[PATCH] mm: Replace READ_ONCE() with standard page table accessors

[PATCH] mm: Replace READ_ONCE() with standard page table accessors

[Anshuman Khandual]

Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get()
that defaults into READ_ONCE() in cases where platform does not override.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 mm/gup.c            | 10 +++++-----
 mm/hmm.c            |  2 +-
 mm/memory.c         |  4 ++--
 mm/mprotect.c       |  2 +-
 mm/sparse-vmemmap.c |  2 +-
 mm/vmscan.c         |  2 +-
 6 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 0bc4d140fc07..37e2af5ed96d 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -964,7 +964,7 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
 	struct mm_struct *mm = vma->vm_mm;
 
 	pudp = pud_offset(p4dp, address);
-	pud = READ_ONCE(*pudp);
+	pud = pudp_get(pudp);
 	if (!pud_present(pud))
 		return no_page_table(vma, flags, address);
 	if (pud_leaf(pud)) {
@@ -989,7 +989,7 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
 	p4d_t *p4dp, p4d;
 
 	p4dp = p4d_offset(pgdp, address);
-	p4d = READ_ONCE(*p4dp);
+	p4d = p4dp_get(p4dp);
 	BUILD_BUG_ON(p4d_leaf(p4d));
 
 	if (!p4d_present(p4d) || p4d_bad(p4d))
@@ -3080,7 +3080,7 @@ static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,
 
 	pudp = pud_offset_lockless(p4dp, p4d, addr);
 	do {
-		pud_t pud = READ_ONCE(*pudp);
+		pud_t pud = pudp_get(pudp);
 
 		next = pud_addr_end(addr, end);
 		if (unlikely(!pud_present(pud)))
@@ -3106,7 +3106,7 @@ static int gup_fast_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr,
 
 	p4dp = p4d_offset_lockless(pgdp, pgd, addr);
 	do {
-		p4d_t p4d = READ_ONCE(*p4dp);
+		p4d_t p4d = p4dp_get(p4dp);
 
 		next = p4d_addr_end(addr, end);
 		if (!p4d_present(p4d))
@@ -3128,7 +3128,7 @@ static void gup_fast_pgd_range(unsigned long addr, unsigned long end,
 
 	pgdp = pgd_offset(current->mm, addr);
 	do {
-		pgd_t pgd = READ_ONCE(*pgdp);
+		pgd_t pgd = pgdp_get(pgdp);
 
 		next = pgd_addr_end(addr, end);
 		if (pgd_none(pgd))
diff --git a/mm/hmm.c b/mm/hmm.c
index d545e2494994..126c3f42e525 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -431,7 +431,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 	/* Normally we don't want to split the huge page */
 	walk->action = ACTION_CONTINUE;
 
-	pud = READ_ONCE(*pudp);
+	pud = pudp_get(pudp);
 	if (!pud_present(pud)) {
 		spin_unlock(ptl);
 		return hmm_vma_walk_hole(start, end, -1, walk);
diff --git a/mm/memory.c b/mm/memory.c
index 0ba4f6b71847..50f841ee6e84 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6549,12 +6549,12 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 		goto out;
 
 	p4dp = p4d_offset(pgdp, address);
-	p4d = READ_ONCE(*p4dp);
+	p4d = p4dp_get(p4dp);
 	if (p4d_none(p4d) || unlikely(p4d_bad(p4d)))
 		goto out;
 
 	pudp = pud_offset(p4dp, address);
-	pud = READ_ONCE(*pudp);
+	pud = pudp_get(pudp);
 	if (pud_none(pud))
 		goto out;
 	if (pud_leaf(pud)) {
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 113b48985834..988c366137d5 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -599,7 +599,7 @@ static inline long change_pud_range(struct mmu_gather *tlb,
 			break;
 		}
 
-		pud = READ_ONCE(*pudp);
+		pud = pudp_get(pudp);
 		if (pud_none(pud))
 			continue;
 
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index dbd8daccade2..37522d6cb398 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -439,7 +439,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
 			return -ENOMEM;
 
 		pmd = pmd_offset(pud, addr);
-		if (pmd_none(READ_ONCE(*pmd))) {
+		if (pmd_none(pmdp_get(pmd))) {
 			void *p;
 
 			p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 674999999cd0..14c2722b955b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3772,7 +3772,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end,
 	pud = pud_offset(p4d, start & P4D_MASK);
 restart:
 	for (i = pud_index(start), addr = start; addr != end; i++, addr = next) {
-		pud_t val = READ_ONCE(pud[i]);
+		pud_t val = pudp_get(pud + i);
 
 		next = pud_addr_end(addr, end);
 
-- 
2.30.2

Re: [PATCH] mm: Replace READ_ONCE() with standard page table accessors

[Dev Jain]

On 07/10/25 12:01 pm, Anshuman Khandual wrote:
> Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get()
> that defaults into READ_ONCE() in cases where platform does not override.

Does any platform override into something else currently? The way you write
the description implies that.

>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>   mm/gup.c            | 10 +++++-----
>   mm/hmm.c            |  2 +-
>   mm/memory.c         |  4 ++--
>   mm/mprotect.c       |  2 +-
>   mm/sparse-vmemmap.c |  2 +-
>   mm/vmscan.c         |  2 +-
>   6 files changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 0bc4d140fc07..37e2af5ed96d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -964,7 +964,7 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
>   	struct mm_struct *mm = vma->vm_mm;
>   
>   	pudp = pud_offset(p4dp, address);
> -	pud = READ_ONCE(*pudp);
> +	pud = pudp_get(pudp);
>   	if (!pud_present(pud))
>   		return no_page_table(vma, flags, address);
>   	if (pud_leaf(pud)) {
> @@ -989,7 +989,7 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
>   	p4d_t *p4dp, p4d;
>   
>   	p4dp = p4d_offset(pgdp, address);
> -	p4d = READ_ONCE(*p4dp);
> +	p4d = p4dp_get(p4dp);
>   	BUILD_BUG_ON(p4d_leaf(p4d));
>   
>   	if (!p4d_present(p4d) || p4d_bad(p4d))
> @@ -3080,7 +3080,7 @@ static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,
>   
>   	pudp = pud_offset_lockless(p4dp, p4d, addr);
>   	do {
> -		pud_t pud = READ_ONCE(*pudp);
> +		pud_t pud = pudp_get(pudp);
>   
>   		next = pud_addr_end(addr, end);
>   		if (unlikely(!pud_present(pud)))
> @@ -3106,7 +3106,7 @@ static int gup_fast_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr,
>   
>   	p4dp = p4d_offset_lockless(pgdp, pgd, addr);
>   	do {
> -		p4d_t p4d = READ_ONCE(*p4dp);
> +		p4d_t p4d = p4dp_get(p4dp);
>   
>   		next = p4d_addr_end(addr, end);
>   		if (!p4d_present(p4d))
> @@ -3128,7 +3128,7 @@ static void gup_fast_pgd_range(unsigned long addr, unsigned long end,
>   
>   	pgdp = pgd_offset(current->mm, addr);
>   	do {
> -		pgd_t pgd = READ_ONCE(*pgdp);
> +		pgd_t pgd = pgdp_get(pgdp);
>   
>   		next = pgd_addr_end(addr, end);
>   		if (pgd_none(pgd))
> diff --git a/mm/hmm.c b/mm/hmm.c
> index d545e2494994..126c3f42e525 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -431,7 +431,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
>   	/* Normally we don't want to split the huge page */
>   	walk->action = ACTION_CONTINUE;
>   
> -	pud = READ_ONCE(*pudp);
> +	pud = pudp_get(pudp);
>   	if (!pud_present(pud)) {
>   		spin_unlock(ptl);
>   		return hmm_vma_walk_hole(start, end, -1, walk);
> diff --git a/mm/memory.c b/mm/memory.c
> index 0ba4f6b71847..50f841ee6e84 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6549,12 +6549,12 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
>   		goto out;
>   
>   	p4dp = p4d_offset(pgdp, address);
> -	p4d = READ_ONCE(*p4dp);
> +	p4d = p4dp_get(p4dp);
>   	if (p4d_none(p4d) || unlikely(p4d_bad(p4d)))
>   		goto out;
>   
>   	pudp = pud_offset(p4dp, address);
> -	pud = READ_ONCE(*pudp);
> +	pud = pudp_get(pudp);
>   	if (pud_none(pud))
>   		goto out;
>   	if (pud_leaf(pud)) {
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 113b48985834..988c366137d5 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -599,7 +599,7 @@ static inline long change_pud_range(struct mmu_gather *tlb,
>   			break;
>   		}
>   
> -		pud = READ_ONCE(*pudp);
> +		pud = pudp_get(pudp);
>   		if (pud_none(pud))
>   			continue;
>   
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index dbd8daccade2..37522d6cb398 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -439,7 +439,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>   			return -ENOMEM;
>   
>   		pmd = pmd_offset(pud, addr);
> -		if (pmd_none(READ_ONCE(*pmd))) {
> +		if (pmd_none(pmdp_get(pmd))) {

I believe sparse-vmemmap is only for 64 bit arches so we are safe.

>   			void *p;
>   
>   			p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 674999999cd0..14c2722b955b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3772,7 +3772,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end,
>   	pud = pud_offset(p4d, start & P4D_MASK);
>   restart:
>   	for (i = pud_index(start), addr = start; addr != end; i++, addr = next) {
> -		pud_t val = READ_ONCE(pud[i]);
> +		pud_t val = pudp_get(pud + i);
>   
>   		next = pud_addr_end(addr, end);
>   

Re: [PATCH] mm: Replace READ_ONCE() with standard page table accessors

[Anshuman Khandual]

On 07/10/25 12:41 PM, Dev Jain wrote:
> 
> On 07/10/25 12:01 pm, Anshuman Khandual wrote:
>> Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get()
>> that defaults into READ_ONCE() in cases where platform does not override.
> 
> Does any platform override into something else currently? The way you write
> the description implies that.

That's how the callbacks have been designed to be overridden when required.
> 
>>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>>   mm/gup.c            | 10 +++++-----
>>   mm/hmm.c            |  2 +-
>>   mm/memory.c         |  4 ++--
>>   mm/mprotect.c       |  2 +-
>>   mm/sparse-vmemmap.c |  2 +-
>>   mm/vmscan.c         |  2 +-
>>   6 files changed, 11 insertions(+), 11 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 0bc4d140fc07..37e2af5ed96d 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -964,7 +964,7 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
>>       struct mm_struct *mm = vma->vm_mm;
>>         pudp = pud_offset(p4dp, address);
>> -    pud = READ_ONCE(*pudp);
>> +    pud = pudp_get(pudp);
>>       if (!pud_present(pud))
>>           return no_page_table(vma, flags, address);
>>       if (pud_leaf(pud)) {
>> @@ -989,7 +989,7 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
>>       p4d_t *p4dp, p4d;
>>         p4dp = p4d_offset(pgdp, address);
>> -    p4d = READ_ONCE(*p4dp);
>> +    p4d = p4dp_get(p4dp);
>>       BUILD_BUG_ON(p4d_leaf(p4d));
>>         if (!p4d_present(p4d) || p4d_bad(p4d))
>> @@ -3080,7 +3080,7 @@ static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,
>>         pudp = pud_offset_lockless(p4dp, p4d, addr);
>>       do {
>> -        pud_t pud = READ_ONCE(*pudp);
>> +        pud_t pud = pudp_get(pudp);
>>             next = pud_addr_end(addr, end);
>>           if (unlikely(!pud_present(pud)))
>> @@ -3106,7 +3106,7 @@ static int gup_fast_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr,
>>         p4dp = p4d_offset_lockless(pgdp, pgd, addr);
>>       do {
>> -        p4d_t p4d = READ_ONCE(*p4dp);
>> +        p4d_t p4d = p4dp_get(p4dp);
>>             next = p4d_addr_end(addr, end);
>>           if (!p4d_present(p4d))
>> @@ -3128,7 +3128,7 @@ static void gup_fast_pgd_range(unsigned long addr, unsigned long end,
>>         pgdp = pgd_offset(current->mm, addr);
>>       do {
>> -        pgd_t pgd = READ_ONCE(*pgdp);
>> +        pgd_t pgd = pgdp_get(pgdp);
>>             next = pgd_addr_end(addr, end);
>>           if (pgd_none(pgd))
>> diff --git a/mm/hmm.c b/mm/hmm.c
>> index d545e2494994..126c3f42e525 100644
>> --- a/mm/hmm.c
>> +++ b/mm/hmm.c
>> @@ -431,7 +431,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
>>       /* Normally we don't want to split the huge page */
>>       walk->action = ACTION_CONTINUE;
>>   -    pud = READ_ONCE(*pudp);
>> +    pud = pudp_get(pudp);
>>       if (!pud_present(pud)) {
>>           spin_unlock(ptl);
>>           return hmm_vma_walk_hole(start, end, -1, walk);
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 0ba4f6b71847..50f841ee6e84 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6549,12 +6549,12 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
>>           goto out;
>>         p4dp = p4d_offset(pgdp, address);
>> -    p4d = READ_ONCE(*p4dp);
>> +    p4d = p4dp_get(p4dp);
>>       if (p4d_none(p4d) || unlikely(p4d_bad(p4d)))
>>           goto out;
>>         pudp = pud_offset(p4dp, address);
>> -    pud = READ_ONCE(*pudp);
>> +    pud = pudp_get(pudp);
>>       if (pud_none(pud))
>>           goto out;
>>       if (pud_leaf(pud)) {
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index 113b48985834..988c366137d5 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -599,7 +599,7 @@ static inline long change_pud_range(struct mmu_gather *tlb,
>>               break;
>>           }
>>   -        pud = READ_ONCE(*pudp);
>> +        pud = pudp_get(pudp);
>>           if (pud_none(pud))
>>               continue;
>>   diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index dbd8daccade2..37522d6cb398 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -439,7 +439,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>>               return -ENOMEM;
>>             pmd = pmd_offset(pud, addr);
>> -        if (pmd_none(READ_ONCE(*pmd))) {
>> +        if (pmd_none(pmdp_get(pmd))) {
> 
> I believe sparse-vmemmap is only for 64 bit arches so we are safe.
> 
>>               void *p;
>>                 p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 674999999cd0..14c2722b955b 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -3772,7 +3772,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end,
>>       pud = pud_offset(p4d, start & P4D_MASK);
>>   restart:
>>       for (i = pud_index(start), addr = start; addr != end; i++, addr = next) {
>> -        pud_t val = READ_ONCE(pud[i]);
>> +        pud_t val = pudp_get(pud + i);
>>             next = pud_addr_end(addr, end);
>>   

Re: [PATCH] mm: Replace READ_ONCE() with standard page table accessors

[Dev Jain]

On 07/10/25 1:24 pm, Anshuman Khandual wrote:
>
> On 07/10/25 12:41 PM, Dev Jain wrote:
>> On 07/10/25 12:01 pm, Anshuman Khandual wrote:
>>> Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get()
>>> that defaults into READ_ONCE() in cases where platform does not override.
>> Does any platform override into something else currently? The way you write
>> the description implies that.
> That's how the callbacks have been designed to be overridden when required.

What I mean is, if currently some arch is overriding into something else,
this patch will be a functional change. So it is beneficial to say
in the description that the patch is a safeguard for when the platform
chooses to override, and no functional change is intended.

Reviewed-by: Dev Jain <dev.jain@arm.com>

>>

Re: [PATCH] mm: Replace READ_ONCE() with standard page table accessors

[David Hildenbrand]

On 07.10.25 08:31, Anshuman Khandual wrote:
> Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get()
> that defaults into READ_ONCE() in cases where platform does not override.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb

Re: [PATCH] mm: Replace READ_ONCE() with standard page table accessors

[Lance Yang]

On Tue, Oct 7, 2025 at 2:31 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
> Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get()
> that defaults into READ_ONCE() in cases where platform does not override.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---

LGTM. I still assume it is a no-op change :)

Reviewed-by: Lance Yang <lance.yang@linux.dev>

Re: [PATCH] mm: Replace READ_ONCE() with standard page table accessors

[Wei Yang]

On Tue, Oct 07, 2025 at 07:31:00AM +0100, Anshuman Khandual wrote:
>Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get()
>that defaults into READ_ONCE() in cases where platform does not override.
>
>Cc: Andrew Morton <akpm@linux-foundation.org>
>Cc: David Hildenbrand <david@redhat.com>
>Cc: linux-mm@kvack.org
>Cc: linux-kernel@vger.kernel.org
>Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

-- 
Wei Yang
Help you, Help me