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Subject: Re: [PATCH v4 11/13] mm/debug_vm_pgtable/pmd_clear: Don't use
 pmd/pud_clear on pte entries
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>, linux-mm@kvack.org,
 akpm@linux-foundation.org
References: <20200902114222.181353-1-aneesh.kumar@linux.ibm.com>
 <20200902114222.181353-12-aneesh.kumar@linux.ibm.com>
From: Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <8251b4b6-63ee-cbf8-63dd-c0031a010a4b@arm.com>
Date: Fri, 4 Sep 2020 11:33:04 +0530
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On 09/02/2020 05:12 PM, Aneesh Kumar K.V wrote:
> pmd_clear() should not be used to clear pmd level pte entries.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  mm/debug_vm_pgtable.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 26023d990bd0..b53903fdee85 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -196,6 +196,8 @@ static void __init pmd_advanced_tests(struct mm_struct *mm,
>  	pmd = READ_ONCE(*pmdp);
>  	WARN_ON(pmd_young(pmd));
>  
> +	/*  Clear the pte entries  */
> +	pmdp_huge_get_and_clear(mm, vaddr, pmdp);
>  	pgtable = pgtable_trans_huge_withdraw(mm, pmdp);
>  }
>  
> @@ -319,6 +321,8 @@ static void __init pud_advanced_tests(struct mm_struct *mm,
>  	pudp_test_and_clear_young(vma, vaddr, pudp);
>  	pud = READ_ONCE(*pudp);
>  	WARN_ON(pud_young(pud));
> +
> +	pudp_huge_get_and_clear(mm, vaddr, pudp);
>  }
>  
>  static void __init pud_leaf_tests(unsigned long pfn, pgprot_t prot)
> @@ -442,8 +446,6 @@ static void __init pud_populate_tests(struct mm_struct *mm, pud_t *pudp,
>  	 * This entry points to next level page table page.
>  	 * Hence this must not qualify as pud_bad().
>  	 */
> -	pmd_clear(pmdp);
> -	pud_clear(pudp);
>  	pud_populate(mm, pudp, pmdp);
>  	pud = READ_ONCE(*pudp);
>  	WARN_ON(pud_bad(pud));
> @@ -575,7 +577,6 @@ static void __init pmd_populate_tests(struct mm_struct *mm, pmd_t *pmdp,
>  	 * This entry points to next level page table page.
>  	 * Hence this must not qualify as pmd_bad().
>  	 */
> -	pmd_clear(pmdp);
>  	pmd_populate(mm, pmdp, pgtable);
>  	pmd = READ_ONCE(*pmdp);
>  	WARN_ON(pmd_bad(pmd));
> 

Why pxxp_huge_get_and_clear() cannot be called inside pxx_populate_tests()
functions itself ? Nonetheless, this does not seem to cause any problem.