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

[Aneesh Kumar K.V]

On 8/13/20 10:57 AM, Anshuman Khandual wrote:
> 
> 
> 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.
> 

pmd_clear is implemented such that we don't use that to clear a huge pte 
entry. We use pmdp_huge_get_and_clear() for that. Hence we have check in 
pmd_clear which add a WARN if we find a _PAGE_PTE set on the entry.

In the test we follow a hugepmd usage with a pmd_clear. We should 
instead at the end of the advanced pmd test use pmdp_huge_get_and_clear().



>>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at 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 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));
>>