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

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	linux-mm@kvack.org, akpm@linux-foundation.org
Cc: mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org, linux-s390@vger.kernel.org,
	linux-snps-arc@lists.infradead.org, x86@kernel.org,
	linux-arch@vger.kernel.org,
	Gerald Schaefer <gerald.schaefer@de.ibm.com>,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	Vineet Gupta <vgupta@synopsys.com>,
	Mike Rapoport <rppt@linux.ibm.com>, Qian Cai <cai@lca.pw>
Subject: Re: [PATCH v3 13/13] mm/debug_vm_pgtable: populate a pte entry before fetching it
Date: Tue, 1 Sep 2020 12:07:10 +0530	[thread overview]
Message-ID: <abef1791-8779-6b34-3178-3bf3ab36d42b@linux.ibm.com> (raw)
In-Reply-To: <edc68223-7f8a-13df-68eb-9682f585adb8@arm.com>

On 9/1/20 8:55 AM, Anshuman Khandual wrote:
> 
> 
> 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@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.
> 

and also fetch the pte value which is used further.

>>   
>>   	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()

and fetch the old value set previously.

> - 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.
> 