Re: [RFC PATCH 05/16] mm: thp: handling 1GB THP reference bit.

[Zi Yan]

On 9 Sep 2020, at 10:09, Kirill A. Shutemov wrote:

> On Wed, Sep 02, 2020 at 02:06:17PM -0400, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> Add PUD-level TLB flush ops and teach page_vma_mapped_talk about 1GB
>> THPs.
>>
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/pgtable.h |  3 +++
>>  arch/x86/mm/pgtable.c          | 13 +++++++++++++
>>  include/linux/mmu_notifier.h   | 13 +++++++++++++
>>  include/linux/pgtable.h        | 14 ++++++++++++++
>>  include/linux/rmap.h           |  1 +
>>  mm/page_vma_mapped.c           | 33 +++++++++++++++++++++++++++++----
>>  mm/rmap.c                      | 12 +++++++++---
>>  7 files changed, 82 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index 26255cac78c0..15334f5ba172 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -1127,6 +1127,9 @@ extern int pudp_test_and_clear_young(struct vm_area_struct *vma,
>>  extern int pmdp_clear_flush_young(struct vm_area_struct *vma,
>>  				  unsigned long address, pmd_t *pmdp);
>>
>> +#define __HAVE_ARCH_PUDP_CLEAR_YOUNG_FLUSH
>> +extern int pudp_clear_flush_young(struct vm_area_struct *vma,
>> +				  unsigned long address, pud_t *pudp);
>>
>>  #define pmd_write pmd_write
>>  static inline int pmd_write(pmd_t pmd)
>> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
>> index 7be73aee6183..e4a2dffcc418 100644
>> --- a/arch/x86/mm/pgtable.c
>> +++ b/arch/x86/mm/pgtable.c
>> @@ -633,6 +633,19 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma,
>>
>>  	return young;
>>  }
>> +int pudp_clear_flush_young(struct vm_area_struct *vma,
>> +			   unsigned long address, pud_t *pudp)
>> +{
>> +	int young;
>> +
>> +	VM_BUG_ON(address & ~HPAGE_PUD_MASK);
>> +
>> +	young = pudp_test_and_clear_young(vma, address, pudp);
>> +	if (young)
>> +		flush_tlb_range(vma, address, address + HPAGE_PUD_SIZE);
>> +
>> +	return young;
>> +}
>>  #endif
>>
>>  /**
>> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
>> index b8200782dede..4ffa179e654f 100644
>> --- a/include/linux/mmu_notifier.h
>> +++ b/include/linux/mmu_notifier.h
>> @@ -557,6 +557,19 @@ static inline void mmu_notifier_range_init_migrate(
>>  	__young;							\
>>  })
>>
>> +#define pudp_clear_flush_young_notify(__vma, __address, __pudp)		\
>> +({									\
>> +	int __young;							\
>> +	struct vm_area_struct *___vma = __vma;				\
>> +	unsigned long ___address = __address;				\
>> +	__young = pudp_clear_flush_young(___vma, ___address, __pudp);	\
>> +	__young |= mmu_notifier_clear_flush_young(___vma->vm_mm,	\
>> +						  ___address,		\
>> +						  ___address +		\
>> +							PUD_SIZE);	\
>> +	__young;							\
>> +})
>> +
>>  #define ptep_clear_young_notify(__vma, __address, __ptep)		\
>>  ({									\
>>  	int __young;							\
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index 255275d5b73e..8ef358c386af 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -240,6 +240,20 @@ static inline int pmdp_clear_flush_young(struct vm_area_struct *vma,
>>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>  #endif
>>
>> +#ifndef __HAVE_ARCH_PUDP_CLEAR_YOUNG_FLUSH
>> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
>> +extern int pudp_clear_flush_young(struct vm_area_struct *vma,
>> +				  unsigned long address, pud_t *pudp);
>> +#else
>> +int pudp_clear_flush_young(struct vm_area_struct *vma,
>> +				  unsigned long address, pud_t *pudp)
>> +{
>> +	BUILD_BUG();
>> +	return 0;
>> +}
>> +#endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD  */
>> +#endif
>> +
>>  #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>>  static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>>  				       unsigned long address,
>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>> index 3a6adfa70fb0..0af61dd193d2 100644
>> --- a/include/linux/rmap.h
>> +++ b/include/linux/rmap.h
>> @@ -206,6 +206,7 @@ struct page_vma_mapped_walk {
>>  	struct page *page;
>>  	struct vm_area_struct *vma;
>>  	unsigned long address;
>> +	pud_t *pud;
>>  	pmd_t *pmd;
>>  	pte_t *pte;
>>  	spinlock_t *ptl;
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index 5e77b269c330..d9d39ec06e21 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -145,9 +145,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>  	struct page *page = pvmw->page;
>>  	pgd_t *pgd;
>>  	p4d_t *p4d;
>> -	pud_t *pud;
>> +	pud_t pude;
>>  	pmd_t pmde;
>>
>> +	if (!pvmw->pte && !pvmw->pmd && pvmw->pud)
>> +		return not_found(pvmw);
>> +
>>  	/* The only possible pmd mapping has been handled on last iteration */
>>  	if (pvmw->pmd && !pvmw->pte)
>>  		return not_found(pvmw);
>> @@ -174,10 +177,31 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>  	p4d = p4d_offset(pgd, pvmw->address);
>>  	if (!p4d_present(*p4d))
>>  		return false;
>> -	pud = pud_offset(p4d, pvmw->address);
>> -	if (!pud_present(*pud))
>> +	pvmw->pud = pud_offset(p4d, pvmw->address);
>> +
>> +	/*
>> +	 * Make sure the pud value isn't cached in a register by the
>> +	 * compiler and used as a stale value after we've observed a
>> +	 * subsequent update.
>> +	 */
>> +	pude = READ_ONCE(*pvmw->pud);
>> +	if (pud_trans_huge(pude)) {
>> +		pvmw->ptl = pud_lock(mm, pvmw->pud);
>> +		if (likely(pud_trans_huge(*pvmw->pud))) {
>> +			if (pvmw->flags & PVMW_MIGRATION)
>> +				return not_found(pvmw);
>> +			if (pud_page(*pvmw->pud) != page)
>> +				return not_found(pvmw);
>> +			return true;
>> +		} else {
>> +			/* THP pud was split under us: handle on pmd level */
>> +			spin_unlock(pvmw->ptl);
>> +			pvmw->ptl = NULL;
>
> Hm. What makes you sure the pmd table is established here?
>
> I have not looked at PUD THP handling of  MADV_DONTNEED yet, but for PMD
> THP can became pmd_none() at any point (unless ptl is locked).

You are right. I need to check pud_present here and only
go to pmd level when pud is present, otherwise just return with not_found.

Thanks.


>
>> +		}
>> +	} else if (!pud_present(pude))
>>  		return false;
>> -	pvmw->pmd = pmd_offset(pud, pvmw->address);
>> +
>> +	pvmw->pmd = pmd_offset(pvmw->pud, pvmw->address);
>>  	/*
>>  	 * Make sure the pmd value isn't cached in a register by the
>>  	 * compiler and used as a stale value after we've observed a
>> @@ -213,6 +237,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>  	} else if (!pmd_present(pmde)) {
>>  		return false;
>>  	}
>> +
>>  	if (!map_pte(pvmw))
>>  		goto next_pte;
>>  	while (1) {
>> diff --git a/mm/rmap.c b/mm/rmap.c
>
> Why?
>

The extra newline? Will remove it.

>> index 10195a2421cf..77cec0658b76 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -803,9 +803,15 @@ static bool page_referenced_one(struct page *page, struct vm_area_struct *vma,
>>  					referenced++;
>>  			}
>>  		} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
>> -			if (pmdp_clear_flush_young_notify(vma, address,
>> -						pvmw.pmd))
>> -				referenced++;
>> +			if (pvmw.pmd) {
>> +				if (pmdp_clear_flush_young_notify(vma, address,
>> +							pvmw.pmd))
>> +					referenced++;
>> +			} else if (pvmw.pud) {
>> +				if (pudp_clear_flush_young_notify(vma, address,
>> +							pvmw.pud))
>> +					referenced++;
>> +			}
>>  		} else {
>>  			/* unexpected pmd-mapped page? */
>>  			WARN_ON_ONCE(1);
>> -- 
>> 2.28.0
>>
>>
>
> -- 
>  Kirill A. Shutemov


—
Best Regards,
Yan Zi

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