won't ever 
miss a TLB flush - we will only have extras. It is the responsibility of 
the caller to do the TLBI, the platform API is only checking whether we 
need some more TLBIs.

> 
>> I'll explain some more in the changelog next version.
>>
>>>
>>>>
>>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>>> ---
>>>>    arch/arm64/include/asm/pgtable.h |  5 +++++
>>>>    arch/arm64/mm/mmu.c              | 12 +++++++++---
>>>>    include/linux/pgtable.h          |  4 ++++
>>>>    mm/pgtable-generic.c             | 16 +++++++++++-----
>>>>    4 files changed, 29 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>>> index 2a77f11b78d5..8872ea5f0642 100644
>>>> --- a/arch/arm64/include/asm/pgtable.h
>>>> +++ b/arch/arm64/include/asm/pgtable.h
>>>> @@ -1553,6 +1553,11 @@ extern void ptep_modify_prot_commit(struct vm_area_struct *vma,
>>>>                        unsigned long addr, pte_t *ptep,
>>>>                        pte_t old_pte, pte_t new_pte);
>>>>    +#define modify_prot_start_ptes modify_prot_start_ptes
>>>> +extern pte_t modify_prot_start_ptes(struct vm_area_struct *vma,
>>>> +                    unsigned long addr, pte_t *ptep,
>>>> +                    unsigned int nr);
>>>> +
>>>>    #ifdef CONFIG_ARM64_CONTPTE
>>>>      /*
>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>>> index 8fcf59ba39db..fe60be8774f4 100644
>>>> --- a/arch/arm64/mm/mmu.c
>>>> +++ b/arch/arm64/mm/mmu.c
>>>> @@ -1523,7 +1523,8 @@ static int __init prevent_bootmem_remove_init(void)
>>>>    early_initcall(prevent_bootmem_remove_init);
>>>>    #endif
>>>>    -pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
>>>> +pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,
>>>> +                 pte_t *ptep, unsigned int nr)
>>>>    {
>>>>        if (alternative_has_cap_unlikely(ARM64_WORKAROUND_2645198)) {
>>>>            /*
>>>> @@ -1532,9 +1533,14 @@ pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte
>>>>             * in cases where cpu is affected with errata #2645198.
>>>>             */
>>>>            if (pte_user_exec(ptep_get(ptep)))
>>>> -            return ptep_clear_flush(vma, addr, ptep);
>>>> +            return clear_flush_ptes(vma, addr, ptep, nr);
>>>>        }
>>>> -    return ptep_get_and_clear(vma->vm_mm, addr, ptep);
>>>> +    return get_and_clear_full_ptes(vma->vm_mm, addr, ptep, nr, 0);
>>>> +}
>>>> +
>>>> +pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
>>>> +{
>>>> +    return modify_prot_start_ptes(vma, addr, ptep, 1);
>>>>    }
>>>>      void ptep_modify_prot_commit(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep,
>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>> index ed287289335f..10cdb87ccecf 100644
>>>> --- a/include/linux/pgtable.h
>>>> +++ b/include/linux/pgtable.h
>>>> @@ -828,6 +828,10 @@ extern pte_t ptep_clear_flush(struct vm_area_struct *vma,
>>>>                      pte_t *ptep);
>>>>    #endif
>>>>    +extern pte_t clear_flush_ptes(struct vm_area_struct *vma,
>>>> +                  unsigned long address,
>>>> +                  pte_t *ptep, unsigned int nr);
>>>> +
>>>>    #ifndef __HAVE_ARCH_PMDP_HUGE_CLEAR_FLUSH
>>>>    extern pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma,
>>>>                      unsigned long address,
>>>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>>>> index 5a882f2b10f9..e238f88c3cac 100644
>>>> --- a/mm/pgtable-generic.c
>>>> +++ b/mm/pgtable-generic.c
>>>> @@ -90,17 +90,23 @@ int ptep_clear_flush_young(struct vm_area_struct *vma,
>>>>    }
>>>>    #endif
>>>>    -#ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
>>>> -pte_t ptep_clear_flush(struct vm_area_struct *vma, unsigned long address,
>>>> -               pte_t *ptep)
>>>> +pte_t clear_flush_ptes(struct vm_area_struct *vma, unsigned long address,
>>>> +               pte_t *ptep, unsigned int nr)
>>>>    {
>>>>        struct mm_struct *mm = (vma)->vm_mm;
>>>>        pte_t pte;
>>>> -    pte = ptep_get_and_clear(mm, address, ptep);
>>>> +    pte = get_and_clear_full_ptes(mm, address, ptep, nr, 0);
>>>>        if (pte_accessible(mm, pte))
>>>> -        flush_tlb_page(vma, address);
>>>> +        flush_tlb_range(vma, address, address + nr * PAGE_SIZE);
>>>>        return pte;
>>>>    }
>>>> +
>>>> +#ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
>>>> +pte_t ptep_clear_flush(struct vm_area_struct *vma, unsigned long address,
>>>> +               pte_t *ptep)
>>>> +{
>>>> +    return clear_flush_ptes(vma, address, ptep, 1);
>>>> +}
>>>>    #endif
>>>>      #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>
>