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Message-ID: <584fea72-e564-4cf6-9435-ce4b03f2fd65@arm.com>
Date: Tue, 7 Oct 2025 12:41:22 +0530
From: Dev Jain <dev.jain@....com>
To: Anshuman Khandual <anshuman.khandual@....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 12:01 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@...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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