[PATCH 7.2 v16 04/13] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

Nico Pache
2026-04-19T19:00:04Z

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

The current mechanism for determining collapse with the
khugepaged_max_ptes_none value is not designed with mTHP in mind. This
raises a key design issue: if we support user defined max_pte_none values
(even those scaled by order), a collapse of a lower order can introduces
an feedback loop, or "creep", when max_ptes_none is set to a value greater
than HPAGE_PMD_NR / 2.

With this configuration, a successful collapse to order N will populate
enough pages to satisfy the collapse condition on order N+1 on the next
scan. This leads to unnecessary work and memory churn.

To fix this issue introduce a helper function that will limit mTHP
collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
This effectively supports two modes:

- max_ptes_none=0: never introduce new none-pages for mTHP collapse.
- max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
available mTHP order.

This removes the possiblilty of "creep", while not modifying any uAPI
expectations. A warning will be emitted if any non-supported
max_ptes_none value is configured with mTHP enabled.

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; however it defines future behavior
for mTHP collapse.

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 | 124 ++++++++++++++++++++++++++++++++++--------------
1 file changed, 88 insertions(+), 36 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f42b55421191..283bb63854a5 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -352,51 +352,86 @@ static bool pte_none_or_zero(pte_t pte)
* collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
* @cc: The collapse control struct
* @vma: The vma to check for userfaultfd
+ * @order: The folio order being collapsed to
*
* If we are not in khugepaged mode use HPAGE_PMD_NR to allow any
- * empty page.
+ * empty page. For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the
+ * configured khugepaged_max_ptes_none value.
+ *
+ * For mTHP collapses, we currently only support khugepaged_max_pte_none values
+ * of 0 or (KHUGEPAGED_MAX_PTES_LIMIT). Any other value will emit a warning and
+ * no mTHP collapse will be attempted
*
* 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)
+static int collapse_max_ptes_none(struct collapse_control *cc,
+ struct vm_area_struct *vma, unsigned int order)
{
if (vma && userfaultfd_armed(vma))
return 0;
if (!cc->is_khugepaged)
return HPAGE_PMD_NR;
- return khugepaged_max_ptes_none;
+ if (is_pmd_order(order))
+ return khugepaged_max_ptes_none;
+ /* Zero/non-present collapse disabled. */
+ if (!khugepaged_max_ptes_none)
+ return 0;
+ if (khugepaged_max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
+ return (1 << order) - 1;
+
+ pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %u\n",
+ KHUGEPAGED_MAX_PTES_LIMIT);
+ return -EINVAL;
}

/**
* collapse_max_ptes_shared - Calculate maximum allowed shared PTEs for collapse
* @cc: The collapse control struct
+ * @order: The folio order being collapsed to
*
* If we are not in khugepaged mode use HPAGE_PMD_NR to allow any
* shared page.
*
+ * For mTHP collapses, we currently dont support collapsing memory with
+ * shared memory.
+ *
* Return: Maximum number of shared PTEs allowed for the collapse operation
*/
-static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
+static unsigned int collapse_max_ptes_shared(struct collapse_control *cc,
+ unsigned int order)
{
if (!cc->is_khugepaged)
return HPAGE_PMD_NR;
+ if (!is_pmd_order(order))
+ return 0;
+
return khugepaged_max_ptes_shared;
}

/**
* collapse_max_ptes_swap - Calculate maximum allowed swap PTEs for collapse
* @cc: The collapse control struct
+ * @order: The folio order being collapsed to
*
* If we are not in khugepaged mode use HPAGE_PMD_NR to allow any
* swap page.
*
+ * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured
+ * khugepaged_max_ptes_swap value.
+ *
+ * For mTHP collapses, we currently dont support collapsing memory with
+ * swapped out memory.
+ *
* Return: Maximum number of swap PTEs allowed for the collapse operation
*/
-static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
+static unsigned int collapse_max_ptes_swap(struct collapse_control *cc,
+ unsigned int order)
{
if (!cc->is_khugepaged)
return HPAGE_PMD_NR;
+ if (!is_pmd_order(order))
+ return 0;
+
return khugepaged_max_ptes_swap;
}

@@ -590,18 +625,22 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,

static enum scan_result __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 int order, struct list_head *compound_pagelist)
{
+ const unsigned long nr_pages = 1UL << order;
struct page *page = NULL;
struct folio *folio = NULL;
unsigned long addr = start_addr;
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);
+ int max_ptes_none = collapse_max_ptes_none(cc, vma, order);
+ unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, order);
+
+ if (max_ptes_none < 0)
+ return result;

- 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)) {
@@ -734,18 +773,18 @@ static enum scan_result __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;
+ const unsigned long nr_pages = 1UL << order;
+ unsigned long end = address + (PAGE_SIZE << order);
struct folio *src, *tmp;
pte_t pteval;
pte_t *_pte;
unsigned int nr_ptes;

- 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);
@@ -798,13 +837,11 @@ 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)
{
+ const unsigned long nr_pages = 1UL << order;
spinlock_t *pmd_ptl;
-
/*
* Re-establish the PMD to point to the original page table
* entry. Restoring PMD needs to be done prior to releasing
@@ -818,7 +855,7 @@ 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);
}

/*
@@ -838,16 +875,16 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
*/
static enum scan_result __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)
{
+ const unsigned long nr_pages = 1UL << order;
unsigned int i;
enum scan_result result = SCAN_SUCCEED;
-
/*
* 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;
@@ -866,10 +903,10 @@ static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *foli

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;
}
@@ -1040,12 +1077,12 @@ static enum scan_result check_pmd_still_valid(struct mm_struct *mm,
* Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
*/
static enum scan_result __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);
enum scan_result result;
pte_t *pte = NULL;
spinlock_t *ptl;
@@ -1077,6 +1114,19 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
pte_present(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 (!is_pmd_order(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);
@@ -1196,7 +1246,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
* 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;
}
@@ -1244,6 +1294,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
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 {
@@ -1274,6 +1325,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a

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))
@@ -1318,9 +1370,9 @@ 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);
+ int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
+ unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
+ unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);

VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);

@@ -2371,8 +2423,8 @@ 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);
+ int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER);
+ unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);

present = 0;
swap = 0;
--
2.53.0