[PATCH v12 mm-new 05/15] khugepaged: generalize __collapse_huge_page_* for mTHP support

Nico Pache
2025-10-22T18:39:17Z

generalize the order of the __collapse_huge_page_* functions
to support future mTHP collapse.

mTHP collapse will not honor the khugepaged_max_ptes_shared or
khugepaged_max_ptes_swap parameters, and will fail if it encounters a
shared or swapped entry.

No functional changes in this patch.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
mm/khugepaged.c | 78 ++++++++++++++++++++++++++++++-------------------
1 file changed, 48 insertions(+), 30 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 36ee659acfbb..4ccebf5dda97 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -537,25 +537,25 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
}

static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
- unsigned long start_addr,
- pte_t *pte,
- struct collapse_control *cc,
- struct list_head *compound_pagelist)
+ unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
+ unsigned int order, struct list_head *compound_pagelist)
{
struct page *page = NULL;
struct folio *folio = NULL;
unsigned long addr = start_addr;
pte_t *_pte;
int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
+ const unsigned long nr_pages = 1UL << order;
+ int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);

- for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
+ for (_pte = pte; _pte < pte + nr_pages;
_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)) {
+ none_or_zero <= max_ptes_none)) {
continue;
} else {
result = SCAN_EXCEED_NONE_PTE;
@@ -583,8 +583,14 @@ static int __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) {
+ /*
+ * TODO: Support shared pages without leading to further
+ * mTHP collapses. Currently bringing in new pages via
+ * shared may cause a future higher order collapse on a
+ * rescan of the same range.
+ */
+ if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
+ shared > khugepaged_max_ptes_shared)) {
result = SCAN_EXCEED_SHARED_PTE;
count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
goto out;
@@ -677,18 +683,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
}

static void __collapse_huge_page_copy_succeeded(pte_t *pte,
- struct vm_area_struct *vma,
- unsigned long address,
- spinlock_t *ptl,
- struct list_head *compound_pagelist)
+ struct vm_area_struct *vma, unsigned long address,
+ spinlock_t *ptl, unsigned int order,
+ struct list_head *compound_pagelist)
{
- unsigned long end = address + HPAGE_PMD_SIZE;
+ unsigned long end = address + (PAGE_SIZE << order);
struct folio *src, *tmp;
pte_t pteval;
pte_t *_pte;
unsigned int nr_ptes;
+ const unsigned long nr_pages = 1UL << order;

- for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
+ for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes,
address += nr_ptes * PAGE_SIZE) {
nr_ptes = 1;
pteval = ptep_get(_pte);
@@ -741,13 +747,11 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
}

static void __collapse_huge_page_copy_failed(pte_t *pte,
- pmd_t *pmd,
- pmd_t orig_pmd,
- struct vm_area_struct *vma,
- struct list_head *compound_pagelist)
+ pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
+ unsigned int order, struct list_head *compound_pagelist)
{
spinlock_t *pmd_ptl;
-
+ const unsigned long nr_pages = 1UL << order;
/*
* Re-establish the PMD to point to the original page table
* entry. Restoring PMD needs to be done prior to releasing
@@ -761,7 +765,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
* Release both raw and compound pages isolated
* in __collapse_huge_page_isolate.
*/
- release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
+ release_pte_pages(pte, pte + nr_pages, compound_pagelist);
}

/*
@@ -781,16 +785,16 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
*/
static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
- unsigned long address, spinlock_t *ptl,
+ unsigned long address, spinlock_t *ptl, unsigned int order,
struct list_head *compound_pagelist)
{
unsigned int i;
int result = SCAN_SUCCEED;
-
+ const unsigned long nr_pages = 1UL << order;
/*
* Copying pages' contents is subject to memory poison at any iteration.
*/
- for (i = 0; i < HPAGE_PMD_NR; i++) {
+ for (i = 0; i < nr_pages; i++) {
pte_t pteval = ptep_get(pte + i);
struct page *page = folio_page(folio, i);
unsigned long src_addr = address + i * PAGE_SIZE;
@@ -809,10 +813,10 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,

if (likely(result == SCAN_SUCCEED))
__collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
- compound_pagelist);
+ order, compound_pagelist);
else
__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
- compound_pagelist);
+ order, compound_pagelist);

return result;
}
@@ -985,13 +989,12 @@ static int check_pmd_still_valid(struct mm_struct *mm,
* Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
*/
static int __collapse_huge_page_swapin(struct mm_struct *mm,
- struct vm_area_struct *vma,
- unsigned long start_addr, pmd_t *pmd,
- int referenced)
+ struct vm_area_struct *vma, unsigned long start_addr,
+ pmd_t *pmd, int referenced, unsigned int order)
{
int swapped_in = 0;
vm_fault_t ret = 0;
- unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE);
+ unsigned long addr, end = start_addr + (PAGE_SIZE << order);
int result;
pte_t *pte = NULL;
spinlock_t *ptl;
@@ -1022,6 +1025,19 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
if (!is_swap_pte(vmf.orig_pte))
continue;

+ /*
+ * TODO: Support swapin without leading to further mTHP
+ * collapses. Currently bringing in new pages via swapin may
+ * cause a future higher order collapse on a rescan of the same
+ * range.
+ */
+ if (order != HPAGE_PMD_ORDER) {
+ pte_unmap(pte);
+ mmap_read_unlock(mm);
+ result = SCAN_EXCEED_SWAP_PTE;
+ goto out;
+ }
+
vmf.pte = pte;
vmf.ptl = ptl;
ret = do_swap_page(&vmf);
@@ -1142,7 +1158,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
* that case. Continuing to collapse causes inconsistency.
*/
result = __collapse_huge_page_swapin(mm, vma, address, pmd,
- referenced);
+ referenced, HPAGE_PMD_ORDER);
if (result != SCAN_SUCCEED)
goto out_nolock;
}
@@ -1190,6 +1206,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
if (pte) {
result = __collapse_huge_page_isolate(vma, address, pte, cc,
+ HPAGE_PMD_ORDER,
&compound_pagelist);
spin_unlock(pte_ptl);
} else {
@@ -1220,6 +1237,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,

result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
vma, address, pte_ptl,
+ HPAGE_PMD_ORDER,
&compound_pagelist);
pte_unmap(pte);
if (unlikely(result != SCAN_SUCCEED))
--
2.51.0