[PATCH mm-unstable v17 00/14] khugepaged: mTHP support

[PATCH mm-unstable v17 00/14] khugepaged: mTHP support

[Nico Pache]

The following series provides khugepaged with the capability to collapse
anonymous memory regions to mTHPs.

To achieve this we generalize the khugepaged functions to no longer depend
on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
pages that are occupied (!none/zero). After the PMD scan is done, we use
the bitmap to find the optimal mTHP sizes for the PMD range. The
restriction on max_ptes_none is removed during the scan, to make sure we
account for the whole PMD range in the bitmap. When no mTHP size is
enabled, the legacy behavior of khugepaged is maintained.

We currently only support max_ptes_none values of 0 or HPAGE_PMD_NR - 1
(ie 511). If any other value is specified, the kernel will emit a warning
and no mTHP collapse will be attempted. If a mTHP collapse is attempted,
but contains swapped out, or shared pages, we don't perform the collapse.
It is now also possible to collapse to mTHPs without requiring the PMD THP
size to be enabled. These limitations are to prevent collapse "creep"
behavior. This prevents constantly promoting mTHPs to the next available
size, which would occur because a collapse introduces more non-zero pages
that would satisfy the promotion condition on subsequent scans.

Patch 1-2:   Generalize hugepage_vma_revalidate and alloc_charge_folio
	     for arbitrary orders.
Patch 3:     Rework max_ptes_* handling into helper functions
Patch 4:     Generalize __collapse_huge_page_* for mTHP support
Patch 5:     Require collapse_huge_page to enter/exit with the lock dropped
Patch 6:     Generalize collapse_huge_page for mTHP collapse
Patch 7:     Skip collapsing mTHP to smaller orders
Patch 8-9:   Add per-order mTHP statistics and tracepoints
Patch 10:    Introduce collapse_allowable_orders helper function
Patch 11-13: Introduce bitmap and mTHP collapse support, fully enabled
Patch 14:    Documentation

Testing:
- Built for x86_64, aarch64, ppc64le, and s390x
- ran all arches on test suites provided by the kernel-tests project
- internal testing suites: functional testing and performance testing
- selftests mm
- I created a test script that I used to push khugepaged to its limits
   while monitoring a number of stats and tracepoints. The code is
   available here[1] (Run in legacy mode for these changes and set mthp
   sizes to inherit)
   The summary from my testings was that there was no significant
   regression noticed through this test. In some cases my changes had
   better collapse latencies, and was able to scan more pages in the same
   amount of time/work, but for the most part the results were consistent.
- redis testing. I did some testing with these changes along with my defer
  changes (see followup [2] post for more details). We've decided to get
  the mTHP changes merged first before attempting the defer series.
- some basic testing on 64k page size.
- lots of general use.

V17 Changes:
- Added Acks/RB
- New patch(5): split the mmap_read_unlock() locking contract change out of
  "generalize collapse_huge_page" into its own patch; add a comment
  documenting the enter/exit-with-lock-dropped contract (Usama, David)
- [patch 03] Add const to max_ptes_none/shared/swap variables; improve the
  three helper docstrings; replace the paragraphs with inline comments;
  note that sysctl values are now snapshotted once per scan (Usama, David)
- [patch 04] Add SCAN_INVALID_PTES_NONE result code and return it instead
  of SCAN_FAIL when collapse_max_ptes_none() returns -EINVAL (Usama);
  snapshot khugepaged_max_ptes_none into a local variable to fix race on
  the two comparisons (Usama); clean up mTHP docstring paragraphs into
  inline comments; fix commit message wording (David)
- [patch 06] Remove /* PMD collapse */ and /* mTHP collapse */ comments
  (David); move const declarations to top of variable list (David); add
  comment explaining that map_anon_folio_pte_nopf() calls set_ptes under
  pmd_ptl and is safe because PMD is expected to be none (Usama)
- [patch 08] Shorten sysfs counter documentation for
  collapse_exceed_swap/shared_pte to concise one-liners; trim
  collapse_exceed_none_pte description; fix "dont" → "do not" (David)
- [patch 10] Keep vm_flags parameter in khugepaged_enter_vma() and
  collapse_allowable_orders() rather than dropping it and reading
  vma->vm_flags internally; pass vm_flags explicitly at all three
  collapse_allowable_orders() call sites (David, sashskio)
- [patch 11] Fix MTHP_STACK_SIZE: was exponential (~128); correct formula
  is (height + 1) for a DFS on a binary tree rewrite comment to explain
  the DFS sizing (sashskio)
- [patch 12] Replace SCAN_PAGE_LRU with SCAN_PAGE_LAZYFREE in the
  "goto next_order" early-bail cases; non-LRU page failures cannot be
  recovered at any order and belong in the default (return) path
- [patch 13] Use tva_flags == TVA_KHUGEPAGED (strict equality) instead of
  tva_flags & TVA_KHUGEPAGED; flatten nested if into single condition;
  retain vm_flags parameter; pass vm_flags to collapse_allowable_orders()

V16: https://lore.kernel.org/all/20260419185750.260784-1-npache@redhat.com
V15: https://lore.kernel.org/all/20260226031741.230674-1-npache@redhat.com
V14: https://lore.kernel.org/all/20260122192841.128719-1-npache@redhat.com
V13: https://lore.kernel.org/all/20251201174627.23295-1-npache@redhat.com
V12: https://lore.kernel.org/all/20251022183717.70829-1-npache@redhat.com
V11: https://lore.kernel.org/all/20250912032810.197475-1-npache@redhat.com
V10: https://lore.kernel.org/all/20250819134205.622806-1-npache@redhat.com
V9 : https://lore.kernel.org/all/20250714003207.113275-1-npache@redhat.com
V8 : https://lore.kernel.org/all/20250702055742.102808-1-npache@redhat.com
V7 : https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com
V6 : https://lore.kernel.org/all/20250515030312.125567-1-npache@redhat.com
V5 : https://lore.kernel.org/all/20250428181218.85925-1-npache@redhat.com
V4 : https://lore.kernel.org/all/20250417000238.74567-1-npache@redhat.com
V3 : https://lore.kernel.org/all/20250414220557.35388-1-npache@redhat.com
V2 : https://lore.kernel.org/all/20250211003028.213461-1-npache@redhat.com
V1 : https://lore.kernel.org/all/20250108233128.14484-1-npache@redhat.com

Baolin Wang (1):
  mm/khugepaged: run khugepaged for all orders

Dev Jain (1):
  mm/khugepaged: generalize alloc_charge_folio()

Nico Pache (12):
  mm/khugepaged: generalize hugepage_vma_revalidate for mTHP support
  mm/khugepaged: rework max_ptes_* handling with helper functions
  mm/khugepaged: generalize __collapse_huge_page_* for mTHP support
  mm/khugepaged: require collapse_huge_page to enter/exit with the lock
    dropped
  mm/khugepaged: generalize collapse_huge_page for mTHP collapse
  mm/khugepaged: skip collapsing mTHP to smaller orders
  mm/khugepaged: add per-order mTHP collapse failure statistics
  mm/khugepaged: improve tracepoints for mTHP orders
  mm/khugepaged: introduce collapse_allowable_orders helper function
  mm/khugepaged: Introduce mTHP collapse support
  mm/khugepaged: avoid unnecessary mTHP collapse attempts
  Documentation: mm: update the admin guide for mTHP collapse

 Documentation/admin-guide/mm/transhuge.rst |  71 ++-
 include/linux/huge_mm.h                    |   5 +
 include/trace/events/huge_memory.h         |  37 +-
 mm/huge_memory.c                           |  11 +
 mm/khugepaged.c                            | 625 ++++++++++++++++-----
 5 files changed, 579 insertions(+), 170 deletions(-)


base-commit: e9dd96806dbc2d50a66770b6a86962bd5d601153
-- 
2.54.0

[PATCH mm-unstable v17 01/14] mm/khugepaged: generalize hugepage_vma_revalidate for mTHP support

[Nico Pache]

For khugepaged to support different mTHP orders, we must generalize this
to check if the PMD is not shared by another VMA and that the order is
enabled.

No functional change in this patch. Also correct a comment about the
functionality of the revalidation and fix a double space issues.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
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 | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 28a843f30b32..979885694351 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -902,12 +902,13 @@ static int collapse_find_target_node(struct collapse_control *cc)
 
 /*
  * If mmap_lock temporarily dropped, revalidate vma
- * before taking mmap_lock.
+ * after taking the mmap_lock again.
  * Returns enum scan_result value.
  */
 
 static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
-		bool expect_anon, struct vm_area_struct **vmap, struct collapse_control *cc)
+		bool expect_anon, struct vm_area_struct **vmap,
+		struct collapse_control *cc, unsigned int order)
 {
 	struct vm_area_struct *vma;
 	enum tva_type type = cc->is_khugepaged ? TVA_KHUGEPAGED :
@@ -920,15 +921,16 @@ static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned l
 	if (!vma)
 		return SCAN_VMA_NULL;
 
+	/* Always check the PMD order to ensure its not shared by another VMA */
 	if (!thp_vma_suitable_order(vma, address, PMD_ORDER))
 		return SCAN_ADDRESS_RANGE;
-	if (!thp_vma_allowable_order(vma, vma->vm_flags, type, PMD_ORDER))
+	if (!thp_vma_allowable_orders(vma, vma->vm_flags, type, BIT(order)))
 		return SCAN_VMA_CHECK;
 	/*
 	 * Anon VMA expected, the address may be unmapped then
 	 * remapped to file after khugepaged reaquired the mmap_lock.
 	 *
-	 * thp_vma_allowable_order may return true for qualified file
+	 * thp_vma_allowable_orders may return true for qualified file
 	 * vmas.
 	 */
 	if (expect_anon && (!(*vmap)->anon_vma || !vma_is_anonymous(*vmap)))
@@ -1121,7 +1123,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 		goto out_nolock;
 
 	mmap_read_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
+	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
+					 HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
 		goto out_nolock;
@@ -1155,7 +1158,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * mmap_lock.
 	 */
 	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
+	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
+					 HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 	/* check if the pmd is still valid */
@@ -2858,8 +2862,8 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			mmap_unlocked = false;
 			*lock_dropped = true;
 			result = hugepage_vma_revalidate(mm, addr, false, &vma,
-							 cc);
-			if (result  != SCAN_SUCCEED) {
+							 cc, HPAGE_PMD_ORDER);
+			if (result != SCAN_SUCCEED) {
 				last_fail = result;
 				goto out_nolock;
 			}
-- 
2.54.0

[PATCH mm-unstable v17 02/14] mm/khugepaged: generalize alloc_charge_folio()

[Nico Pache]

From: Dev Jain <dev.jain@arm.com>

Pass order to alloc_charge_folio() and update mTHP statistics.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Co-developed-by: Nico Pache <npache@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst |  8 ++++++++
 include/linux/huge_mm.h                    |  2 ++
 mm/huge_memory.c                           |  4 ++++
 mm/khugepaged.c                            | 17 +++++++++++------
 4 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 5fbc3d89bb07..c51932e6275d 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -639,6 +639,14 @@ anon_fault_fallback_charge
 	instead falls back to using huge pages with lower orders or
 	small pages even though the allocation was successful.
 
+collapse_alloc
+	is incremented every time a huge page is successfully allocated for a
+	khugepaged collapse.
+
+collapse_alloc_failed
+	is incremented every time a huge page allocation fails during a
+	khugepaged collapse.
+
 zswpout
 	is incremented every time a huge page is swapped out to zswap in one
 	piece without splitting.
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2949e5acff35..ba7ae6808544 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -128,6 +128,8 @@ enum mthp_stat_item {
 	MTHP_STAT_ANON_FAULT_ALLOC,
 	MTHP_STAT_ANON_FAULT_FALLBACK,
 	MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
+	MTHP_STAT_COLLAPSE_ALLOC,
+	MTHP_STAT_COLLAPSE_ALLOC_FAILED,
 	MTHP_STAT_ZSWPOUT,
 	MTHP_STAT_SWPIN,
 	MTHP_STAT_SWPIN_FALLBACK,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e9d499da0ac7..05f482a72a89 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -699,6 +699,8 @@ static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
 DEFINE_MTHP_STAT_ATTR(anon_fault_alloc, MTHP_STAT_ANON_FAULT_ALLOC);
 DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
 DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
+DEFINE_MTHP_STAT_ATTR(collapse_alloc, MTHP_STAT_COLLAPSE_ALLOC);
+DEFINE_MTHP_STAT_ATTR(collapse_alloc_failed, MTHP_STAT_COLLAPSE_ALLOC_FAILED);
 DEFINE_MTHP_STAT_ATTR(zswpout, MTHP_STAT_ZSWPOUT);
 DEFINE_MTHP_STAT_ATTR(swpin, MTHP_STAT_SWPIN);
 DEFINE_MTHP_STAT_ATTR(swpin_fallback, MTHP_STAT_SWPIN_FALLBACK);
@@ -764,6 +766,8 @@ static struct attribute *any_stats_attrs[] = {
 #endif
 	&split_attr.attr,
 	&split_failed_attr.attr,
+	&collapse_alloc_attr.attr,
+	&collapse_alloc_failed_attr.attr,
 	NULL,
 };
 
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 979885694351..f0e29d5c7b1f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1068,21 +1068,26 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 }
 
 static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
-		struct collapse_control *cc)
+		struct collapse_control *cc, unsigned int order)
 {
 	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
 		     GFP_TRANSHUGE);
 	int node = collapse_find_target_node(cc);
 	struct folio *folio;
 
-	folio = __folio_alloc(gfp, HPAGE_PMD_ORDER, node, &cc->alloc_nmask);
+	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask);
 	if (!folio) {
 		*foliop = NULL;
-		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
+		if (is_pmd_order(order))
+			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
+		count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC_FAILED);
 		return SCAN_ALLOC_HUGE_PAGE_FAIL;
 	}
 
-	count_vm_event(THP_COLLAPSE_ALLOC);
+	if (is_pmd_order(order))
+		count_vm_event(THP_COLLAPSE_ALLOC);
+	count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC);
+
 	if (unlikely(mem_cgroup_charge(folio, mm, gfp))) {
 		folio_put(folio);
 		*foliop = NULL;
@@ -1118,7 +1123,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 */
 	mmap_read_unlock(mm);
 
-	result = alloc_charge_folio(&folio, mm, cc);
+	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out_nolock;
 
@@ -1899,7 +1904,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
 	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
 
-	result = alloc_charge_folio(&new_folio, mm, cc);
+	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out;
 
-- 
2.54.0

[PATCH mm-unstable v17 03/14] mm/khugepaged: rework max_ptes_* handling with helper functions

[Nico Pache]

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 be used later in this series to cleanly restrict the mTHP collapse
behavior.

No functional change is intended; however, we are now only reading the
sysfs variables once per scan, whereas before these variables were being
read on each loop iteration.

Suggested-by: David Hildenbrand <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 118 +++++++++++++++++++++++++++++++++---------------
 1 file changed, 82 insertions(+), 36 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f0e29d5c7b1f..f68853b3caa7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -348,6 +348,62 @@ 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 none-page or zero-page
+ * PTEs for the given collapse operation.
+ * @cc: The collapse control struct
+ * @vma: The vma to check for userfaultfd
+ *
+ * Return: Maximum number of none-page or zero-page PTEs allowed for the
+ * collapse operation.
+ */
+static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
+		struct vm_area_struct *vma)
+{
+	// If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs.
+	if (vma && userfaultfd_armed(vma))
+		return 0;
+	// for MADV_COLLAPSE, allow any none-page or zero-page PTEs.
+	if (!cc->is_khugepaged)
+		return HPAGE_PMD_NR;
+	// For all other cases repect the user defined maximum.
+	return khugepaged_max_ptes_none;
+}
+
+/**
+ * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
+ * anonymous pages for the given collapse operation.
+ * @cc: The collapse control struct
+ *
+ * Return: Maximum number of PTEs that map shared anonymous pages for the
+ * collapse operation
+ */
+static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
+{
+	// for MADV_COLLAPSE, do not restrict the number of PTEs that map shared
+	// anonymous pages.
+	if (!cc->is_khugepaged)
+		return HPAGE_PMD_NR;
+	return khugepaged_max_ptes_shared;
+}
+
+/**
+ * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
+ * maximum allowed non-present pagecache entries for the given collapse operation.
+ * @cc: The collapse control struct
+ *
+ * Return: Maximum number of non-present PTEs or the maximum allowed non-present
+ * pagecache entries for the collapse operation.
+ */
+static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
+{
+	// for MADV_COLLAPSE, do not restrict the number PTEs entries or
+	// pagecache entries that are non-present.
+	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 +602,19 @@ 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 +645,7 @@ 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;
@@ -1261,6 +1313,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		struct vm_area_struct *vma, unsigned long start_addr,
 		bool *lock_dropped, struct collapse_control *cc)
 {
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
 	pmd_t *pmd;
 	pte_t *pte, *_pte;
 	int none_or_zero = 0, shared = 0, referenced = 0;
@@ -1294,36 +1349,29 @@ 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 +1414,7 @@ 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;
@@ -2323,6 +2369,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 		unsigned long addr, struct file *file, pgoff_t start,
 		struct collapse_control *cc)
 {
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
 	struct folio *folio = NULL;
 	struct address_space *mapping = file->f_mapping;
 	XA_STATE(xas, &mapping->i_pages, start);
@@ -2341,8 +2389,7 @@ 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 +2460,7 @@ 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 {
-- 
2.54.0

Re: [PATCH mm-unstable v17 03/14] mm/khugepaged: rework max_ptes_* handling with helper functions

[Lance Yang]

On Mon, May 11, 2026 at 12:58:03PM -0600, Nico Pache wrote:
>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

Nit: s/dont/do not/

>will be used later in this series to cleanly restrict the mTHP collapse
>behavior.
>
>No functional change is intended; however, we are now only reading the
>sysfs variables once per scan, whereas before these variables were being
>read on each loop iteration.
>
>Suggested-by: David Hildenbrand <david@kernel.org>
>Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>Acked-by: Usama Arif <usama.arif@linux.dev>
>Signed-off-by: Nico Pache <npache@redhat.com>
>---
> mm/khugepaged.c | 118 +++++++++++++++++++++++++++++++++---------------
> 1 file changed, 82 insertions(+), 36 deletions(-)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index f0e29d5c7b1f..f68853b3caa7 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -348,6 +348,62 @@ 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 none-page or zero-page
>+ * PTEs for the given collapse operation.
>+ * @cc: The collapse control struct
>+ * @vma: The vma to check for userfaultfd
>+ *
>+ * Return: Maximum number of none-page or zero-page PTEs allowed for the
>+ * collapse operation.
>+ */
>+static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
>+		struct vm_area_struct *vma)
>+{
>+	// If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs.
>+	if (vma && userfaultfd_armed(vma))
>+		return 0;
>+	// for MADV_COLLAPSE, allow any none-page or zero-page PTEs.
>+	if (!cc->is_khugepaged)
>+		return HPAGE_PMD_NR;
>+	// For all other cases repect the user defined maximum.
>+	return khugepaged_max_ptes_none;

Nit: kernel code usually uses C-style comments. This could be:

/* For all other cases, respect the user-defined maximum. */

Also, s/repect/respect/.

>+}
>+
>+/**
>+ * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
>+ * anonymous pages for the given collapse operation.
>+ * @cc: The collapse control struct
>+ *
>+ * Return: Maximum number of PTEs that map shared anonymous pages for the
>+ * collapse operation
>+ */
>+static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
>+{
>+	// for MADV_COLLAPSE, do not restrict the number of PTEs that map shared
>+	// anonymous pages.

Ditto.

>+	if (!cc->is_khugepaged)
>+		return HPAGE_PMD_NR;
>+	return khugepaged_max_ptes_shared;
>+}
>+
>+/**
>+ * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
>+ * maximum allowed non-present pagecache entries for the given collapse operation.
>+ * @cc: The collapse control struct
>+ *
>+ * Return: Maximum number of non-present PTEs or the maximum allowed non-present
>+ * pagecache entries for the collapse operation.
>+ */
>+static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
>+{
>+	// for MADV_COLLAPSE, do not restrict the number PTEs entries or
>+	// pagecache entries that are non-present.

Same here.

>+	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 +602,19 @@ 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);

Nit: could these be const, as David suggested earlier?

Nothing else jumped out at me. LGTM!

Reviewed-by: Lance Yang <lance.yang@linux.dev>

Re: [PATCH mm-unstable v17 03/14] mm/khugepaged: rework max_ptes_* handling with helper functions

[David Hildenbrand (Arm)]

On 5/11/26 20:58, Nico Pache wrote:
> 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 be used later in this series to cleanly restrict the mTHP collapse
> behavior.
> 
> No functional change is intended; however, we are now only reading the
> sysfs variables once per scan, whereas before these variables were being
> read on each loop iteration.
> 
> Suggested-by: David Hildenbrand <david@kernel.org>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Some nits when re-reading:

> Acked-by: Usama Arif <usama.arif@linux.dev>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 118 +++++++++++++++++++++++++++++++++---------------
>  1 file changed, 82 insertions(+), 36 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index f0e29d5c7b1f..f68853b3caa7 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -348,6 +348,62 @@ 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 none-page or zero-page

I know, it's painful, but ...

There is no "none-page".

Calculate maximum allowed empty PTEs or PTEs mapping the shared zeropage ... ?

> + * PTEs for the given collapse operation.

We usually indent here (second line of subject), I think. Same applies to the
other doc below.

> + * @cc: The collapse control struct
> + * @vma: The vma to check for userfaultfd
> + *
> + * Return: Maximum number of none-page or zero-page PTEs allowed for the
> + * collapse operation.

Same here.

> + */
> +static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
> +		struct vm_area_struct *vma)
> +{
> +	// If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs.

Lance commented on the comment style.

Is this comment really required? It's pretty self-documenting already.

> +	if (vma && userfaultfd_armed(vma))
> +		return 0;
> +	// for MADV_COLLAPSE, allow any none-page or zero-page PTEs.
> +	if (!cc->is_khugepaged)
> +		return HPAGE_PMD_NR;
> +	// For all other cases repect the user defined maximum.
> +	return khugepaged_max_ptes_none;
> +}
> +
-- 
Cheers,

David

[PATCH mm-unstable v17 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Nico Pache]

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. [1]

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: [2]

- max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
  that maps the shared zeropage. Consequently, no memory bloat.
- 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.

[1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com
[2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@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>
---
 include/trace/events/huge_memory.h |   3 +-
 mm/khugepaged.c                    | 117 ++++++++++++++++++++---------
 2 files changed, 85 insertions(+), 35 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index bcdc57eea270..443e0bd13fdb 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -39,7 +39,8 @@
 	EM( SCAN_STORE_FAILED,		"store_failed")			\
 	EM( SCAN_COPY_MC,		"copy_poisoned_page")		\
 	EM( SCAN_PAGE_FILLED,		"page_filled")			\
-	EMe(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")
+	EM(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")	\
+	EMe(SCAN_INVALID_PTES_NONE,	"invalid_ptes_none")
 
 #undef EM
 #undef EMe
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f68853b3caa7..27465161fa6d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -61,6 +61,7 @@ enum scan_result {
 	SCAN_COPY_MC,
 	SCAN_PAGE_FILLED,
 	SCAN_PAGE_DIRTY_OR_WRITEBACK,
+	SCAN_INVALID_PTES_NONE,
 };
 
 #define CREATE_TRACE_POINTS
@@ -353,37 +354,60 @@ static bool pte_none_or_zero(pte_t pte)
  * PTEs for the given collapse operation.
  * @cc: The collapse control struct
  * @vma: The vma to check for userfaultfd
+ * @order: The folio order being collapsed to
  *
  * Return: Maximum number of none-page or zero-page 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)
 {
+	unsigned int max_ptes_none = khugepaged_max_ptes_none;
 	// If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs.
 	if (vma && userfaultfd_armed(vma))
 		return 0;
 	// for MADV_COLLAPSE, allow any none-page or zero-page PTEs.
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
-	// For all other cases repect the user defined maximum.
-	return khugepaged_max_ptes_none;
+	// for PMD collapse, respect the user defined maximum.
+	if (is_pmd_order(order))
+		return max_ptes_none;
+	/* Zero/non-present collapse disabled. */
+	if (!max_ptes_none)
+		return 0;
+	// for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT,
+	// scale the maximum number of PTEs to the order of the collapse.
+	if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
+		return (1 << order) - 1;
+
+	// We currently only support max_ptes_none values of 0 or KHUGEPAGED_MAX_PTES_LIMIT.
+	// Emit a warning and return -EINVAL.
+	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 PTEs that map shared
  * anonymous pages for the given collapse operation.
  * @cc: The collapse control struct
+ * @order: The folio order being collapsed to
  *
  * Return: Maximum number of PTEs that map shared anonymous pages 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)
 {
 	// for MADV_COLLAPSE, do not restrict the number of PTEs that map shared
 	// anonymous pages.
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
+	// for mTHP collapse do not allow collapsing anonymous memory pages that
+	// are shared between processes.
+	if (!is_pmd_order(order))
+		return 0;
+	// for PMD collapse, respect the user defined maximum.
 	return khugepaged_max_ptes_shared;
 }
 
@@ -391,16 +415,22 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
  * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
  * maximum allowed non-present pagecache entries for the given collapse operation.
  * @cc: The collapse control struct
+ * @order: The folio order being collapsed to
  *
  * Return: Maximum number of non-present PTEs or the maximum allowed non-present
  * pagecache entries 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)
 {
 	// for MADV_COLLAPSE, do not restrict the number PTEs entries or
 	// pagecache entries that are non-present.
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
+	// for mTHP collapse do not allow any non-present PTEs or pagecache entries.
+	if (!is_pmd_order(order))
+		return 0;
+	// for PMD collapse, respect the user defined maximum.
 	return khugepaged_max_ptes_swap;
 }
 
@@ -594,18 +624,22 @@ 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 SCAN_INVALID_PTES_NONE;
 
-	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)) {
@@ -738,18 +772,18 @@ 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);
@@ -802,11 +836,10 @@ 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;
 
 	/*
@@ -822,7 +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);
 }
 
 /*
@@ -842,16 +875,17 @@ 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;
@@ -870,10 +904,10 @@ 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;
 }
@@ -1044,12 +1078,12 @@ 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;
@@ -1081,6 +1115,19 @@ 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);
@@ -1200,7 +1247,7 @@ 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;
 	}
@@ -1248,6 +1295,7 @@ 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 {
@@ -1278,6 +1326,7 @@ 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))
@@ -1313,9 +1362,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		struct vm_area_struct *vma, unsigned long start_addr,
 		bool *lock_dropped, struct collapse_control *cc)
 {
-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
-	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
-	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
+	const int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
 	pmd_t *pmd;
 	pte_t *pte, *_pte;
 	int none_or_zero = 0, shared = 0, referenced = 0;
@@ -2369,8 +2418,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 		unsigned long addr, struct file *file, pgoff_t start,
 		struct collapse_control *cc)
 {
-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
-	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
+	const int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
 	struct folio *folio = NULL;
 	struct address_space *mapping = file->f_mapping;
 	XA_STATE(xas, &mapping->i_pages, start);
-- 
2.54.0

Re: [PATCH mm-unstable v17 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Lance Yang]

On Mon, May 11, 2026 at 12:58:04PM -0600, Nico Pache wrote:
>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. [1]
>
>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: [2]
>
>- max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
>  that maps the shared zeropage. Consequently, no memory bloat.
>- 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.
>
>[1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com
>[2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@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>
>---
> include/trace/events/huge_memory.h |   3 +-
> mm/khugepaged.c                    | 117 ++++++++++++++++++++---------
> 2 files changed, 85 insertions(+), 35 deletions(-)
>
>diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
>index bcdc57eea270..443e0bd13fdb 100644
>--- a/include/trace/events/huge_memory.h
>+++ b/include/trace/events/huge_memory.h
>@@ -39,7 +39,8 @@
> 	EM( SCAN_STORE_FAILED,		"store_failed")			\
> 	EM( SCAN_COPY_MC,		"copy_poisoned_page")		\
> 	EM( SCAN_PAGE_FILLED,		"page_filled")			\
>-	EMe(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")
>+	EM(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")	\
>+	EMe(SCAN_INVALID_PTES_NONE,	"invalid_ptes_none")
> 
> #undef EM
> #undef EMe
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index f68853b3caa7..27465161fa6d 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -61,6 +61,7 @@ enum scan_result {
> 	SCAN_COPY_MC,
> 	SCAN_PAGE_FILLED,
> 	SCAN_PAGE_DIRTY_OR_WRITEBACK,
>+	SCAN_INVALID_PTES_NONE,
> };
> 
> #define CREATE_TRACE_POINTS
>@@ -353,37 +354,60 @@ static bool pte_none_or_zero(pte_t pte)
>  * PTEs for the given collapse operation.
>  * @cc: The collapse control struct
>  * @vma: The vma to check for userfaultfd
>+ * @order: The folio order being collapsed to
>  *
>  * Return: Maximum number of none-page or zero-page 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)
> {
>+	unsigned int max_ptes_none = khugepaged_max_ptes_none;
> 	// If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs.

One thing I still want to call out: kernel code usually uses C-style
comments :)

> 	if (vma && userfaultfd_armed(vma))
> 		return 0;
> 	// for MADV_COLLAPSE, allow any none-page or zero-page PTEs.
> 	if (!cc->is_khugepaged)
> 		return HPAGE_PMD_NR;
>-	// For all other cases repect the user defined maximum.
>-	return khugepaged_max_ptes_none;
>+	// for PMD collapse, respect the user defined maximum.
>+	if (is_pmd_order(order))
>+		return max_ptes_none;
>+	/* Zero/non-present collapse disabled. */
>+	if (!max_ptes_none)
>+		return 0;
>+	// for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT,
>+	// scale the maximum number of PTEs to the order of the collapse.
>+	if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
>+		return (1 << order) - 1;
>+
>+	// We currently only support max_ptes_none values of 0 or KHUGEPAGED_MAX_PTES_LIMIT.
>+	// Emit a warning and return -EINVAL.
>+	pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %u\n",
>+		      KHUGEPAGED_MAX_PTES_LIMIT);

Maybe fallback to 0 instead, as David suggested earlier?

max_ptes_none is mostly legacy PMD THP behavior. mTHP is new, and any
intermediate value in (0, KHUGEPAGED_MAX_PTES_LIMIT) would implicitly
disable it :(

Treating those values as 0 feels like the least surprising behavior,
IMHO. It also gives mTHP a cleaner staring point, rather than carry over
all the old PMD knob semantics :)

Otherwise, LGTM!
Reviewed-by: Lance Yang <lance.yang@linux.dev>

>+	return -EINVAL;
> }
> 
> /**
>  * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
>  * anonymous pages for the given collapse operation.
>  * @cc: The collapse control struct
>+ * @order: The folio order being collapsed to
>  *
>  * Return: Maximum number of PTEs that map shared anonymous pages 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)
> {
> 	// for MADV_COLLAPSE, do not restrict the number of PTEs that map shared
> 	// anonymous pages.
> 	if (!cc->is_khugepaged)
> 		return HPAGE_PMD_NR;
>+	// for mTHP collapse do not allow collapsing anonymous memory pages that
>+	// are shared between processes.
>+	if (!is_pmd_order(order))
>+		return 0;
>+	// for PMD collapse, respect the user defined maximum.
> 	return khugepaged_max_ptes_shared;
> }
> 
>@@ -391,16 +415,22 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
>  * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
>  * maximum allowed non-present pagecache entries for the given collapse operation.
>  * @cc: The collapse control struct
>+ * @order: The folio order being collapsed to
>  *
>  * Return: Maximum number of non-present PTEs or the maximum allowed non-present
>  * pagecache entries 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)
> {
> 	// for MADV_COLLAPSE, do not restrict the number PTEs entries or
> 	// pagecache entries that are non-present.
> 	if (!cc->is_khugepaged)
> 		return HPAGE_PMD_NR;
>+	// for mTHP collapse do not allow any non-present PTEs or pagecache entries.
>+	if (!is_pmd_order(order))
>+		return 0;
>+	// for PMD collapse, respect the user defined maximum.
> 	return khugepaged_max_ptes_swap;
> }
> 
>@@ -594,18 +624,22 @@ 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 SCAN_INVALID_PTES_NONE;
> 
>-	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)) {
>@@ -738,18 +772,18 @@ 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);
>@@ -802,11 +836,10 @@ 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;
> 
> 	/*
>@@ -822,7 +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);
> }
> 
> /*
>@@ -842,16 +875,17 @@ 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;
>@@ -870,10 +904,10 @@ 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;
> }
>@@ -1044,12 +1078,12 @@ 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;
>@@ -1081,6 +1115,19 @@ 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);
>@@ -1200,7 +1247,7 @@ 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;
> 	}
>@@ -1248,6 +1295,7 @@ 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 {
>@@ -1278,6 +1326,7 @@ 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))
>@@ -1313,9 +1362,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> 		struct vm_area_struct *vma, unsigned long start_addr,
> 		bool *lock_dropped, struct collapse_control *cc)
> {
>-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
>-	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
>-	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
>+	const int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
>+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
>+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
> 	pmd_t *pmd;
> 	pte_t *pte, *_pte;
> 	int none_or_zero = 0, shared = 0, referenced = 0;
>@@ -2369,8 +2418,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
> 		unsigned long addr, struct file *file, pgoff_t start,
> 		struct collapse_control *cc)
> {
>-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
>-	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
>+	const int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER);
>+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
> 	struct folio *folio = NULL;
> 	struct address_space *mapping = file->f_mapping;
> 	XA_STATE(xas, &mapping->i_pages, start);
>-- 
>2.54.0
>
>

[PATCH mm-unstable v17 05/14] mm/khugepaged: require collapse_huge_page to enter/exit with the lock dropped

[Nico Pache]

Currently the collapse_huge_page function requires the mmap_read_lock to
enter with it held, and exit with it dropped. This function moves the
unlock into its parent caller, and changes this semantic to requiring it
to enter/exit with it always unlocked.

In future patches, we need this expectation, as for in mTHP collapse, we
may have already have dropped the lock, and do not want to conditionally
check for this by passing through the lock_dropped variable.

No functional change is expected as one of the first things the
collapse_huge_page function does is drop this lock before allocating the
hugepage.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 27465161fa6d..37a5f6791816 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1199,6 +1199,14 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
 	return SCAN_SUCCEED;
 }
 
+/*
+ * collapse_huge_page expects the mmap_read_lock to be dropped before
+ * entering this function. The function will also always return with the lock
+ * dropped. The function starts by allocation a folio, which can potentially
+ * take a long time if it involves sync compaction, and we do not need to hold
+ * the mmap_lock during that. We must recheck the vma after taking it again in
+ * write mode.
+ */
 static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		int referenced, int unmapped, struct collapse_control *cc)
 {
@@ -1214,14 +1222,6 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
-	/*
-	 * Before allocating the hugepage, release the mmap_lock read lock.
-	 * The allocation can take potentially a long time if it involves
-	 * sync compaction, and we do not need to hold the mmap_lock during
-	 * that. We will recheck the vma after taking it again in write mode.
-	 */
-	mmap_read_unlock(mm);
-
 	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out_nolock;
@@ -1526,6 +1526,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
 	if (result == SCAN_SUCCEED) {
+		/* collapse_huge_page expects the lock to be dropped before calling */
+		mmap_read_unlock(mm);
 		result = collapse_huge_page(mm, start_addr, referenced,
 					    unmapped, cc);
 		/* collapse_huge_page will return with the mmap_lock released */
-- 
2.54.0

Re: [PATCH mm-unstable v17 05/14] mm/khugepaged: require collapse_huge_page to enter/exit with the lock dropped

[David Hildenbrand (Arm)]

On 5/11/26 20:58, Nico Pache wrote:
> Currently the collapse_huge_page function requires the mmap_read_lock to
> enter with it held, and exit with it dropped. This function moves the
> unlock into its parent caller, and changes this semantic to requiring it
> to enter/exit with it always unlocked.
> 
> In future patches, we need this expectation, as for in mTHP collapse, we
> may have already have dropped the lock, and do not want to conditionally
> check for this by passing through the lock_dropped variable.
> 
> No functional change is expected as one of the first things the
> collapse_huge_page function does is drop this lock before allocating the
> hugepage.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 27465161fa6d..37a5f6791816 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1199,6 +1199,14 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
>  	return SCAN_SUCCEED;
>  }
>  
> +/*
> + * collapse_huge_page expects the mmap_read_lock to be dropped before
> + * entering this function. 

"before entering." Talking about "this function" after naming it sounds odd.

Also, there is only an "mmap_lock".

> The function will also always return with the lock
> + * dropped. 

"collapse_huge_page expects the mmap_lock to be unlocked before entering and
will always return with the lock unlocked."

Or something simple like that?

> The function starts by allocation a folio, which can potentially
> + * take a long time if it involves sync compaction, and we do not need to hold
> + * the mmap_lock during that. We must recheck the vma after taking it again in
> + * write mode.
> + */

"... to avoid holding the mmap_lock while allocating a THP, as that could
trigger direct reclaim/compaction. Note that the VMA must be rechecked after
grabbing the mmap_lock again."

?

Ending up with something like

"collapse_huge_page expects the mmap_lock to be unlocked before entering and
will always return with the lock unlocked, to avoid holding the mmap_lock while
allocating a THP, as that could trigger direct reclaim/compaction.  Note that
the VMA must be rechecked after grabbing the mmap_lock again."

>  static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
>  		int referenced, int unmapped, struct collapse_control *cc)
>  {
> @@ -1214,14 +1222,6 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>  
> -	/*
> -	 * Before allocating the hugepage, release the mmap_lock read lock.
> -	 * The allocation can take potentially a long time if it involves
> -	 * sync compaction, and we do not need to hold the mmap_lock during
> -	 * that. We will recheck the vma after taking it again in write mode.
> -	 */
> -	mmap_read_unlock(mm);
> -
>  	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
>  	if (result != SCAN_SUCCEED)
>  		goto out_nolock;
> @@ -1526,6 +1526,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  out_unmap:
>  	pte_unmap_unlock(pte, ptl);
>  	if (result == SCAN_SUCCEED) {
> +		/* collapse_huge_page expects the lock to be dropped before calling */
> +		mmap_read_unlock(mm);
>  		result = collapse_huge_page(mm, start_addr, referenced,
>  					    unmapped, cc);
>  		/* collapse_huge_page will return with the mmap_lock released */


Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

[PATCH mm-unstable v17 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[Nico Pache]

Pass an order and offset to collapse_huge_page to support collapsing anon
memory to arbitrary orders within a PMD. order indicates what mTHP size we
are attempting to collapse to, and offset indicates were in the PMD to
start the collapse attempt.

For non-PMD collapse we must leave the anon VMA write locked until after
we collapse the mTHP-- in the PMD case all the pages are isolated, but in
the mTHP case this is not true, and we must keep the lock to prevent
access/changes to the page tables. This can happen if the rmap walkers hit
a pmd_none while the PMD entry is currently unavailable due to being
temporarily removed during the collapse phase.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 93 +++++++++++++++++++++++++++++--------------------
 1 file changed, 55 insertions(+), 38 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 37a5f6791816..f49bef78cf51 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1207,34 +1207,36 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
  * the mmap_lock during that. We must recheck the vma after taking it again in
  * write mode.
  */
-static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
-		int referenced, int unmapped, struct collapse_control *cc)
+static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
+		int referenced, int unmapped, struct collapse_control *cc,
+		unsigned int order)
 {
+	const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK;
+	const unsigned long end_addr = start_addr + (PAGE_SIZE << order);
 	LIST_HEAD(compound_pagelist);
 	pmd_t *pmd, _pmd;
-	pte_t *pte;
+	pte_t *pte = NULL;
 	pgtable_t pgtable;
 	struct folio *folio;
 	spinlock_t *pmd_ptl, *pte_ptl;
 	enum scan_result result = SCAN_FAIL;
 	struct vm_area_struct *vma;
 	struct mmu_notifier_range range;
+	bool anon_vma_locked = false;
 
-	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
-
-	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
+	result = alloc_charge_folio(&folio, mm, cc, order);
 	if (result != SCAN_SUCCEED)
 		goto out_nolock;
 
 	mmap_read_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
-					 HPAGE_PMD_ORDER);
+	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
+					 &vma, cc, order);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
 		goto out_nolock;
 	}
 
-	result = find_pmd_or_thp_or_none(mm, address, &pmd);
+	result = find_pmd_or_thp_or_none(mm, pmd_addr, &pmd);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
 		goto out_nolock;
@@ -1246,8 +1248,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 		 * released when it fails. So we jump out_nolock directly in
 		 * that case.  Continuing to collapse causes inconsistency.
 		 */
-		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
-						     referenced, HPAGE_PMD_ORDER);
+		result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd,
+						     referenced, order);
 		if (result != SCAN_SUCCEED)
 			goto out_nolock;
 	}
@@ -1262,20 +1264,21 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * mmap_lock.
 	 */
 	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
-					 HPAGE_PMD_ORDER);
+	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
+					 &vma, cc, order);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 	/* check if the pmd is still valid */
 	vma_start_write(vma);
-	result = check_pmd_still_valid(mm, address, pmd);
+	result = check_pmd_still_valid(mm, pmd_addr, pmd);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 
 	anon_vma_lock_write(vma->anon_vma);
+	anon_vma_locked = true;
 
-	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
-				address + HPAGE_PMD_SIZE);
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, start_addr,
+				end_addr);
 	mmu_notifier_invalidate_range_start(&range);
 
 	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
@@ -1287,26 +1290,23 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * Parallel GUP-fast is fine since GUP-fast will back off when
 	 * it detects PMD is changed.
 	 */
-	_pmd = pmdp_collapse_flush(vma, address, pmd);
+	_pmd = pmdp_collapse_flush(vma, pmd_addr, pmd);
 	spin_unlock(pmd_ptl);
 	mmu_notifier_invalidate_range_end(&range);
 	tlb_remove_table_sync_one();
 
-	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
+	pte = pte_offset_map_lock(mm, &_pmd, start_addr, &pte_ptl);
 	if (pte) {
-		result = __collapse_huge_page_isolate(vma, address, pte, cc,
-						      HPAGE_PMD_ORDER,
-						      &compound_pagelist);
+		result = __collapse_huge_page_isolate(vma, start_addr, pte, cc,
+						      order, &compound_pagelist);
 		spin_unlock(pte_ptl);
 	} else {
 		result = SCAN_NO_PTE_TABLE;
 	}
 
 	if (unlikely(result != SCAN_SUCCEED)) {
-		if (pte)
-			pte_unmap(pte);
 		spin_lock(pmd_ptl);
-		BUG_ON(!pmd_none(*pmd));
+		WARN_ON_ONCE(!pmd_none(*pmd));
 		/*
 		 * We can only use set_pmd_at when establishing
 		 * hugepmds and never for establishing regular pmds that
@@ -1314,21 +1314,24 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 		 */
 		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
 		spin_unlock(pmd_ptl);
-		anon_vma_unlock_write(vma->anon_vma);
 		goto out_up_write;
 	}
 
 	/*
-	 * All pages are isolated and locked so anon_vma rmap
-	 * can't run anymore.
+	 * For PMD collapse all pages are isolated and locked so anon_vma
+	 * rmap can't run anymore. For mTHP collapse the PMD entry has been
+	 * removed and not all pages are isolated and locked, so we must hold
+	 * the lock to prevent neighboring folios from attempting to access
+	 * this PMD until its reinstalled.
 	 */
-	anon_vma_unlock_write(vma->anon_vma);
+	if (is_pmd_order(order)) {
+		anon_vma_unlock_write(vma->anon_vma);
+		anon_vma_locked = false;
+	}
 
 	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
-					   vma, address, pte_ptl,
-					   HPAGE_PMD_ORDER,
-					   &compound_pagelist);
-	pte_unmap(pte);
+					   vma, start_addr, pte_ptl,
+					   order, &compound_pagelist);
 	if (unlikely(result != SCAN_SUCCEED))
 		goto out_up_write;
 
@@ -1338,18 +1341,32 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * write.
 	 */
 	__folio_mark_uptodate(folio);
-	pgtable = pmd_pgtable(_pmd);
-
 	spin_lock(pmd_ptl);
-	BUG_ON(!pmd_none(*pmd));
-	pgtable_trans_huge_deposit(mm, pmd, pgtable);
-	map_anon_folio_pmd_nopf(folio, pmd, vma, address);
+	WARN_ON_ONCE(!pmd_none(*pmd));
+	if (is_pmd_order(order)) {
+		pgtable = pmd_pgtable(_pmd);
+		pgtable_trans_huge_deposit(mm, pmd, pgtable);
+		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
+	} else {
+		/*
+		 * set_ptes is called in map_anon_folio_pte_nopf with the
+		 * pmd_ptl lock still held; this is safe as the PMD is expected
+		 * to be none. The pmd entry is then repopulated below.
+		 */
+		map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false);
+		smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
+		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
+	}
 	spin_unlock(pmd_ptl);
 
 	folio = NULL;
 
 	result = SCAN_SUCCEED;
 out_up_write:
+	if (anon_vma_locked)
+		anon_vma_unlock_write(vma->anon_vma);
+	if (pte)
+		pte_unmap(pte);
 	mmap_write_unlock(mm);
 out_nolock:
 	if (folio)
@@ -1529,7 +1546,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		/* collapse_huge_page expects the lock to be dropped before calling */
 		mmap_read_unlock(mm);
 		result = collapse_huge_page(mm, start_addr, referenced,
-					    unmapped, cc);
+					    unmapped, cc, HPAGE_PMD_ORDER);
 		/* collapse_huge_page will return with the mmap_lock released */
 		*lock_dropped = true;
 	}
-- 
2.54.0

[PATCH mm-unstable v17 07/14] mm/khugepaged: skip collapsing mTHP to smaller orders

[Nico Pache]

khugepaged may try to collapse a mTHP to a smaller mTHP, resulting in
some pages being unmapped. Skip these cases until we have a way to check
if its ok to collapse to a smaller mTHP size (like in the case of a
partially mapped folio). This check is also not done during the scan phase
as the current collapse order is unknown at that time.

This patch is inspired by Dev Jain's work on khugepaged mTHP support [1].

[1] https://lore.kernel.org/lkml/20241216165105.56185-11-dev.jain@arm.com/

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
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 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f49bef78cf51..ba21b134fc86 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -685,6 +685,14 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 				goto out;
 			}
 		}
+		/*
+		 * TODO: In some cases of partially-mapped folios, we'd actually
+		 * want to collapse.
+		 */
+		if (!is_pmd_order(order) && folio_order(folio) >= order) {
+			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			goto out;
+		}
 
 		if (folio_test_large(folio)) {
 			struct folio *f;
-- 
2.54.0

[PATCH mm-unstable v17 08/14] mm/khugepaged: add per-order mTHP collapse failure statistics

[Nico Pache]

Add three new mTHP statistics to track collapse failures for different
orders when encountering swap PTEs, excessive none PTEs, and shared PTEs:

- collapse_exceed_swap_pte: Increment when mTHP collapse fails due to
	encountering a swap PTE.

- collapse_exceed_none_pte: Counts when mTHP collapse fails due to
  	exceeding the none PTE threshold for the given order

- collapse_exceed_shared_pte: Counts when mTHP collapse fails due to
	encountering a shared PTE.

These statistics complement the existing THP_SCAN_EXCEED_* events by
providing per-order granularity for mTHP collapse attempts. The stats are
exposed via sysfs under
`/sys/kernel/mm/transparent_hugepage/hugepages-*/stats/` for each
supported hugepage size.

As we currently do not support collapsing mTHPs that contain a swap or
shared entry, those statistics keep track of how often we are
encountering failed mTHP collapses due to these restrictions.

We will add support for mTHP collapse for anonymous pages next; lets also
track when this happens at the PMD level within the per-mTHP stats.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 14 ++++++++++++++
 include/linux/huge_mm.h                    |  3 +++
 mm/huge_memory.c                           |  7 +++++++
 mm/khugepaged.c                            | 21 +++++++++++++++++++--
 4 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index c51932e6275d..80a4d0bed70b 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -714,6 +714,20 @@ nr_anon_partially_mapped
        an anonymous THP as "partially mapped" and count it here, even though it
        is not actually partially mapped anymore.
 
+collapse_exceed_none_pte
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_none threshold.
+
+collapse_exceed_swap_pte
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_swap threshold. For non-PMD orders this occurs if a mTHP range
+       contains at least one swap PTE.
+
+collapse_exceed_shared_pte
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_shared threshold. For non-PMD orders this occurs if a mTHP range
+       contains at least one shared PTE.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in ``/proc/vmstat`` to help
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index ba7ae6808544..48496f09909b 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -144,6 +144,9 @@ enum mthp_stat_item {
 	MTHP_STAT_SPLIT_DEFERRED,
 	MTHP_STAT_NR_ANON,
 	MTHP_STAT_NR_ANON_PARTIALLY_MAPPED,
+	MTHP_STAT_COLLAPSE_EXCEED_SWAP,
+	MTHP_STAT_COLLAPSE_EXCEED_NONE,
+	MTHP_STAT_COLLAPSE_EXCEED_SHARED,
 	__MTHP_STAT_COUNT
 };
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 05f482a72a89..3e9eabc74c6c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -717,6 +717,10 @@ DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
 DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
 DEFINE_MTHP_STAT_ATTR(nr_anon, MTHP_STAT_NR_ANON);
 DEFINE_MTHP_STAT_ATTR(nr_anon_partially_mapped, MTHP_STAT_NR_ANON_PARTIALLY_MAPPED);
+DEFINE_MTHP_STAT_ATTR(collapse_exceed_swap_pte, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
+DEFINE_MTHP_STAT_ATTR(collapse_exceed_none_pte, MTHP_STAT_COLLAPSE_EXCEED_NONE);
+DEFINE_MTHP_STAT_ATTR(collapse_exceed_shared_pte, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
+
 
 static struct attribute *anon_stats_attrs[] = {
 	&anon_fault_alloc_attr.attr,
@@ -733,6 +737,9 @@ static struct attribute *anon_stats_attrs[] = {
 	&split_deferred_attr.attr,
 	&nr_anon_attr.attr,
 	&nr_anon_partially_mapped_attr.attr,
+	&collapse_exceed_swap_pte_attr.attr,
+	&collapse_exceed_none_pte_attr.attr,
+	&collapse_exceed_shared_pte_attr.attr,
 	NULL,
 };
 
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ba21b134fc86..27654ea3f5ca 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -645,7 +645,9 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		if (pte_none_or_zero(pteval)) {
 			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
-				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				if (is_pmd_order(order))
+					count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_NONE);
 				goto out;
 			}
 			continue;
@@ -679,9 +681,17 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		/* See collapse_scan_pmd(). */
 		if (folio_maybe_mapped_shared(folio)) {
+			/*
+			 * 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 (++shared > max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
-				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				if (is_pmd_order(order))
+					count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
 				goto out;
 			}
 		}
@@ -1130,6 +1140,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 		 * range.
 		 */
 		if (!is_pmd_order(order)) {
+			count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
 			pte_unmap(pte);
 			mmap_read_unlock(mm);
 			result = SCAN_EXCEED_SWAP_PTE;
@@ -1426,6 +1437,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
 				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				count_mthp_stat(HPAGE_PMD_ORDER,
+						MTHP_STAT_COLLAPSE_EXCEED_NONE);
 				goto out_unmap;
 			}
 			continue;
@@ -1434,6 +1447,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			if (++unmapped > max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
+				count_mthp_stat(HPAGE_PMD_ORDER,
+						MTHP_STAT_COLLAPSE_EXCEED_SWAP);
 				goto out_unmap;
 			}
 			/*
@@ -1491,6 +1506,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			if (++shared > max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				count_mthp_stat(HPAGE_PMD_ORDER,
+						MTHP_STAT_COLLAPSE_EXCEED_SHARED);
 				goto out_unmap;
 			}
 		}
-- 
2.54.0

[PATCH mm-unstable v17 09/14] mm/khugepaged: improve tracepoints for mTHP orders

[Nico Pache]

Add the order to the mm_collapse_huge_page<_swapin,_isolate> tracepoints to
give better insight into what order is being operated at for.

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 include/trace/events/huge_memory.h | 34 +++++++++++++++++++-----------
 mm/khugepaged.c                    |  9 ++++----
 2 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 443e0bd13fdb..70c25136e7e8 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -90,40 +90,44 @@ TRACE_EVENT(mm_khugepaged_scan_pmd,
 
 TRACE_EVENT(mm_collapse_huge_page,
 
-	TP_PROTO(struct mm_struct *mm, int isolated, int status),
+	TP_PROTO(struct mm_struct *mm, int isolated, int status, unsigned int order),
 
-	TP_ARGS(mm, isolated, status),
+	TP_ARGS(mm, isolated, status, order),
 
 	TP_STRUCT__entry(
 		__field(struct mm_struct *, mm)
 		__field(int, isolated)
 		__field(int, status)
+		__field(unsigned int, order)
 	),
 
 	TP_fast_assign(
 		__entry->mm = mm;
 		__entry->isolated = isolated;
 		__entry->status = status;
+		__entry->order = order;
 	),
 
-	TP_printk("mm=%p, isolated=%d, status=%s",
+	TP_printk("mm=%p, isolated=%d, status=%s, order=%u",
 		__entry->mm,
 		__entry->isolated,
-		__print_symbolic(__entry->status, SCAN_STATUS))
+		__print_symbolic(__entry->status, SCAN_STATUS),
+		__entry->order)
 );
 
 TRACE_EVENT(mm_collapse_huge_page_isolate,
 
 	TP_PROTO(struct folio *folio, int none_or_zero,
-		 int referenced, int status),
+		 int referenced, int status, unsigned int order),
 
-	TP_ARGS(folio, none_or_zero, referenced, status),
+	TP_ARGS(folio, none_or_zero, referenced, status, order),
 
 	TP_STRUCT__entry(
 		__field(unsigned long, pfn)
 		__field(int, none_or_zero)
 		__field(int, referenced)
 		__field(int, status)
+		__field(unsigned int, order)
 	),
 
 	TP_fast_assign(
@@ -131,26 +135,30 @@ TRACE_EVENT(mm_collapse_huge_page_isolate,
 		__entry->none_or_zero = none_or_zero;
 		__entry->referenced = referenced;
 		__entry->status = status;
+		__entry->order = order;
 	),
 
-	TP_printk("scan_pfn=0x%lx, none_or_zero=%d, referenced=%d, status=%s",
+	TP_printk("scan_pfn=0x%lx, none_or_zero=%d, referenced=%d, status=%s, order=%u",
 		__entry->pfn,
 		__entry->none_or_zero,
 		__entry->referenced,
-		__print_symbolic(__entry->status, SCAN_STATUS))
+		__print_symbolic(__entry->status, SCAN_STATUS),
+		__entry->order)
 );
 
 TRACE_EVENT(mm_collapse_huge_page_swapin,
 
-	TP_PROTO(struct mm_struct *mm, int swapped_in, int referenced, int ret),
+	TP_PROTO(struct mm_struct *mm, int swapped_in, int referenced, int ret,
+		 unsigned int order),
 
-	TP_ARGS(mm, swapped_in, referenced, ret),
+	TP_ARGS(mm, swapped_in, referenced, ret, order),
 
 	TP_STRUCT__entry(
 		__field(struct mm_struct *, mm)
 		__field(int, swapped_in)
 		__field(int, referenced)
 		__field(int, ret)
+		__field(unsigned int, order)
 	),
 
 	TP_fast_assign(
@@ -158,13 +166,15 @@ TRACE_EVENT(mm_collapse_huge_page_swapin,
 		__entry->swapped_in = swapped_in;
 		__entry->referenced = referenced;
 		__entry->ret = ret;
+		__entry->order = order;
 	),
 
-	TP_printk("mm=%p, swapped_in=%d, referenced=%d, ret=%d",
+	TP_printk("mm=%p, swapped_in=%d, referenced=%d, ret=%d, order=%u",
 		__entry->mm,
 		__entry->swapped_in,
 		__entry->referenced,
-		__entry->ret)
+		__entry->ret,
+		__entry->order)
 );
 
 TRACE_EVENT(mm_khugepaged_scan_file,
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 27654ea3f5ca..f28066069437 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -779,13 +779,13 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	} else {
 		result = SCAN_SUCCEED;
 		trace_mm_collapse_huge_page_isolate(folio, none_or_zero,
-						    referenced, result);
+						    referenced, result, order);
 		return result;
 	}
 out:
 	release_pte_pages(pte, _pte, compound_pagelist);
 	trace_mm_collapse_huge_page_isolate(folio, none_or_zero,
-					    referenced, result);
+					    referenced, result, order);
 	return result;
 }
 
@@ -1181,7 +1181,8 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 
 	result = SCAN_SUCCEED;
 out:
-	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, result);
+	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, result,
+					   order);
 	return result;
 }
 
@@ -1390,7 +1391,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 out_nolock:
 	if (folio)
 		folio_put(folio);
-	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
+	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result, order);
 	return result;
 }
 
-- 
2.54.0

[PATCH mm-unstable v17 10/14] mm/khugepaged: introduce collapse_allowable_orders helper function

[Nico Pache]

Add collapse_allowable_orders() to generalize THP order eligibility. The
function determines which THP orders are permitted based on collapse
context (khugepaged vs madv_collapse).

This consolidates collapse configuration logic and provides a clean
interface for future mTHP collapse support where the orders may be
different.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f28066069437..3492b135d667 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -545,12 +545,21 @@ void __khugepaged_enter(struct mm_struct *mm)
 		wake_up_interruptible(&khugepaged_wait);
 }
 
+/* Check what orders are allowed based on the vma and collapse type */
+static unsigned long collapse_allowable_orders(struct vm_area_struct *vma,
+		vm_flags_t vm_flags, enum tva_type tva_flags)
+{
+	unsigned long orders = BIT(HPAGE_PMD_ORDER);
+
+	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
+}
+
 void khugepaged_enter_vma(struct vm_area_struct *vma,
 			  vm_flags_t vm_flags)
 {
 	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
 	    hugepage_pmd_enabled()) {
-		if (thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
+		if (collapse_allowable_orders(vma, vm_flags, TVA_KHUGEPAGED))
 			__khugepaged_enter(vma->vm_mm);
 	}
 }
@@ -2673,7 +2682,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
 			cc->progress++;
 			break;
 		}
-		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
+		if (!collapse_allowable_orders(vma, vma->vm_flags, TVA_KHUGEPAGED)) {
 			cc->progress++;
 			continue;
 		}
@@ -2983,7 +2992,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 	BUG_ON(vma->vm_start > start);
 	BUG_ON(vma->vm_end < end);
 
-	if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER))
+	if (!collapse_allowable_orders(vma, vma->vm_flags, TVA_FORCED_COLLAPSE))
 		return -EINVAL;
 
 	cc = kmalloc_obj(*cc);
-- 
2.54.0

[PATCH mm-unstable v17 11/14] mm/khugepaged: Introduce mTHP collapse support

[Nico Pache]

Enable khugepaged to collapse to mTHP orders. This patch implements the
main scanning logic using a bitmap to track occupied pages and a stack
structure that allows us to find optimal collapse sizes.

Previous to this patch, PMD collapse had 3 main phases, a light weight
scanning phase (mmap_read_lock) that determines a potential PMD
collapse, an alloc phase (mmap unlocked), then finally heavier collapse
phase (mmap_write_lock).

To enabled mTHP collapse we make the following changes:

During PMD scan phase, track occupied pages in a bitmap. When mTHP
orders are enabled, we remove the restriction of max_ptes_none during the
scan phase to avoid missing potential mTHP collapse candidates. Once we
have scanned the full PMD range and updated the bitmap to track occupied
pages, we use the bitmap to find the optimal mTHP size.

Implement collapse_scan_bitmap() to perform binary recursion on the bitmap
and determine the best eligible order for the collapse. A stack structure
is used instead of traditional recursion to manage the search. This also
prevents a traditional recursive approach when the kernel stack struct is
limited. The algorithm recursively splits the bitmap into smaller chunks to
find the highest order mTHPs that satisfy the collapse criteria. We start
by attempting the PMD order, then moved on the consecutively lower orders
(mTHP collapse). The stack maintains a pair of variables (offset, order),
indicating the number of PTEs from the start of the PMD, and the order of
the potential collapse candidate.

The algorithm for consuming the bitmap works as such:
    1) push (0, HPAGE_PMD_ORDER) onto the stack
    2) pop the stack
    3) check if the number of set bits in that (offset,order) pair
       statisfy the max_ptes_none threshold for that order
    4) if yes, attempt collapse
    5) if no (or collapse fails), push two new stack items representing
       the left and right halves of the current bitmap range, at the
       next lower order
    6) repeat at step (2) until stack is empty.

Below is a diagram representing the algorithm and stack items:

                            offset   mid_offset
                            |        |
                            |        |
                            v        v
          ____________________________________
         |          PTE Page Table            |
         --------------------------------------
			    <-------><------->
                             order-1  order-1

mTHP collapses reject regions containing swapped out or shared pages.
This is because adding new entries can lead to new none pages, and these
may lead to constant promotion into a higher order mTHP. A similar
issue can occur with "max_ptes_none > HPAGE_PMD_NR/2" due to a collapse
introducing at least 2x the number of pages, and on a future scan will
satisfy the promotion condition once again. This issue is prevented via
the collapse_max_ptes_none() function which imposes the max_ptes_none
restrictions above.

We currently only support mTHP collapse for max_ptes_none values of 0
and HPAGE_PMD_NR - 1. resulting in the following behavior:

    - max_ptes_none=0: Never introduce new empty pages during collapse
    - max_ptes_none=HPAGE_PMD_NR-1: Always try collapse to the highest
      available mTHP order

Any other max_ptes_none value will emit a warning and skip mTHP collapse
attempts. There should be no behavior change for PMD collapse.

Once we determine what mTHP sizes fits best in that PMD range a collapse
is attempted. A minimum collapse order of 2 is used as this is the lowest
order supported by anon memory as defined by THP_ORDERS_ALL_ANON.

Currently madv_collapse is not supported and will only attempt PMD
collapse.

We can also remove the check for is_khugepaged inside the PMD scan as
the collapse_max_ptes_none() function handles this logic now.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 182 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 174 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 3492b135d667..39bf7ea8a6e8 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -100,6 +100,30 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
 
 static struct kmem_cache *mm_slot_cache __ro_after_init;
 
+#define KHUGEPAGED_MIN_MTHP_ORDER	2
+/*
+ * mthp_collapse() does an iterative DFS over a binary tree, from
+ * HPAGE_PMD_ORDER down to KHUGEPAGED_MIN_MTHP_ORDER. The max stack
+ * size needed for a DFS on a binary tree is height + 1, where
+ * height = HPAGE_PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER.
+ *
+ * ilog2 is used in place of HPAGE_PMD_ORDER because some architectures
+ * (e.g. ppc64le) do not define HPAGE_PMD_ORDER until after build time.
+ */
+#define MTHP_STACK_SIZE	(ilog2(MAX_PTRS_PER_PTE) - KHUGEPAGED_MIN_MTHP_ORDER + 1)
+
+/*
+ * Defines a range of PTE entries in a PTE page table which are being
+ * considered for mTHP collapse.
+ *
+ * @offset: the offset of the first PTE entry in a PMD range.
+ * @order: the order of the PTE entries being considered for collapse.
+ */
+struct mthp_range {
+	u16 offset;
+	u8 order;
+};
+
 struct collapse_control {
 	bool is_khugepaged;
 
@@ -111,6 +135,12 @@ struct collapse_control {
 
 	/* nodemask for allocation fallback */
 	nodemask_t alloc_nmask;
+
+	/* Each bit represents a single occupied (!none/zero) page. */
+	DECLARE_BITMAP(mthp_bitmap, MAX_PTRS_PER_PTE);
+	/* A mask of the current range being considered for mTHP collapse. */
+	DECLARE_BITMAP(mthp_bitmap_mask, MAX_PTRS_PER_PTE);
+	struct mthp_range mthp_bitmap_stack[MTHP_STACK_SIZE];
 };
 
 /**
@@ -1404,20 +1434,140 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 	return result;
 }
 
+static void collapse_mthp_stack_push(struct collapse_control *cc, int *stack_size,
+				     u16 offset, u8 order)
+{
+	const int size = *stack_size;
+	struct mthp_range *stack = &cc->mthp_bitmap_stack[size];
+
+	VM_WARN_ON_ONCE(size >= MTHP_STACK_SIZE);
+	stack->order = order;
+	stack->offset = offset;
+	(*stack_size)++;
+}
+
+static struct mthp_range collapse_mthp_stack_pop(struct collapse_control *cc,
+						 int *stack_size)
+{
+	const int size = *stack_size;
+
+	VM_WARN_ON_ONCE(size <= 0);
+	(*stack_size)--;
+	return cc->mthp_bitmap_stack[size - 1];
+}
+
+static unsigned int collapse_mthp_count_present(struct collapse_control *cc,
+						u16 offset, unsigned int nr_ptes)
+{
+	bitmap_zero(cc->mthp_bitmap_mask, MAX_PTRS_PER_PTE);
+	bitmap_set(cc->mthp_bitmap_mask, offset, nr_ptes);
+	return bitmap_weight_and(cc->mthp_bitmap, cc->mthp_bitmap_mask, MAX_PTRS_PER_PTE);
+}
+
+/*
+ * mthp_collapse() consumes the bitmap that is generated during
+ * collapse_scan_pmd() to determine what regions and mTHP orders fit best.
+ *
+ * Each bit in cc->mthp_bitmap represents a single occupied (!none/zero) page.
+ * A stack structure cc->mthp_bitmap_stack is used to check different regions
+ * of the bitmap for collapse eligibility. The stack maintains a pair of
+ * variables (offset, order), indicating the number of PTEs from the start of
+ * the PMD, and the order of the potential collapse candidate respectively. We
+ * start at the PMD order and check if it is eligible for collapse; if not, we
+ * add two entries to the stack at a lower order to represent the left and right
+ * halves of the PTE page table we are examining.
+ *
+ *                         offset       mid_offset
+ *                         |         |
+ *                         |         |
+ *                         v         v
+ *      --------------------------------------
+ *      |          cc->mthp_bitmap            |
+ *      --------------------------------------
+ *                         <-------><------->
+ *                          order-1  order-1
+ *
+ * For each of these, we determine how many PTE entries are occupied in the
+ * range of PTE entries we propose to collapse, then we compare this to a
+ * threshold number of PTE entries which would need to be occupied for a
+ * collapse to be permitted at that order (accounting for max_ptes_none).
+ *
+ * If a collapse is permitted, we attempt to collapse the PTE range into a
+ * mTHP.
+ */
+static int mthp_collapse(struct mm_struct *mm, unsigned long address,
+		int referenced, int unmapped, struct collapse_control *cc,
+		unsigned long enabled_orders)
+{
+	unsigned int nr_occupied_ptes, nr_ptes;
+	int max_ptes_none, collapsed = 0, stack_size = 0;
+	unsigned long collapse_address;
+	struct mthp_range range;
+	u16 offset;
+	u8 order;
+
+	collapse_mthp_stack_push(cc, &stack_size, 0, HPAGE_PMD_ORDER);
+
+	while (stack_size) {
+		range = collapse_mthp_stack_pop(cc, &stack_size);
+		order = range.order;
+		offset = range.offset;
+		nr_ptes = 1UL << order;
+
+		if (!test_bit(order, &enabled_orders))
+			goto next_order;
+
+		max_ptes_none = collapse_max_ptes_none(cc, NULL, order);
+
+		if (max_ptes_none < 0)
+			return collapsed;
+
+		nr_occupied_ptes = collapse_mthp_count_present(cc, offset,
+							       nr_ptes);
+
+		if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
+			int ret;
+
+			collapse_address = address + offset * PAGE_SIZE;
+			ret = collapse_huge_page(mm, collapse_address, referenced,
+						 unmapped, cc, order);
+			if (ret == SCAN_SUCCEED) {
+				collapsed += nr_ptes;
+				continue;
+			}
+		}
+
+next_order:
+		if (order > KHUGEPAGED_MIN_MTHP_ORDER) {
+			const u8 next_order = order - 1;
+			const u16 mid_offset = offset + (nr_ptes / 2);
+
+			collapse_mthp_stack_push(cc, &stack_size, mid_offset,
+						 next_order);
+			collapse_mthp_stack_push(cc, &stack_size, offset,
+						 next_order);
+		}
+	}
+	return collapsed;
+}
+
 static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		struct vm_area_struct *vma, unsigned long start_addr,
 		bool *lock_dropped, struct collapse_control *cc)
 {
-	const int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
+	int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
 	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
 	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
+	enum tva_type tva_flags = cc->is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
 	pmd_t *pmd;
-	pte_t *pte, *_pte;
-	int none_or_zero = 0, shared = 0, referenced = 0;
+	pte_t *pte, *_pte, pteval;
+	int i;
+	int none_or_zero = 0, shared = 0, nr_collapsed = 0, referenced = 0;
 	enum scan_result result = SCAN_FAIL;
 	struct page *page = NULL;
 	struct folio *folio = NULL;
 	unsigned long addr;
+	unsigned long enabled_orders;
 	spinlock_t *ptl;
 	int node = NUMA_NO_NODE, unmapped = 0;
 
@@ -1429,8 +1579,19 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		goto out;
 	}
 
+	bitmap_zero(cc->mthp_bitmap, MAX_PTRS_PER_PTE);
 	memset(cc->node_load, 0, sizeof(cc->node_load));
 	nodes_clear(cc->alloc_nmask);
+
+	enabled_orders = collapse_allowable_orders(vma, vma->vm_flags, tva_flags);
+
+	/*
+	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
+	 * scan all pages to populate the bitmap for mTHP collapse.
+	 */
+	if (enabled_orders != BIT(HPAGE_PMD_ORDER))
+		max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
+
 	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
 	if (!pte) {
 		cc->progress++;
@@ -1438,11 +1599,13 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		goto out;
 	}
 
-	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
-	     _pte++, addr += PAGE_SIZE) {
+	for (i = 0; i < HPAGE_PMD_NR; i++) {
+		_pte = pte + i;
+		addr = start_addr + i * PAGE_SIZE;
+		pteval = ptep_get(_pte);
+
 		cc->progress++;
 
-		pte_t pteval = ptep_get(_pte);
 		if (pte_none_or_zero(pteval)) {
 			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -1522,6 +1685,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			}
 		}
 
+		/* Set bit for occupied pages */
+		__set_bit(i, cc->mthp_bitmap);
 		/*
 		 * Record which node the original page is from and save this
 		 * information to cc->node_load[].
@@ -1580,10 +1745,11 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 	if (result == SCAN_SUCCEED) {
 		/* collapse_huge_page expects the lock to be dropped before calling */
 		mmap_read_unlock(mm);
-		result = collapse_huge_page(mm, start_addr, referenced,
-					    unmapped, cc, HPAGE_PMD_ORDER);
+		nr_collapsed = mthp_collapse(mm, start_addr, referenced, unmapped,
+					      cc, enabled_orders);
 		/* collapse_huge_page will return with the mmap_lock released */
 		*lock_dropped = true;
+		result = nr_collapsed ? SCAN_SUCCEED : SCAN_FAIL;
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
-- 
2.54.0

Re: [PATCH mm-unstable v17 11/14] mm/khugepaged: Introduce mTHP collapse support

[Wei Yang]

On Mon, May 11, 2026 at 12:58:11PM -0600, Nico Pache wrote:
>Enable khugepaged to collapse to mTHP orders. This patch implements the
>main scanning logic using a bitmap to track occupied pages and a stack
>structure that allows us to find optimal collapse sizes.
>
>Previous to this patch, PMD collapse had 3 main phases, a light weight
>scanning phase (mmap_read_lock) that determines a potential PMD
>collapse, an alloc phase (mmap unlocked), then finally heavier collapse
>phase (mmap_write_lock).
>
>To enabled mTHP collapse we make the following changes:
>
>During PMD scan phase, track occupied pages in a bitmap. When mTHP
>orders are enabled, we remove the restriction of max_ptes_none during the
>scan phase to avoid missing potential mTHP collapse candidates. Once we
>have scanned the full PMD range and updated the bitmap to track occupied
>pages, we use the bitmap to find the optimal mTHP size.
>
>Implement collapse_scan_bitmap() to perform binary recursion on the bitmap
>and determine the best eligible order for the collapse. A stack structure
>is used instead of traditional recursion to manage the search. This also
>prevents a traditional recursive approach when the kernel stack struct is
>limited. The algorithm recursively splits the bitmap into smaller chunks to
>find the highest order mTHPs that satisfy the collapse criteria. We start
>by attempting the PMD order, then moved on the consecutively lower orders
>(mTHP collapse). The stack maintains a pair of variables (offset, order),
>indicating the number of PTEs from the start of the PMD, and the order of
>the potential collapse candidate.
>
>The algorithm for consuming the bitmap works as such:
>    1) push (0, HPAGE_PMD_ORDER) onto the stack
>    2) pop the stack
>    3) check if the number of set bits in that (offset,order) pair
>       statisfy the max_ptes_none threshold for that order
>    4) if yes, attempt collapse
>    5) if no (or collapse fails), push two new stack items representing
>       the left and right halves of the current bitmap range, at the
>       next lower order
>    6) repeat at step (2) until stack is empty.
>
>Below is a diagram representing the algorithm and stack items:
>
>                            offset   mid_offset
>                            |        |
>                            |        |
>                            v        v
>          ____________________________________
>         |          PTE Page Table            |
>         --------------------------------------
>			    <-------><------->
>                             order-1  order-1
>
>mTHP collapses reject regions containing swapped out or shared pages.
>This is because adding new entries can lead to new none pages, and these
>may lead to constant promotion into a higher order mTHP. A similar
>issue can occur with "max_ptes_none > HPAGE_PMD_NR/2" due to a collapse
>introducing at least 2x the number of pages, and on a future scan will
>satisfy the promotion condition once again. This issue is prevented via
>the collapse_max_ptes_none() function which imposes the max_ptes_none
>restrictions above.
>
>We currently only support mTHP collapse for max_ptes_none values of 0
>and HPAGE_PMD_NR - 1. resulting in the following behavior:
>
>    - max_ptes_none=0: Never introduce new empty pages during collapse
>    - max_ptes_none=HPAGE_PMD_NR-1: Always try collapse to the highest
>      available mTHP order
>
>Any other max_ptes_none value will emit a warning and skip mTHP collapse
>attempts. There should be no behavior change for PMD collapse.
>
>Once we determine what mTHP sizes fits best in that PMD range a collapse
>is attempted. A minimum collapse order of 2 is used as this is the lowest
>order supported by anon memory as defined by THP_ORDERS_ALL_ANON.
>
>Currently madv_collapse is not supported and will only attempt PMD
>collapse.
>
>We can also remove the check for is_khugepaged inside the PMD scan as
>the collapse_max_ptes_none() function handles this logic now.
>
>Signed-off-by: Nico Pache <npache@redhat.com>

[...]

>+static int mthp_collapse(struct mm_struct *mm, unsigned long address,
>+		int referenced, int unmapped, struct collapse_control *cc,
>+		unsigned long enabled_orders)
>+{
>+	unsigned int nr_occupied_ptes, nr_ptes;
>+	int max_ptes_none, collapsed = 0, stack_size = 0;
>+	unsigned long collapse_address;
>+	struct mthp_range range;
>+	u16 offset;
>+	u8 order;
>+
>+	collapse_mthp_stack_push(cc, &stack_size, 0, HPAGE_PMD_ORDER);
>+
>+	while (stack_size) {
>+		range = collapse_mthp_stack_pop(cc, &stack_size);
>+		order = range.order;
>+		offset = range.offset;
>+		nr_ptes = 1UL << order;
>+
>+		if (!test_bit(order, &enabled_orders))
>+			goto next_order;
>+
>+		max_ptes_none = collapse_max_ptes_none(cc, NULL, order);

I am thinking whether there is a behavioral change for userfaultfd_armed(vma).

collapse_single_pmd()
    collapse_scan_pmd
        max_ptes_none = collapse_max_ptes_none(cc, vma)
        max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT                --- (1)
        mthp_collapse
            max_ptes_none = collapse_max_ptes_none(cc, NULL)     --- (2)
            collapse_huge_page(mm)
                hugepage_vma_revalidate(&vma)
                __collapse_huge_page_isolate(vma)
                    max_ptes_none = collapse_max_ptes_none(cc, vma)

Before mthp_collapse() introduced, userfaultfd_armed(vma) is skipped if there
is any pte_none_or_zero() in collapse_scan_pmd().

But now, max_ptes_none could be set to KHUGEPAGED_MAX_PTES_LIMIT at (1), so
that we can scan all the pte to get the bitmap. This means
userfaultfd_armed(vma) could continue even with pte_none_or_zero().

Then in mthp_collapse(), collapse_max_ptes_none() at (2) ignores
userfaultfd_armed(vma), which means it will continue to collapse a
userfaultfd_armed(vma) when there is pte_none_or_zero(). 

The good news is we will stop at __collapse_huge_page_isolate(), where we
get collapse_max_ptes_none() with vma. But we already did a lot of work.

Not sure if I missed something.

>+
>+		if (max_ptes_none < 0)
>+			return collapsed;
>+
>+		nr_occupied_ptes = collapse_mthp_count_present(cc, offset,
>+							       nr_ptes);
>+
>+		if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
>+			int ret;
>+
>+			collapse_address = address + offset * PAGE_SIZE;
>+			ret = collapse_huge_page(mm, collapse_address, referenced,
>+						 unmapped, cc, order);
>+			if (ret == SCAN_SUCCEED) {
>+				collapsed += nr_ptes;
>+				continue;
>+			}
>+		}
>+
>+next_order:
>+		if (order > KHUGEPAGED_MIN_MTHP_ORDER) {
>+			const u8 next_order = order - 1;
>+			const u16 mid_offset = offset + (nr_ptes / 2);
>+
>+			collapse_mthp_stack_push(cc, &stack_size, mid_offset,
>+						 next_order);
>+			collapse_mthp_stack_push(cc, &stack_size, offset,
>+						 next_order);
>+		}
>+	}
>+	return collapsed;
>+}
>+
> static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> 		struct vm_area_struct *vma, unsigned long start_addr,
> 		bool *lock_dropped, struct collapse_control *cc)
> {
>-	const int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
>+	int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
> 	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
> 	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
>+	enum tva_type tva_flags = cc->is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
> 	pmd_t *pmd;
>-	pte_t *pte, *_pte;
>-	int none_or_zero = 0, shared = 0, referenced = 0;
>+	pte_t *pte, *_pte, pteval;
>+	int i;
>+	int none_or_zero = 0, shared = 0, nr_collapsed = 0, referenced = 0;
> 	enum scan_result result = SCAN_FAIL;
> 	struct page *page = NULL;
> 	struct folio *folio = NULL;
> 	unsigned long addr;
>+	unsigned long enabled_orders;
> 	spinlock_t *ptl;
> 	int node = NUMA_NO_NODE, unmapped = 0;
> 
>@@ -1429,8 +1579,19 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> 		goto out;
> 	}
> 
>+	bitmap_zero(cc->mthp_bitmap, MAX_PTRS_PER_PTE);
> 	memset(cc->node_load, 0, sizeof(cc->node_load));
> 	nodes_clear(cc->alloc_nmask);
>+
>+	enabled_orders = collapse_allowable_orders(vma, vma->vm_flags, tva_flags);

Would it be 0 at this point?

>+
>+	/*
>+	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
>+	 * scan all pages to populate the bitmap for mTHP collapse.
>+	 */
>+	if (enabled_orders != BIT(HPAGE_PMD_ORDER))
>+		max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
>+
> 	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
> 	if (!pte) {
> 		cc->progress++;
>@@ -1438,11 +1599,13 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> 		goto out;
> 	}
> 
>-	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
>-	     _pte++, addr += PAGE_SIZE) {
>+	for (i = 0; i < HPAGE_PMD_NR; i++) {
>+		_pte = pte + i;
>+		addr = start_addr + i * PAGE_SIZE;
>+		pteval = ptep_get(_pte);
>+
> 		cc->progress++;
> 
>-		pte_t pteval = ptep_get(_pte);
> 		if (pte_none_or_zero(pteval)) {
> 			if (++none_or_zero > max_ptes_none) {
> 				result = SCAN_EXCEED_NONE_PTE;
>@@ -1522,6 +1685,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> 			}
> 		}
> 
>+		/* Set bit for occupied pages */
>+		__set_bit(i, cc->mthp_bitmap);
> 		/*
> 		 * Record which node the original page is from and save this
> 		 * information to cc->node_load[].
>@@ -1580,10 +1745,11 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> 	if (result == SCAN_SUCCEED) {
> 		/* collapse_huge_page expects the lock to be dropped before calling */
> 		mmap_read_unlock(mm);
>-		result = collapse_huge_page(mm, start_addr, referenced,
>-					    unmapped, cc, HPAGE_PMD_ORDER);
>+		nr_collapsed = mthp_collapse(mm, start_addr, referenced, unmapped,
>+					      cc, enabled_orders);
> 		/* collapse_huge_page will return with the mmap_lock released */

collapse_huge_page will return with mmap_lock released, but mthp_collapse()
may not?

> 		*lock_dropped = true;
>+		result = nr_collapsed ? SCAN_SUCCEED : SCAN_FAIL;
> 	}
> out:
> 	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
>-- 
>2.54.0

-- 
Wei Yang
Help you, Help me

[PATCH mm-unstable v17 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[Nico Pache]

There are cases where, if an attempted collapse fails, all subsequent
orders are guaranteed to also fail. Avoid these collapse attempts by
bailing out early.

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 39bf7ea8a6e8..f0ae02936638 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1531,9 +1531,31 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address,
 			collapse_address = address + offset * PAGE_SIZE;
 			ret = collapse_huge_page(mm, collapse_address, referenced,
 						 unmapped, cc, order);
-			if (ret == SCAN_SUCCEED) {
+
+			switch (ret) {
+			/* Cases where we continue to next collapse candidate */
+			case SCAN_SUCCEED:
 				collapsed += nr_ptes;
+				fallthrough;
+			case SCAN_PTE_MAPPED_HUGEPAGE:
 				continue;
+			/* Cases where lower orders might still succeed */
+			case SCAN_LACK_REFERENCED_PAGE:
+			case SCAN_EXCEED_NONE_PTE:
+			case SCAN_EXCEED_SWAP_PTE:
+			case SCAN_EXCEED_SHARED_PTE:
+			case SCAN_PAGE_LOCK:
+			case SCAN_PAGE_COUNT:
+			case SCAN_PAGE_NULL:
+			case SCAN_DEL_PAGE_LRU:
+			case SCAN_PTE_NON_PRESENT:
+			case SCAN_PTE_UFFD_WP:
+			case SCAN_ALLOC_HUGE_PAGE_FAIL:
+			case SCAN_PAGE_LAZYFREE:
+				goto next_order;
+			/* Cases where no further collapse is possible */
+			default:
+				return collapsed;
 			}
 		}
 
-- 
2.54.0

[PATCH mm-unstable v17 13/14] mm/khugepaged: run khugepaged for all orders

[Nico Pache]

From: Baolin Wang <baolin.wang@linux.alibaba.com>

If any order (m)THP is enabled we should allow running khugepaged to
attempt scanning and collapsing mTHPs. In order for khugepaged to operate
when only mTHP sizes are specified in sysfs, we must modify the predicate
function that determines whether it ought to run to do so.

This function is currently called hugepage_pmd_enabled(), this patch
renames it to hugepage_enabled() and updates the logic to check to
determine whether any valid orders may exist which would justify
khugepaged running.

We must also update collapse_allowable_orders() to check all orders if
the vma is anonymous and the collapse is khugepaged.

After this patch khugepaged mTHP collapse is fully enabled.

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f0ae02936638..5ba298d420b7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -522,23 +522,23 @@ static inline int collapse_test_exit_or_disable(struct mm_struct *mm)
 		mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm);
 }
 
-static bool hugepage_pmd_enabled(void)
+static bool hugepage_enabled(void)
 {
 	/*
 	 * We cover the anon, shmem and the file-backed case here; file-backed
 	 * hugepages, when configured in, are determined by the global control.
-	 * Anon pmd-sized hugepages are determined by the pmd-size control.
+	 * Anon hugepages are determined by its per-size mTHP control.
 	 * Shmem pmd-sized hugepages are also determined by its pmd-size control,
 	 * except when the global shmem_huge is set to SHMEM_HUGE_DENY.
 	 */
 	if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
 	    hugepage_global_enabled())
 		return true;
-	if (test_bit(PMD_ORDER, &huge_anon_orders_always))
+	if (READ_ONCE(huge_anon_orders_always))
 		return true;
-	if (test_bit(PMD_ORDER, &huge_anon_orders_madvise))
+	if (READ_ONCE(huge_anon_orders_madvise))
 		return true;
-	if (test_bit(PMD_ORDER, &huge_anon_orders_inherit) &&
+	if (READ_ONCE(huge_anon_orders_inherit) &&
 	    hugepage_global_enabled())
 		return true;
 	if (IS_ENABLED(CONFIG_SHMEM) && shmem_hpage_pmd_enabled())
@@ -579,7 +579,13 @@ void __khugepaged_enter(struct mm_struct *mm)
 static unsigned long collapse_allowable_orders(struct vm_area_struct *vma,
 		vm_flags_t vm_flags, enum tva_type tva_flags)
 {
-	unsigned long orders = BIT(HPAGE_PMD_ORDER);
+	unsigned long orders;
+
+	/* If khugepaged is scanning an anonymous vma, allow mTHP collapse */
+	if ((tva_flags == TVA_KHUGEPAGED) && vma_is_anonymous(vma))
+		orders = THP_ORDERS_ALL_ANON;
+	else
+		orders = BIT(HPAGE_PMD_ORDER);
 
 	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
 }
@@ -588,10 +594,9 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
 			  vm_flags_t vm_flags)
 {
 	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
-	    hugepage_pmd_enabled()) {
-		if (collapse_allowable_orders(vma, vm_flags, TVA_KHUGEPAGED))
-			__khugepaged_enter(vma->vm_mm);
-	}
+	    collapse_allowable_orders(vma, vm_flags, TVA_KHUGEPAGED) &&
+	    hugepage_enabled())
+		__khugepaged_enter(vma->vm_mm);
 }
 
 void __khugepaged_exit(struct mm_struct *mm)
@@ -2945,7 +2950,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
 
 static int khugepaged_has_work(void)
 {
-	return !list_empty(&khugepaged_scan.mm_head) && hugepage_pmd_enabled();
+	return !list_empty(&khugepaged_scan.mm_head) && hugepage_enabled();
 }
 
 static int khugepaged_wait_event(void)
@@ -3018,7 +3023,7 @@ static void khugepaged_wait_work(void)
 		return;
 	}
 
-	if (hugepage_pmd_enabled())
+	if (hugepage_enabled())
 		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
 }
 
@@ -3049,7 +3054,7 @@ void set_recommended_min_free_kbytes(void)
 	int nr_zones = 0;
 	unsigned long recommended_min;
 
-	if (!hugepage_pmd_enabled()) {
+	if (!hugepage_enabled()) {
 		calculate_min_free_kbytes();
 		goto update_wmarks;
 	}
@@ -3099,7 +3104,7 @@ int start_stop_khugepaged(void)
 	int err = 0;
 
 	mutex_lock(&khugepaged_mutex);
-	if (hugepage_pmd_enabled()) {
+	if (hugepage_enabled()) {
 		if (!khugepaged_thread)
 			khugepaged_thread = kthread_run(khugepaged, NULL,
 							"khugepaged");
@@ -3125,7 +3130,7 @@ int start_stop_khugepaged(void)
 void khugepaged_min_free_kbytes_update(void)
 {
 	mutex_lock(&khugepaged_mutex);
-	if (hugepage_pmd_enabled() && khugepaged_thread)
+	if (hugepage_enabled() && khugepaged_thread)
 		set_recommended_min_free_kbytes();
 	mutex_unlock(&khugepaged_mutex);
 }
-- 
2.54.0

[PATCH mm-unstable v17 14/14] Documentation: mm: update the admin guide for mTHP collapse

[Nico Pache]

Now that we can collapse to mTHPs lets update the admin guide to
reflect these changes and provide proper guidance on how to utilize it.

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 49 +++++++++++++---------
 1 file changed, 29 insertions(+), 20 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 80a4d0bed70b..fc0127a36ef6 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -63,7 +63,8 @@ often.
 THP can be enabled system wide or restricted to certain tasks or even
 memory ranges inside task's address space. Unless THP is completely
 disabled, there is ``khugepaged`` daemon that scans memory and
-collapses sequences of basic pages into PMD-sized huge pages.
+collapses sequences of basic pages into huge pages of either PMD size
+or mTHP sizes, if the system is configured to do so.
 
 The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
 interface and using madvise(2) and prctl(2) system calls.
@@ -219,10 +220,10 @@ this behaviour by writing 0 to shrink_underused, and enable it by writing
 	echo 0 > /sys/kernel/mm/transparent_hugepage/shrink_underused
 	echo 1 > /sys/kernel/mm/transparent_hugepage/shrink_underused
 
-khugepaged will be automatically started when PMD-sized THP is enabled
+khugepaged will be automatically started when any THP size is enabled
 (either of the per-size anon control or the top-level control are set
 to "always" or "madvise"), and it'll be automatically shutdown when
-PMD-sized THP is disabled (when both the per-size anon control and the
+all THP sizes are disabled (when both the per-size anon control and the
 top-level control are "never")
 
 process THP controls
@@ -264,11 +265,6 @@ support the following arguments::
 Khugepaged controls
 -------------------
 
-.. note::
-   khugepaged currently only searches for opportunities to collapse to
-   PMD-sized THP and no attempt is made to collapse to other THP
-   sizes.
-
 khugepaged runs usually at low frequency so while one may not want to
 invoke defrag algorithms synchronously during the page faults, it
 should be worth invoking defrag at least in khugepaged. However it's
@@ -296,11 +292,11 @@ allocation failure to throttle the next allocation attempt::
 The khugepaged progress can be seen in the number of pages collapsed (note
 that this counter may not be an exact count of the number of pages
 collapsed, since "collapsed" could mean multiple things: (1) A PTE mapping
-being replaced by a PMD mapping, or (2) All 4K physical pages replaced by
-one 2M hugepage. Each may happen independently, or together, depending on
-the type of memory and the failures that occur. As such, this value should
-be interpreted roughly as a sign of progress, and counters in /proc/vmstat
-consulted for more accurate accounting)::
+being replaced by a PMD mapping, or (2) physical pages replaced by one
+hugepage of various sizes (PMD-sized or mTHP). Each may happen independently,
+or together, depending on the type of memory and the failures that occur.
+As such, this value should be interpreted roughly as a sign of progress,
+and counters in /proc/vmstat consulted for more accurate accounting)::
 
 	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
 
@@ -308,16 +304,20 @@ for each pass::
 
 	/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
 
-``max_ptes_none`` specifies how many extra small pages (that are
-not already mapped) can be allocated when collapsing a group
-of small pages into one large page::
+``max_ptes_none`` specifies how many empty (none/zero) pages are allowed
+when collapsing a group of small pages into one large page::
 
 	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
 
-A higher value leads to use additional memory for programs.
-A lower value leads to gain less thp performance. Value of
-max_ptes_none can waste cpu time very little, you can
-ignore it.
+For PMD-sized THP collapse, this directly limits the number of empty pages
+allowed in the 2MB region.
+
+For mTHP collapse, only 0 or (HPAGE_PMD_NR - 1) are supported. Any other value
+will emit a warning and no mTHP collapse will be attempted.
+
+A higher value allows more empty pages, potentially leading to more memory
+usage but better THP performance. A lower value is more conservative and
+may result in fewer THP collapses.
 
 ``max_ptes_swap`` specifies how many pages can be brought in from
 swap when collapsing a group of pages into a transparent huge page::
@@ -337,6 +337,15 @@ that THP is shared. Exceeding the number would block the collapse::
 
 A higher value may increase memory footprint for some workloads.
 
+.. note::
+   For mTHP collapse, khugepaged does not support collapsing regions that
+   contain shared or swapped out pages, as this could lead to continuous
+   promotion to higher orders. The collapse will fail if any shared or
+   swapped PTEs are encountered during the scan.
+
+   Currently, madvise_collapse only supports collapsing to PMD-sized THPs
+   and does not attempt mTHP collapses.
+
 Boot parameters
 ===============
 
-- 
2.54.0

Re: [PATCH mm-unstable v17 00/14] khugepaged: mTHP support

[Andrew Morton]

On Mon, 11 May 2026 12:58:00 -0600 Nico Pache <npache@redhat.com> wrote:

> The following series provides khugepaged with the capability to collapse
> anonymous memory regions to mTHPs.

Thanks, I've updated mm.git's mm-new branch to this version.

> V17 Changes:
> - Added Acks/RB
> - New patch(5): split the mmap_read_unlock() locking contract change out of
>   "generalize collapse_huge_page" into its own patch; add a comment
>   documenting the enter/exit-with-lock-dropped contract (Usama, David)
> - [patch 03] Add const to max_ptes_none/shared/swap variables; improve the
>   three helper docstrings; replace the paragraphs with inline comments;
>   note that sysctl values are now snapshotted once per scan (Usama, David)
> - [patch 04] Add SCAN_INVALID_PTES_NONE result code and return it instead
>   of SCAN_FAIL when collapse_max_ptes_none() returns -EINVAL (Usama);
>   snapshot khugepaged_max_ptes_none into a local variable to fix race on
>   the two comparisons (Usama); clean up mTHP docstring paragraphs into
>   inline comments; fix commit message wording (David)
> - [patch 06] Remove /* PMD collapse */ and /* mTHP collapse */ comments
>   (David); move const declarations to top of variable list (David); add
>   comment explaining that map_anon_folio_pte_nopf() calls set_ptes under
>   pmd_ptl and is safe because PMD is expected to be none (Usama)
> - [patch 08] Shorten sysfs counter documentation for
>   collapse_exceed_swap/shared_pte to concise one-liners; trim
>   collapse_exceed_none_pte description; fix "dont" → "do not" (David)
> - [patch 10] Keep vm_flags parameter in khugepaged_enter_vma() and
>   collapse_allowable_orders() rather than dropping it and reading
>   vma->vm_flags internally; pass vm_flags explicitly at all three
>   collapse_allowable_orders() call sites (David, sashskio)
> - [patch 11] Fix MTHP_STACK_SIZE: was exponential (~128); correct formula
>   is (height + 1) for a DFS on a binary tree rewrite comment to explain
>   the DFS sizing (sashskio)
> - [patch 12] Replace SCAN_PAGE_LRU with SCAN_PAGE_LAZYFREE in the
>   "goto next_order" early-bail cases; non-LRU page failures cannot be
>   recovered at any order and belong in the default (return) path
> - [patch 13] Use tva_flags == TVA_KHUGEPAGED (strict equality) instead of
>   tva_flags & TVA_KHUGEPAGED; flatten nested if into single condition;
>   retain vm_flags parameter; pass vm_flags to collapse_allowable_orders()

Here's how v17 altered mm.git:


 Documentation/admin-guide/mm/transhuge.rst |   24 ---
 include/linux/khugepaged.h                 |    6 
 include/trace/events/huge_memory.h         |    3 
 mm/huge_memory.c                           |    2 
 mm/khugepaged.c                            |  152 ++++++++++---------
 mm/vma.c                                   |    6 
 tools/testing/vma/include/stubs.h          |    3 
 7 files changed, 99 insertions(+), 97 deletions(-)

--- a/Documentation/admin-guide/mm/transhuge.rst~b
+++ a/Documentation/admin-guide/mm/transhuge.rst
@@ -725,27 +725,17 @@ nr_anon_partially_mapped
 
 collapse_exceed_none_pte
        The number of collapse attempts that failed due to exceeding the
-       max_ptes_none threshold. For mTHP collapse, Currently only max_ptes_none
-       values of 0 and (HPAGE_PMD_NR - 1) are supported. Any other value will
-       emit a warning and no mTHP collapse will be attempted. khugepaged will
-       try to collapse to the largest enabled (m)THP size; if it fails, it will
-       try the next lower enabled mTHP size. This counter records the number of
-       times a collapse attempt was skipped for exceeding the max_ptes_none
-       threshold, and khugepaged will move on to the next available mTHP size.
+       max_ptes_none threshold.
 
 collapse_exceed_swap_pte
-       The number of anonymous mTHP PTE ranges which were unable to collapse due
-       to containing at least one swap PTE. Currently khugepaged does not
-       support collapsing mTHP regions that contain a swap PTE. This counter can
-       be used to monitor the number of khugepaged mTHP collapses that failed
-       due to the presence of a swap PTE.
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_swap threshold. For non-PMD orders this occurs if a mTHP range
+       contains at least one swap PTE.
 
 collapse_exceed_shared_pte
-       The number of anonymous mTHP PTE ranges which were unable to collapse due
-       to containing at least one shared PTE. Currently khugepaged does not
-       support collapsing mTHP PTE ranges that contain a shared PTE. This
-       counter can be used to monitor the number of khugepaged mTHP collapses
-       that failed due to the presence of a shared PTE.
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_shared threshold. For non-PMD orders this occurs if a mTHP range
+       contains at least one shared PTE.
 
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
--- a/include/linux/khugepaged.h~b
+++ a/include/linux/khugepaged.h
@@ -13,7 +13,8 @@ extern void khugepaged_destroy(void);
 extern int start_stop_khugepaged(void);
 extern void __khugepaged_enter(struct mm_struct *mm);
 extern void __khugepaged_exit(struct mm_struct *mm);
-extern void khugepaged_enter_vma(struct vm_area_struct *vma);
+extern void khugepaged_enter_vma(struct vm_area_struct *vma,
+				 vm_flags_t vm_flags);
 extern void khugepaged_min_free_kbytes_update(void);
 extern bool current_is_khugepaged(void);
 void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
@@ -37,7 +38,8 @@ static inline void khugepaged_fork(struc
 static inline void khugepaged_exit(struct mm_struct *mm)
 {
 }
-static inline void khugepaged_enter_vma(struct vm_area_struct *vma)
+static inline void khugepaged_enter_vma(struct vm_area_struct *vma,
+					vm_flags_t vm_flags)
 {
 }
 static inline void collapse_pte_mapped_thp(struct mm_struct *mm,
--- a/include/trace/events/huge_memory.h~b
+++ a/include/trace/events/huge_memory.h
@@ -39,7 +39,8 @@
 	EM( SCAN_STORE_FAILED,		"store_failed")			\
 	EM( SCAN_COPY_MC,		"copy_poisoned_page")		\
 	EM( SCAN_PAGE_FILLED,		"page_filled")			\
-	EMe(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")
+	EM(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback")	\
+	EMe(SCAN_INVALID_PTES_NONE,	"invalid_ptes_none")
 
 #undef EM
 #undef EMe
--- a/mm/huge_memory.c~b
+++ a/mm/huge_memory.c
@@ -1571,7 +1571,7 @@ vm_fault_t do_huge_pmd_anonymous_page(st
 	ret = vmf_anon_prepare(vmf);
 	if (ret)
 		return ret;
-	khugepaged_enter_vma(vma);
+	khugepaged_enter_vma(vma, vma->vm_flags);
 
 	if (!(vmf->flags & FAULT_FLAG_WRITE) &&
 			!mm_forbids_zeropage(vma->vm_mm) &&
--- a/mm/khugepaged.c~b
+++ a/mm/khugepaged.c
@@ -61,6 +61,7 @@ enum scan_result {
 	SCAN_COPY_MC,
 	SCAN_PAGE_FILLED,
 	SCAN_PAGE_DIRTY_OR_WRITEBACK,
+	SCAN_INVALID_PTES_NONE,
 };
 
 #define CREATE_TRACE_POINTS
@@ -101,16 +102,15 @@ static struct kmem_cache *mm_slot_cache
 
 #define KHUGEPAGED_MIN_MTHP_ORDER	2
 /*
- * The maximum number of mTHP ranges that can be stored on the stack.
- * This is calculated based on the number of PTE entries in a PTE page table
- * and the minimum mTHP order.
+ * mthp_collapse() does an iterative DFS over a binary tree, from
+ * HPAGE_PMD_ORDER down to KHUGEPAGED_MIN_MTHP_ORDER. The max stack
+ * size needed for a DFS on a binary tree is height + 1, where
+ * height = HPAGE_PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER.
  *
- * ilog2 is needed in place of HPAGE_PMD_ORDER due to some architectures
- * (ie ppc64le) not defining HPAGE_PMD_ORDER until after build time.
- *
- * At most there will be 1 << (PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER) mTHP ranges
+ * ilog2 is used in place of HPAGE_PMD_ORDER because some architectures
+ * (e.g. ppc64le) do not define HPAGE_PMD_ORDER until after build time.
  */
-#define MTHP_STACK_SIZE	(1UL << (ilog2(MAX_PTRS_PER_PTE) - KHUGEPAGED_MIN_MTHP_ORDER))
+#define MTHP_STACK_SIZE	(ilog2(MAX_PTRS_PER_PTE) - KHUGEPAGED_MIN_MTHP_ORDER + 1)
 
 /*
  * Defines a range of PTE entries in a PTE page table which are being
@@ -380,89 +380,87 @@ static bool pte_none_or_zero(pte_t pte)
 }
 
 /**
- * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
+ * collapse_max_ptes_none - Calculate maximum allowed none-page or zero-page
+ * PTEs for the given collapse operation.
  * @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. 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
+ * Return: Maximum number of none-page or zero-page PTEs allowed for the
+ * collapse operation.
  */
 static int collapse_max_ptes_none(struct collapse_control *cc,
 		struct vm_area_struct *vma, unsigned int order)
 {
+	unsigned int max_ptes_none = khugepaged_max_ptes_none;
+	// If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs.
 	if (vma && userfaultfd_armed(vma))
 		return 0;
+	// for MADV_COLLAPSE, allow any none-page or zero-page PTEs.
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
+	// for PMD collapse, respect the user defined maximum.
 	if (is_pmd_order(order))
-		return khugepaged_max_ptes_none;
+		return max_ptes_none;
 	/* Zero/non-present collapse disabled. */
-	if (!khugepaged_max_ptes_none)
+	if (!max_ptes_none)
 		return 0;
-	if (khugepaged_max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
+	// for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT,
+	// scale the maximum number of PTEs to the order of the collapse.
+	if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
 		return (1 << order) - 1;
 
+	// We currently only support max_ptes_none values of 0 or KHUGEPAGED_MAX_PTES_LIMIT.
+	// Emit a warning and return -EINVAL.
 	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
+ * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
+ * anonymous pages for the given collapse operation.
  * @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
+ * Return: Maximum number of PTEs that map shared anonymous pages for the
+ * collapse operation
  */
 static unsigned int collapse_max_ptes_shared(struct collapse_control *cc,
 		unsigned int order)
 {
+	// for MADV_COLLAPSE, do not restrict the number of PTEs that map shared
+	// anonymous pages.
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
+	// for mTHP collapse do not allow collapsing anonymous memory pages that
+	// are shared between processes.
 	if (!is_pmd_order(order))
 		return 0;
-
+	// for PMD collapse, respect the user defined maximum.
 	return khugepaged_max_ptes_shared;
 }
 
 /**
- * collapse_max_ptes_swap - Calculate maximum allowed swap PTEs for collapse
+ * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
+ * maximum allowed non-present pagecache entries for the given collapse operation.
  * @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
+ * Return: Maximum number of non-present PTEs or the maximum allowed non-present
+ * pagecache entries for the collapse operation.
  */
 static unsigned int collapse_max_ptes_swap(struct collapse_control *cc,
 		unsigned int order)
 {
+	// for MADV_COLLAPSE, do not restrict the number PTEs entries or
+	// pagecache entries that are non-present.
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
+	// for mTHP collapse do not allow any non-present PTEs or pagecache entries.
 	if (!is_pmd_order(order))
 		return 0;
-
+	// for PMD collapse, respect the user defined maximum.
 	return khugepaged_max_ptes_swap;
 }
 
@@ -478,7 +476,7 @@ int hugepage_madvise(struct vm_area_stru
 		 * register it here without waiting a page fault that
 		 * may not happen any time soon.
 		 */
-		khugepaged_enter_vma(vma);
+		khugepaged_enter_vma(vma, *vm_flags);
 		break;
 	case MADV_NOHUGEPAGE:
 		*vm_flags &= ~VM_HUGEPAGE;
@@ -579,26 +577,26 @@ void __khugepaged_enter(struct mm_struct
 
 /* Check what orders are allowed based on the vma and collapse type */
 static unsigned long collapse_allowable_orders(struct vm_area_struct *vma,
-		enum tva_type tva_flags)
+		vm_flags_t vm_flags, enum tva_type tva_flags)
 {
 	unsigned long orders;
 
 	/* If khugepaged is scanning an anonymous vma, allow mTHP collapse */
-	if ((tva_flags & TVA_KHUGEPAGED) && vma_is_anonymous(vma))
+	if ((tva_flags == TVA_KHUGEPAGED) && vma_is_anonymous(vma))
 		orders = THP_ORDERS_ALL_ANON;
 	else
 		orders = BIT(HPAGE_PMD_ORDER);
 
-	return thp_vma_allowable_orders(vma, vma->vm_flags, tva_flags, orders);
+	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
 }
 
-void khugepaged_enter_vma(struct vm_area_struct *vma)
+void khugepaged_enter_vma(struct vm_area_struct *vma,
+			  vm_flags_t vm_flags)
 {
 	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
-	    hugepage_enabled()) {
-		if (collapse_allowable_orders(vma, TVA_KHUGEPAGED))
-			__khugepaged_enter(vma->vm_mm);
-	}
+	    collapse_allowable_orders(vma, vm_flags, TVA_KHUGEPAGED) &&
+	    hugepage_enabled())
+		__khugepaged_enter(vma->vm_mm);
 }
 
 void __khugepaged_exit(struct mm_struct *mm)
@@ -683,7 +681,7 @@ static enum scan_result __collapse_huge_
 	unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, order);
 
 	if (max_ptes_none < 0)
-		return result;
+		return SCAN_INVALID_PTES_NONE;
 
 	for (_pte = pte; _pte < pte + nr_pages;
 	     _pte++, addr += PAGE_SIZE) {
@@ -905,6 +903,7 @@ static void __collapse_huge_page_copy_fa
 {
 	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
@@ -944,6 +943,7 @@ static enum scan_result __collapse_huge_
 	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.
 	 */
@@ -1263,10 +1263,20 @@ static enum scan_result alloc_charge_fol
 	return SCAN_SUCCEED;
 }
 
+/*
+ * collapse_huge_page expects the mmap_read_lock to be dropped before
+ * entering this function. The function will also always return with the lock
+ * dropped. The function starts by allocation a folio, which can potentially
+ * take a long time if it involves sync compaction, and we do not need to hold
+ * the mmap_lock during that. We must recheck the vma after taking it again in
+ * write mode.
+ */
 static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
 		int referenced, int unmapped, struct collapse_control *cc,
 		unsigned int order)
 {
+	const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK;
+	const unsigned long end_addr = start_addr + (PAGE_SIZE << order);
 	LIST_HEAD(compound_pagelist);
 	pmd_t *pmd, _pmd;
 	pte_t *pte = NULL;
@@ -1277,8 +1287,6 @@ static enum scan_result collapse_huge_pa
 	struct vm_area_struct *vma;
 	struct mmu_notifier_range range;
 	bool anon_vma_locked = false;
-	const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK;
-	const unsigned long end_addr = start_addr + (PAGE_SIZE << order);
 
 	result = alloc_charge_folio(&folio, mm, cc, order);
 	if (result != SCAN_SUCCEED)
@@ -1399,11 +1407,16 @@ static enum scan_result collapse_huge_pa
 	__folio_mark_uptodate(folio);
 	spin_lock(pmd_ptl);
 	WARN_ON_ONCE(!pmd_none(*pmd));
-	if (is_pmd_order(order)) { /* PMD collapse */
+	if (is_pmd_order(order)) {
 		pgtable = pmd_pgtable(_pmd);
 		pgtable_trans_huge_deposit(mm, pmd, pgtable);
 		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
-	} else { /* mTHP collapse */
+	} else {
+		/*
+		 * set_ptes is called in map_anon_folio_pte_nopf with the
+		 * pmd_ptl lock still held; this is safe as the PMD is expected
+		 * to be none. The pmd entry is then repopulated below.
+		 */
 		map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false);
 		smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
 		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
@@ -1538,12 +1551,12 @@ static int mthp_collapse(struct mm_struc
 			case SCAN_EXCEED_SHARED_PTE:
 			case SCAN_PAGE_LOCK:
 			case SCAN_PAGE_COUNT:
-			case SCAN_PAGE_LRU:
 			case SCAN_PAGE_NULL:
 			case SCAN_DEL_PAGE_LRU:
 			case SCAN_PTE_NON_PRESENT:
 			case SCAN_PTE_UFFD_WP:
 			case SCAN_ALLOC_HUGE_PAGE_FAIL:
+			case SCAN_PAGE_LAZYFREE:
 				goto next_order;
 			/* Cases where no further collapse is possible */
 			default:
@@ -1569,6 +1582,10 @@ static enum scan_result collapse_scan_pm
 		struct vm_area_struct *vma, unsigned long start_addr,
 		bool *lock_dropped, struct collapse_control *cc)
 {
+	int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
+	enum tva_type tva_flags = cc->is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
 	pmd_t *pmd;
 	pte_t *pte, *_pte, pteval;
 	int i;
@@ -1580,10 +1597,6 @@ static enum scan_result collapse_scan_pm
 	unsigned long enabled_orders;
 	spinlock_t *ptl;
 	int node = NUMA_NO_NODE, unmapped = 0;
-	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);
-	enum tva_type tva_flags = cc->is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
 
 	VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
 
@@ -1597,7 +1610,7 @@ static enum scan_result collapse_scan_pm
 	memset(cc->node_load, 0, sizeof(cc->node_load));
 	nodes_clear(cc->alloc_nmask);
 
-	enabled_orders = collapse_allowable_orders(vma, tva_flags);
+	enabled_orders = collapse_allowable_orders(vma, vma->vm_flags, tva_flags);
 
 	/*
 	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
@@ -1757,12 +1770,7 @@ static enum scan_result collapse_scan_pm
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
 	if (result == SCAN_SUCCEED) {
-		/*
-		 * Before allocating the hugepage, release the mmap_lock read lock.
-		 * The allocation can take potentially a long time if it involves
-		 * sync compaction, and we do not need to hold the mmap_lock during
-		 * that. We will recheck the vma after taking it again in write mode.
-		 */
+		/* collapse_huge_page expects the lock to be dropped before calling */
 		mmap_read_unlock(mm);
 		nr_collapsed = mthp_collapse(mm, start_addr, referenced, unmapped,
 					      cc, enabled_orders);
@@ -2657,14 +2665,14 @@ static enum scan_result collapse_scan_fi
 		unsigned long addr, struct file *file, pgoff_t start,
 		struct collapse_control *cc)
 {
+	const int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
 	struct folio *folio = NULL;
 	struct address_space *mapping = file->f_mapping;
 	XA_STATE(xas, &mapping->i_pages, start);
 	int present, swap;
 	int node = NUMA_NO_NODE;
 	enum scan_result result = SCAN_SUCCEED;
-	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;
@@ -2867,7 +2875,7 @@ static void collapse_scan_mm_slot(unsign
 			cc->progress++;
 			break;
 		}
-		if (!collapse_allowable_orders(vma, TVA_KHUGEPAGED)) {
+		if (!collapse_allowable_orders(vma, vma->vm_flags, TVA_KHUGEPAGED)) {
 			cc->progress++;
 			continue;
 		}
@@ -3177,7 +3185,7 @@ int madvise_collapse(struct vm_area_stru
 	BUG_ON(vma->vm_start > start);
 	BUG_ON(vma->vm_end < end);
 
-	if (!collapse_allowable_orders(vma, TVA_FORCED_COLLAPSE))
+	if (!collapse_allowable_orders(vma, vma->vm_flags, TVA_FORCED_COLLAPSE))
 		return -EINVAL;
 
 	cc = kmalloc_obj(*cc);
--- a/mm/vma.c~b
+++ a/mm/vma.c
@@ -989,7 +989,7 @@ static __must_check struct vm_area_struc
 		goto abort;
 
 	vma_set_flags_mask(vmg->target, sticky_flags);
-	khugepaged_enter_vma(vmg->target);
+	khugepaged_enter_vma(vmg->target, vmg->vm_flags);
 	vmg->state = VMA_MERGE_SUCCESS;
 	return vmg->target;
 
@@ -1110,7 +1110,7 @@ struct vm_area_struct *vma_merge_new_ran
 	 * following VMA if we have VMAs on both sides.
 	 */
 	if (vmg->target && !vma_expand(vmg)) {
-		khugepaged_enter_vma(vmg->target);
+		khugepaged_enter_vma(vmg->target, vmg->vm_flags);
 		vmg->state = VMA_MERGE_SUCCESS;
 		return vmg->target;
 	}
@@ -2589,7 +2589,7 @@ static int __mmap_new_vma(struct mmap_st
 	 * call covers the non-merge case.
 	 */
 	if (!vma_is_anonymous(vma))
-		khugepaged_enter_vma(vma);
+		khugepaged_enter_vma(vma, map->vm_flags);
 	*vmap = vma;
 	return 0;
 
--- a/tools/testing/vma/include/stubs.h~b
+++ a/tools/testing/vma/include/stubs.h
@@ -183,7 +183,8 @@ static inline bool mpol_equal(struct mem
 	return true;
 }
 
-static inline void khugepaged_enter_vma(struct vm_area_struct *vma)
+static inline void khugepaged_enter_vma(struct vm_area_struct *vma,
+			  vm_flags_t vm_flags)
 {
 }
 
_