Re: [PATCH 13/16] debug_vm_pgtable/pmd_clear: Don't use pmd/pud_clear on pte entries

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On 08/12/2020 12:03 PM, Aneesh Kumar K.V wrote:
> pmd_clear() should not be used to clear pmd level pte entries.

Could you please elaborate on this. The proposed change set does
not match the description here.

> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> ---
>  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 061c19bba7f0..529892b9be2f 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -191,6 +191,8 @@ 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);
>  }
>  
> @@ -313,6 +315,8 @@ 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)
> @@ -431,8 +435,6 @@ 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);

Both entires are cleared before creating a fresh page table entry.
Why that is a problem.

>  	pud_populate(mm, pudp, pmdp);
>  	pud = READ_ONCE(*pudp);
>  	WARN_ON(pud_bad(pud));
> @@ -564,7 +566,6 @@ 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);

Ditto.

>  	pmd_populate(mm, pmdp, pgtable);
>  	pmd = READ_ONCE(*pmdp);
>  	WARN_ON(pmd_bad(pmd));
> 




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