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

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.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 13:08:08 +0530	[thread overview]
Message-ID: <e3140b44-993e-aa4b-130d-ee2230eff2b5@arm.com> (raw)
In-Reply-To: <abef1791-8779-6b34-3178-3bf3ab36d42b@linux.ibm.com>

On 09/01/2020 12:07 PM, Aneesh Kumar K.V wrote:
> 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.

In that case, please move above two lines i.e

pte = pfn_pte(pte_aligned, prot);
set_pte_at(mm, vaddr, ptep, pte);

from debug_vm_pgtable() to pte_clear_tests() and update it's arguments
as required.