[PATCH v3 13/13] mm/debug_vm_pgtable: populate a pte entry before fetching it

[Anshuman Khandual]

On 08/27/2020 01:34 PM, Aneesh Kumar K.V wrote:
> pte_clear_tests operate on an existing pte entry. Make sure that is not a none
> pte entry.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
> ---
>  mm/debug_vm_pgtable.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 21329c7d672f..8527ebb75f2c 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -546,7 +546,7 @@ static void __init pgd_populate_tests(struct mm_struct *mm, pgd_t *pgdp,
>  static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep,
>  				   unsigned long vaddr)
>  {
> -	pte_t pte = ptep_get(ptep);
> +	pte_t pte =  ptep_get_and_clear(mm, vaddr, ptep);

Seems like ptep_get_and_clear() here just clears the entry in preparation
for a following set_pte_at() which otherwise would have been a problem on
ppc64 as you had pointed out earlier i.e set_pte_at() should not update an
existing valid entry. So the commit message here is bit misleading.

>  
>  	pr_debug("Validating PTE clear\n");
>  	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
> @@ -944,7 +944,7 @@ static int __init debug_vm_pgtable(void)
>  	p4d_t *p4dp, *saved_p4dp;
>  	pud_t *pudp, *saved_pudp;
>  	pmd_t *pmdp, *saved_pmdp, pmd;
> -	pte_t *ptep;
> +	pte_t *ptep, pte;
>  	pgtable_t saved_ptep;
>  	pgprot_t prot, protnone;
>  	phys_addr_t paddr;
> @@ -1049,6 +1049,8 @@ static int __init debug_vm_pgtable(void)
>  	 */
>  
>  	ptep = pte_alloc_map_lock(mm, pmdp, vaddr, &ptl);
> +	pte = pfn_pte(pte_aligned, prot);
> +	set_pte_at(mm, vaddr, ptep, pte);

Not here, creating and populating an entry must be done in respective
test functions itself. Besides, this seems bit redundant as well. The
test pte_clear_tests() with the above change added, already

- Clears the PTEP entry with ptep_get_and_clear()
- Creates and populates the PTEP with set_pte_at()
- Clears with pte_clear()
- Checks for pte_none()

If the objective is to clear the PTEP entry before calling set_pte_at(),
then only the first chunk is required and it should also be merged with
a previous patch. 

[PATCH v3 07/13] mm/debug_vm_pgtable/set_pte/pmd/pud: Don't use set_*_at to update an existing pte entry 


>  	pte_clear_tests(mm, ptep, vaddr);
>  	pte_advanced_tests(mm, vma, ptep, pte_aligned, vaddr, prot);
>  	pte_unmap_unlock(ptep, ptl);
> 

There is a checkpatch.pl warning here.

WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)
#7: 
pte_clear_tests operate on an existing pte entry. Make sure that is not a none

total: 0 errors, 1 warnings, 24 lines checked

There is also a build failure on x86 reported from kernel test robot.