Re: [PATCH 4/5] mm/page_vma_mapped: use huge_ptep_get() for hugetlb

[Lance Yang <lance.yang@linux.dev>]

On Thu, Jun 25, 2026 at 11:29:53AM +0000, Dev Jain wrote:
>check_pte() is the final validation step in page_vma_mapped_walk().
>It reads pvmw->pte with ptep_get() to decide whether the entry maps
>the PFN range being walked. For hugetlb VMAs, that pointer refers
>to a hugetlb entry.
>
>On arches which provide their own huge_ptep_get() to dereference a huge
>pte pointer, accessing via ptep_get() would cause pte_pfn(),
>pte_present() etc to misbehave.
>
>It is not clear whether this has a trivially visible effect to userspace.
>
>Use huge_ptep_get() to dereference a huge pte pointer.
>
>Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
>Cc: stable@vger.kernel.org
>Signed-off-by: Dev Jain <dev.jain@arm.com>
>---
> mm/page_vma_mapped.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
>diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>index 2ccbabfb2cc17..18e1d341f463c 100644
>--- a/mm/page_vma_mapped.c
>+++ b/mm/page_vma_mapped.c
>@@ -107,7 +107,13 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
> {
> 	unsigned long pfn;
>-	pte_t ptent = ptep_get(pvmw->pte);
>+	pte_t ptent;
>+
>+	if (is_vm_hugetlb_page(pvmw->vma))
>+		ptent = huge_ptep_get(pvmw->vma->vm_mm, pvmw->address,
>+				      pvmw->pte);

I think check_pte() can pass a wrong address to huge_ptep_get() ...

Not sure that is wrong in the first place. For memory failure,
page_mapped_in_vma() can be called with a poisoned tail page of a hugetlb
folio. In that case, pvmw->address need not be hugepage-aligned.

@Miaohe

For arm64, CONT_PMD_SIZE is one supported HugeTLB size. With such a VMA,
page_vma_mapped_walk() passes that size to hugetlb_walk():

bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
{
	...
	if (unlikely(is_vm_hugetlb_page(vma))) {
		...
		pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
		...
	}
	...
}

hugetlb_walk() then calls arm64 huge_pte_offset(mm, addr, sz). For
sz == CONT_PMD_SIZE, huge_pte_offset() aligns its local addr before
calculating pmdp:

pte_t *huge_pte_offset(struct mm_struct *mm,
		       unsigned long addr, unsigned long sz)
{
	...
	if (sz == CONT_PMD_SIZE)
		addr &= CONT_PMD_MASK;

	pmdp = pmd_offset(pudp, addr);
	pmd = READ_ONCE(*pmdp);
	...
}

So for that case, pvmw->pte is calculated from the aligned addr, not
necessarily from the original pvmw->address. But check_pte() passes the
original address together with pvmw->pte:

+		ptent = huge_ptep_get(pvmw->vma->vm_mm, pvmw->address,
+				      pvmw->pte);

arm64 then uses that addr again to choose ncontig:

pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
{
	...
	ncontig = find_num_contig(mm, addr, ptep, &pgsize);
	for (i = 0; i < ncontig; i++, ptep++) {
		...
	}
	return orig_pte;
}

static int find_num_contig(struct mm_struct *mm, unsigned long addr,
			   pte_t *ptep, size_t *pgsize)
{
	pgd_t *pgdp = pgd_offset(mm, addr);
	p4d_t *p4dp;
	pud_t *pudp;
	pmd_t *pmdp;

	*pgsize = PAGE_SIZE;
	p4dp = p4d_offset(pgdp, addr);
	pudp = pud_offset(p4dp, addr);
	pmdp = pmd_offset(pudp, addr);
	if ((pte_t *)pmdp == ptep) {
		*pgsize = PMD_SIZE;
		return CONT_PMDS;
	}
	return CONT_PTES;
}

With a tail address, pmdp may no longer point at pvmw->pte, so
find_num_contig() can return CONT_PTES for a CONT_PMD HugeTLB mapping.

On 16K arm64, that changes ncontig from 32 to 128. So huge_ptep_get()
can walk past the CONT_PMD entries, and possibly past the PMD table.

Should check_pte() pass the address matching pvmw->pte, sth like:

---8<---
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 406fd50bbd8f..58463493bd3d 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -109,11 +109,14 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
 	unsigned long pfn;
 	pte_t ptent;

-	if (is_vm_hugetlb_page(pvmw->vma))
-		ptent = huge_ptep_get(pvmw->vma->vm_mm, pvmw->address,
-				      pvmw->pte);
-	else
+	if (is_vm_hugetlb_page(pvmw->vma)) {
+		struct hstate *hstate = hstate_vma(pvmw->vma);
+		unsigned long haddr = pvmw->address & huge_page_mask(hstate);
+
+		ptent = huge_ptep_get(pvmw->vma->vm_mm, haddr, pvmw->pte);
+	} else {
 		ptent = ptep_get(pvmw->pte);
+	}

 	if (pvmw->flags & PVMW_MIGRATION) {
 		const softleaf_t entry = softleaf_from_pte(ptent);
--

while leaving pvmw->address unchanged for page_mapped_in_vma()?

Cheers, Lance

>+	else
>+		ptent = ptep_get(pvmw->pte);
> 
> 	if (pvmw->flags & PVMW_MIGRATION) {
> 		const softleaf_t entry = softleaf_from_pte(ptent);
>-- 
>2.43.0
>
>