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Message-ID: <627da4d5-5c6b-4bf4-9544-9021062d378e@arm.com>
Date: Tue, 7 Oct 2025 13:24:33 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Dev Jain <dev.jain@....com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Replace READ_ONCE() with standard page table
accessors
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@...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/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);
>>
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