Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd()

Nadav Amit
2021-10-25T16:29:15Z

> On Oct 25, 2021, at 3:52 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Oct 21, 2021 at 05:21:09AM -0700, Nadav Amit wrote:
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index 448cd01eb3ec..18c3366f8f4d 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -1146,6 +1146,14 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
>> }
>> }
>> #endif
>> +
>> +#define __HAVE_ARCH_PMDP_INVALIDATE_AD
>> +static inline pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma,
>> + unsigned long address, pmd_t *pmdp)
>> +{
>> + return pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));
>
> Did this want to be something like:
>
> pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));
> if (cpu_feature_enabled(X86_BUG_PTE_LEAK))
> flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> return old;
>
> instead?

Yes. Of course. Where did my code go to? :(

>
>> +}
>> +
>> /*
>> * Page table pages are page-aligned. The lower half of the top
>> * level is used for userspace and the top half for the kernel.
>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index e5ea5f775d5c..435da011b1a2 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1795,10 +1795,11 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>> * The race makes MADV_DONTNEED miss the huge pmd and don't clear it
>> * which may break userspace.
>> *
>> - * pmdp_invalidate() is required to make sure we don't miss
>> - * dirty/young flags set by hardware.
>> + * pmdp_invalidate_ad() is required to make sure we don't miss
>> + * dirty/young flags (which are also known as access/dirty) cannot be
>> + * further modifeid by the hardware.
>
> "modified", I think is the more common spelling.

I tried to start a new trend. I will fix it.

>
>> */
>> - entry = pmdp_invalidate(vma, addr, pmd);
>> + entry = pmdp_invalidate_ad(vma, addr, pmd);
>>
>> entry = pmd_modify(entry, newprot);
>> if (preserve_write)
>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 4e640baf9794..b0ce6c7391bf 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -200,6 +200,14 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>> }
>> #endif
>>
>> +#ifndef __HAVE_ARCH_PMDP_INVALIDATE_AD
>
> /*
> * Does this deserve a comment to explain the intended difference vs
> * pmdp_invalidate() ?
> */

I will add a comment.