linux-kernel - Re: [PATCH v1 2/5] mm: damon must atomically clear young on ptes and pmds

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Message-ID: <c443270d-3d54-7d4f-9162-1fefdbedc92c@arm.com>
Date:   Thu, 11 May 2023 14:14:26 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        SeongJae Park <sj@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        damon@...ts.linux.dev
Subject: Re: [PATCH v1 2/5] mm: damon must atomically clear young on ptes and
 pmds

My appologies for the noise: A blank line between Cc and Subject has broken the
subject and grouping in lore.

Please Ignore this, I will resend.


On 11/05/2023 13:58, Ryan Roberts wrote:
> It is racy to non-atomically read a pte, then clear the young bit, then
> write it back as this could discard dirty information. Further, it is
> bad practice to directly set a pte entry within a table. Instead
> clearing young must go through the arch-provided helper,
> ptep_test_and_clear_young() to ensure it is modified atomically and to
> give the arch code visibility and allow it to validate (and potentially
> modify) the operation.
> 
> Fixes: 46c3a0accdc4 ("mm/damon/vaddr: separate commonly usable functions")
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>  mm/damon/ops-common.c | 16 ++++++----------
>  mm/damon/ops-common.h |  4 ++--
>  mm/damon/paddr.c      |  4 ++--
>  mm/damon/vaddr.c      |  4 ++--
>  4 files changed, 12 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> index cc63cf953636..acc264b97903 100644
> --- a/mm/damon/ops-common.c
> +++ b/mm/damon/ops-common.c
> @@ -37,7 +37,7 @@ struct folio *damon_get_folio(unsigned long pfn)
>  	return folio;
>  }
>  
> -void damon_ptep_mkold(pte_t *pte, struct mm_struct *mm, unsigned long addr)
> +void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr)
>  {
>  	bool referenced = false;
>  	struct folio *folio = damon_get_folio(pte_pfn(*pte));
> @@ -45,13 +45,11 @@ void damon_ptep_mkold(pte_t *pte, struct mm_struct *mm, unsigned long addr)
>  	if (!folio)
>  		return;
>  
> -	if (pte_young(*pte)) {
> +	if (ptep_test_and_clear_young(vma, addr, pte))
>  		referenced = true;
> -		*pte = pte_mkold(*pte);
> -	}
>  
>  #ifdef CONFIG_MMU_NOTIFIER
> -	if (mmu_notifier_clear_young(mm, addr, addr + PAGE_SIZE))
> +	if (mmu_notifier_clear_young(vma->vm_mm, addr, addr + PAGE_SIZE))
>  		referenced = true;
>  #endif /* CONFIG_MMU_NOTIFIER */
>  
> @@ -62,7 +60,7 @@ void damon_ptep_mkold(pte_t *pte, struct mm_struct *mm, unsigned long addr)
>  	folio_put(folio);
>  }
>  
> -void damon_pmdp_mkold(pmd_t *pmd, struct mm_struct *mm, unsigned long addr)
> +void damon_pmdp_mkold(pmd_t *pmd, struct vm_area_struct *vma, unsigned long addr)
>  {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	bool referenced = false;
> @@ -71,13 +69,11 @@ void damon_pmdp_mkold(pmd_t *pmd, struct mm_struct *mm, unsigned long addr)
>  	if (!folio)
>  		return;
>  
> -	if (pmd_young(*pmd)) {
> +	if (pmdp_test_and_clear_young(vma, addr, pmd))
>  		referenced = true;
> -		*pmd = pmd_mkold(*pmd);
> -	}
>  
>  #ifdef CONFIG_MMU_NOTIFIER
> -	if (mmu_notifier_clear_young(mm, addr, addr + HPAGE_PMD_SIZE))
> +	if (mmu_notifier_clear_young(vma->vm_mm, addr, addr + HPAGE_PMD_SIZE))
>  		referenced = true;
>  #endif /* CONFIG_MMU_NOTIFIER */
>  
> diff --git a/mm/damon/ops-common.h b/mm/damon/ops-common.h
> index 14f4bc69f29b..18d837d11bce 100644
> --- a/mm/damon/ops-common.h
> +++ b/mm/damon/ops-common.h
> @@ -9,8 +9,8 @@
>  
>  struct folio *damon_get_folio(unsigned long pfn);
>  
> -void damon_ptep_mkold(pte_t *pte, struct mm_struct *mm, unsigned long addr);
> -void damon_pmdp_mkold(pmd_t *pmd, struct mm_struct *mm, unsigned long addr);
> +void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr);
> +void damon_pmdp_mkold(pmd_t *pmd, struct vm_area_struct *vma, unsigned long addr);
>  
>  int damon_cold_score(struct damon_ctx *c, struct damon_region *r,
>  			struct damos *s);
> diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
> index 467b99166b43..5b3a3463d078 100644
> --- a/mm/damon/paddr.c
> +++ b/mm/damon/paddr.c
> @@ -24,9 +24,9 @@ static bool __damon_pa_mkold(struct folio *folio, struct vm_area_struct *vma,
>  	while (page_vma_mapped_walk(&pvmw)) {
>  		addr = pvmw.address;
>  		if (pvmw.pte)
> -			damon_ptep_mkold(pvmw.pte, vma->vm_mm, addr);
> +			damon_ptep_mkold(pvmw.pte, vma, addr);
>  		else
> -			damon_pmdp_mkold(pvmw.pmd, vma->vm_mm, addr);
> +			damon_pmdp_mkold(pvmw.pmd, vma, addr);
>  	}
>  	return true;
>  }
> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index 1fec16d7263e..37994fb6120c 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
> @@ -311,7 +311,7 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
>  		}
>  
>  		if (pmd_trans_huge(*pmd)) {
> -			damon_pmdp_mkold(pmd, walk->mm, addr);
> +			damon_pmdp_mkold(pmd, walk->vma, addr);
>  			spin_unlock(ptl);
>  			return 0;
>  		}
> @@ -323,7 +323,7 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>  	if (!pte_present(*pte))
>  		goto out;
> -	damon_ptep_mkold(pte, walk->mm, addr);
> +	damon_ptep_mkold(pte, walk->vma, addr);
>  out:
>  	pte_unmap_unlock(pte, ptl);
>  	return 0;