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

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Message-ID: <25653f37-6ba3-442f-9348-d879a8ad4704@redhat.com>
Date: Tue, 30 Sep 2025 16:43:13 +0200
From: David Hildenbrand <david@...hat.com>
To: Anshuman Khandual <anshuman.khandual@....com>, Dev Jain
 <dev.jain@....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 09:00, Anshuman Khandual wrote:
> 
> 
> On 30/09/25 12:11 PM, David Hildenbrand wrote:
>> 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.
>>
> 
> pxdp_get_lockless() not really required here, let's stick with pxdp_get() instead.

I'd suggest that we keep it consistent. That is, also removing the 
ptep_get_lockless() if not really required.

-- 
Cheers

David / dhildenb