[PATCH 7.2 v16 03/13] mm/khugepaged: rework max_ptes_* handling with helper functions

Nico Pache
2026-04-19T18:59:45Z

The following cleanup reworks all the max_ptes_* handling into helper
functions. This increases the code readability and will later be used to
implement the mTHP handling of these variables.

With these changes we abstract all the madvise_collapse() special casing
(dont respect the sysctls) away from the functions that utilize them. And
will later in this series to cleanly restrict mTHP collapses behaviors.

Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Nico Pache <npache@redhat.com>
---
mm/khugepaged.c | 114 +++++++++++++++++++++++++++++++++---------------
1 file changed, 78 insertions(+), 36 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index afac6bc4e76d..f42b55421191 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -348,6 +348,58 @@ static bool pte_none_or_zero(pte_t pte)
return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
}

+/**
+ * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
+ * @cc: The collapse control struct
+ * @vma: The vma to check for userfaultfd
+ *
+ * If we are not in khugepaged mode use HPAGE_PMD_NR to allow any
+ * empty page.
+ *
+ * Return: Maximum number of empty PTEs allowed for the collapse operation
+ */
+static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
+ struct vm_area_struct *vma)
+{
+ if (vma && userfaultfd_armed(vma))
+ return 0;
+ if (!cc->is_khugepaged)
+ return HPAGE_PMD_NR;
+ return khugepaged_max_ptes_none;
+}
+
+/**
+ * collapse_max_ptes_shared - Calculate maximum allowed shared PTEs for collapse
+ * @cc: The collapse control struct
+ *
+ * If we are not in khugepaged mode use HPAGE_PMD_NR to allow any
+ * shared page.
+ *
+ * Return: Maximum number of shared PTEs allowed for the collapse operation
+ */
+static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
+{
+ if (!cc->is_khugepaged)
+ return HPAGE_PMD_NR;
+ return khugepaged_max_ptes_shared;
+}
+
+/**
+ * collapse_max_ptes_swap - Calculate maximum allowed swap PTEs for collapse
+ * @cc: The collapse control struct
+ *
+ * If we are not in khugepaged mode use HPAGE_PMD_NR to allow any
+ * swap page.
+ *
+ * Return: Maximum number of swap PTEs allowed for the collapse operation
+ */
+static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
+{
+ if (!cc->is_khugepaged)
+ return HPAGE_PMD_NR;
+ return khugepaged_max_ptes_swap;
+}
+
int hugepage_madvise(struct vm_area_struct *vma,
vm_flags_t *vm_flags, int advice)
{
@@ -546,21 +598,19 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
pte_t *_pte;
int none_or_zero = 0, shared = 0, referenced = 0;
enum scan_result result = SCAN_FAIL;
+ unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
+ unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);

for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
_pte++, addr += PAGE_SIZE) {
pte_t pteval = ptep_get(_pte);
if (pte_none_or_zero(pteval)) {
- ++none_or_zero;
- if (!userfaultfd_armed(vma) &&
- (!cc->is_khugepaged ||
- none_or_zero <= khugepaged_max_ptes_none)) {
- continue;
- } else {
+ if (++none_or_zero > max_ptes_none) {
result = SCAN_EXCEED_NONE_PTE;
count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
goto out;
}
+ continue;
}
if (!pte_present(pteval)) {
result = SCAN_PTE_NON_PRESENT;
@@ -591,9 +641,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,

/* See collapse_scan_pmd(). */
if (folio_maybe_mapped_shared(folio)) {
- ++shared;
- if (cc->is_khugepaged &&
- shared > khugepaged_max_ptes_shared) {
+ if (++shared > max_ptes_shared) {
result = SCAN_EXCEED_SHARED_PTE;
count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
goto out;
@@ -1270,6 +1318,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
unsigned long addr;
spinlock_t *ptl;
int node = NUMA_NO_NODE, unmapped = 0;
+ unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
+ unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
+ unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);

VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);

@@ -1294,36 +1345,29 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,

pte_t pteval = ptep_get(_pte);
if (pte_none_or_zero(pteval)) {
- ++none_or_zero;
- if (!userfaultfd_armed(vma) &&
- (!cc->is_khugepaged ||
- none_or_zero <= khugepaged_max_ptes_none)) {
- continue;
- } else {
+ if (++none_or_zero > max_ptes_none) {
result = SCAN_EXCEED_NONE_PTE;
count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
goto out_unmap;
}
+ continue;
}
if (!pte_present(pteval)) {
- ++unmapped;
- if (!cc->is_khugepaged ||
- unmapped <= khugepaged_max_ptes_swap) {
- /*
- * Always be strict with uffd-wp
- * enabled swap entries. Please see
- * comment below for pte_uffd_wp().
- */
- if (pte_swp_uffd_wp_any(pteval)) {
- result = SCAN_PTE_UFFD_WP;
- goto out_unmap;
- }
- continue;
- } else {
+ if (++unmapped > max_ptes_swap) {
result = SCAN_EXCEED_SWAP_PTE;
count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
goto out_unmap;
}
+ /*
+ * Always be strict with uffd-wp
+ * enabled swap entries. Please see
+ * comment below for pte_uffd_wp().
+ */
+ if (pte_swp_uffd_wp_any(pteval)) {
+ result = SCAN_PTE_UFFD_WP;
+ goto out_unmap;
+ }
+ continue;
}
if (pte_uffd_wp(pteval)) {
/*
@@ -1366,9 +1410,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
* is shared.
*/
if (folio_maybe_mapped_shared(folio)) {
- ++shared;
- if (cc->is_khugepaged &&
- shared > khugepaged_max_ptes_shared) {
+ if (++shared > max_ptes_shared) {
result = SCAN_EXCEED_SHARED_PTE;
count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
goto out_unmap;
@@ -2329,6 +2371,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
int present, swap;
int node = NUMA_NO_NODE;
enum scan_result result = SCAN_SUCCEED;
+ unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
+ unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);

present = 0;
swap = 0;
@@ -2341,8 +2385,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,

if (xa_is_value(folio)) {
swap += 1 << xas_get_order(&xas);
- if (cc->is_khugepaged &&
- swap > khugepaged_max_ptes_swap) {
+ if (swap > max_ptes_swap) {
result = SCAN_EXCEED_SWAP_PTE;
count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
break;
@@ -2413,8 +2456,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
cc->progress += HPAGE_PMD_NR;

if (result == SCAN_SUCCEED) {
- if (cc->is_khugepaged &&
- present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
+ if (present < HPAGE_PMD_NR - max_ptes_none) {
result = SCAN_EXCEED_NONE_PTE;
count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
} else {
--
2.53.0