linux-kernel - Re: [PATCH v7 1/3] mm/madvise: introduce clear_young_dirty

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Date: Tue, 16 Apr 2024 17:01:32 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: ryan.roberts@....com, 21cnbao@...il.com, mhocko@...e.com,
 fengwei.yin@...el.com, zokeefe@...gle.com, shy828301@...il.com,
 xiehuan09@...il.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com,
 peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/3] mm/madvise: introduce clear_young_dirty_ptes()
 batch helper

On 16.04.24 05:34, Lance Yang wrote:
> This commit introduces clear_young_dirty_ptes() to replace mkold_ptes().
> By doing so, we can use the same function for both use cases
> (madvise_pageout and madvise_free), and it also provides the flexibility
> to only clear the dirty flag in the future if needed.
> 
> Suggested-by: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
>   include/linux/mm_types.h |  9 +++++
>   include/linux/pgtable.h  | 74 ++++++++++++++++++++++++----------------
>   mm/madvise.c             |  3 +-
>   3 files changed, 55 insertions(+), 31 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index c432add95913..28822cd65d2a 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1367,6 +1367,15 @@ enum fault_flag {
>   
>   typedef unsigned int __bitwise zap_flags_t;
>   
> +/* Flags for clear_young_dirty_ptes(). */
> +typedef int __bitwise cydp_t;
> +
> +/* Clear the access bit */
> +#define CYDP_CLEAR_YOUNG		((__force cydp_t)BIT(0))
> +
> +/* Clear the dirty bit */
> +#define CYDP_CLEAR_DIRTY		((__force cydp_t)BIT(1))
> +
>   /*
>    * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
>    * other. Here is what they mean, and how to use them:
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index e2f45e22a6d1..18019f037bae 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -361,36 +361,6 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>   }
>   #endif
>   
> -#ifndef mkold_ptes
> -/**
> - * mkold_ptes - Mark PTEs that map consecutive pages of the same folio as old.
> - * @vma: VMA the pages are mapped into.
> - * @addr: Address the first page is mapped at.
> - * @ptep: Page table pointer for the first entry.
> - * @nr: Number of entries to mark old.
> - *
> - * May be overridden by the architecture; otherwise, implemented as a simple
> - * loop over ptep_test_and_clear_young().
> - *
> - * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> - * some PTEs might be write-protected.
> - *
> - * Context: The caller holds the page table lock.  The PTEs map consecutive
> - * pages that belong to the same folio.  The PTEs are all in the same PMD.
> - */
> -static inline void mkold_ptes(struct vm_area_struct *vma, unsigned long addr,
> -		pte_t *ptep, unsigned int nr)
> -{
> -	for (;;) {
> -		ptep_test_and_clear_young(vma, addr, ptep);
> -		if (--nr == 0)
> -			break;
> -		ptep++;
> -		addr += PAGE_SIZE;
> -	}
> -}
> -#endif
> -
>   #ifndef __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
>   #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG)
>   static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
> @@ -489,6 +459,50 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>   }
>   #endif
>   
> +#ifndef clear_young_dirty_ptes
> +/**
> + * clear_young_dirty_ptes - Mark PTEs that map consecutive pages of the
> + *		same folio as old/clean.
> + * @mm: Address space the pages are mapped into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of entries to mark old/clean.
> + * @flags: Flags to modify the PTE batch semantics.
> + *
> + * May be overridden by the architecture; otherwise, implemented by
> + * get_and_clear/modify/set for each pte in the range.
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might be write-protected.
> + *
> + * Context: The caller holds the page table lock.  The PTEs map consecutive
> + * pages that belong to the same folio.  The PTEs are all in the same PMD.
> + */
> +static inline void clear_young_dirty_ptes(struct vm_area_struct *vma,
> +					  unsigned long addr, pte_t *ptep,
> +					  unsigned int nr, cydp_t flags)
> +{
> +	pte_t pte;
> +
> +	for (;;) {
> +		if (flags == CYDP_CLEAR_YOUNG)
> +			ptep_test_and_clear_young(vma, addr, ptep);
> +		else {
> +			pte = ptep_get_and_clear(vma->vm_mm, addr, ptep);
> +			if (flags & CYDP_CLEAR_YOUNG)
> +				pte = pte_mkold(pte);
> +			if (flags & CYDP_CLEAR_DIRTY)
> +				pte = pte_mkclean(pte);
> +			set_pte_at(vma->vm_mm, addr, ptep, pte);
> +		}
> +		if (--nr == 0)
> +			break;
> +		ptep++;
> +		addr += PAGE_SIZE;
> +	}
> +}

The complier *might* generate a bit faster code if you check for 
CYDP_CLEAR_YOUNG outside of the loop, so you don't have to recheck on 
each loop iteration.

For now, nothing to lose sleep about

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb