linux-kernel - Re: [PATCH] mm/ptdump: Replace READ_ONCE() with standard page table accessors

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Message-ID: <f0f0479a-fdab-4f42-9600-5d7b44a73f4e@redhat.com>
Date: Tue, 30 Sep 2025 08:41:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, Anshuman Khandual
 <anshuman.khandual@....com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/ptdump: Replace READ_ONCE() with standard page table
 accessors

On 30.09.25 06:37, Dev Jain wrote:
> 
> On 30/09/25 8:22 am, Anshuman Khandual wrote:
>> Replace READ_ONCE() with standard page table accessors i.e pxdp_get() which
>> anyways default into READ_ONCE() in cases where platform does not override.
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>    mm/ptdump.c | 8 ++++----
>>    1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/ptdump.c b/mm/ptdump.c
>> index b600c7f864b8..18861501b533 100644
>> --- a/mm/ptdump.c
>> +++ b/mm/ptdump.c
>> @@ -31,7 +31,7 @@ static int ptdump_pgd_entry(pgd_t *pgd, unsigned long addr,
>>    			    unsigned long next, struct mm_walk *walk)
>>    {
>>    	struct ptdump_state *st = walk->private;
>> -	pgd_t val = READ_ONCE(*pgd);
>> +	pgd_t val = pgdp_get(pgd);
>>    
>>    #if CONFIG_PGTABLE_LEVELS > 4 && \
>>    		(defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS))
>> @@ -54,7 +54,7 @@ static int ptdump_p4d_entry(p4d_t *p4d, unsigned long addr,
>>    			    unsigned long next, struct mm_walk *walk)
>>    {
>>    	struct ptdump_state *st = walk->private;
>> -	p4d_t val = READ_ONCE(*p4d);
>> +	p4d_t val = p4dp_get(p4d);
>>    
>>    #if CONFIG_PGTABLE_LEVELS > 3 && \
>>    		(defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS))
>> @@ -77,7 +77,7 @@ static int ptdump_pud_entry(pud_t *pud, unsigned long addr,
>>    			    unsigned long next, struct mm_walk *walk)
>>    {
>>    	struct ptdump_state *st = walk->private;
>> -	pud_t val = READ_ONCE(*pud);
>> +	pud_t val = pudp_get(pud);
>>    
>>    #if CONFIG_PGTABLE_LEVELS > 2 && \
>>    		(defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS))
>> @@ -100,7 +100,7 @@ static int ptdump_pmd_entry(pmd_t *pmd, unsigned long addr,
>>    			    unsigned long next, struct mm_walk *walk)
>>    {
>>    	struct ptdump_state *st = walk->private;
>> -	pmd_t val = READ_ONCE(*pmd);
>> +	pmd_t val = pmdp_get(pmd);
> 
> I believe this should go through pmdp_get_lockless(). I can see in pgtable.h that
> some magic is required on some arches to decode the pmd correctly in case walking
> without locks.

pmdp_get_lockless() is a nasty thingy to handle selected 32bit 
architectures.

But given that we're using ptep_get_lockless() in ptdump_pmd_entry() it 
probably wouldn't hurt to use pmdp_get_lockless() here.

Staring at ARCH_HAS_PTDUMP, I don't think any 32bit arch would actually 
end up compiling ptdump.c.

E.g., on x86 only X86_64 ends up selecting ARCH_HAS_PTDUMP.

-- 
Cheers

David / dhildenb