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

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Message-ID: <1f3c8fe8-cff0-4e3e-bea8-285b00fc7a5d@arm.com>
Date: Tue, 30 Sep 2025 12:30:06 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: David Hildenbrand <david@...hat.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 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.