[PATCH v13 mm-new 00/16] khugepaged: mTHP support

[PATCH v13 mm-new 00/16] 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 limitiations 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:     Refactor/rename hpage_collapse
Patch 2:     Refactoring to combine madvise_collapse and khugepaged
Patch 3-8:   Generalize khugepaged functions for arbitrary orders and
	     introduce some helper functions
Patch 9:     skip collapsing mTHP to smaller orders
Patch 10-11: Add per-order mTHP statistics and tracepoints
Patch 12:    Introduce collapse_allowable_orders
Patch 13-15: Introduce bitmap and mTHP collapse support, fully enabled
Patch 16:    Documentation

---------
 Testing
---------
- Built for x86_64, aarch64, ppc64le, and s390x
- 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 tested 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.

V13 Changes:
- Lots of minor nits, cleanups, comments, and renames
- Bitmap function simplification and more helpers (Wei, Lorenzo)
- Max_ptes_none (0 or 511) restriction
- commit description expansion
- list all reachable enum values in mthp_collapse()
- Fix ppc64 compile error due to using HPAGE_PMD_ORDER (replace with
  ilog2(MAX_PTRS_PER_PTE))

V12: https://lore.kernel.org/lkml/20251022183717.70829-1-npache@redhat.com/
V11: https://lore.kernel.org/lkml/20250912032810.197475-1-npache@redhat.com/
V10: https://lore.kernel.org/lkml/20250819134205.622806-1-npache@redhat.com/
V9 : https://lore.kernel.org/lkml/20250714003207.113275-1-npache@redhat.com/
V8 : https://lore.kernel.org/lkml/20250702055742.102808-1-npache@redhat.com/
V7 : https://lore.kernel.org/lkml/20250515032226.128900-1-npache@redhat.com/
V6 : https://lore.kernel.org/lkml/20250515030312.125567-1-npache@redhat.com/
V5 : https://lore.kernel.org/lkml/20250428181218.85925-1-npache@redhat.com/
V4 : https://lore.kernel.org/lkml/20250417000238.74567-1-npache@redhat.com/
V3 : https://lore.kernel.org/lkml/20250414220557.35388-1-npache@redhat.com/
V2 : https://lore.kernel.org/lkml/20250211003028.213461-1-npache@redhat.com/
V1 : https://lore.kernel.org/lkml/20250108233128.14484-1-npache@redhat.com/

A big thanks to everyone that has reviewed, tested, and participated in
the development process. Its been a great experience working with all of
you on this endeavour.

[1] - https://gitlab.com/npache/khugepaged_mthp_test
[2] - https://lore.kernel.org/lkml/20250515033857.132535-1-npache@redhat.com/

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

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

Nico Pache (14):
  khugepaged: rename hpage_collapse_* to collapse_*
  introduce collapse_single_pmd to unify khugepaged and madvise_collapse
  khugepaged: generalize hugepage_vma_revalidate for mTHP support
  khugepaged: introduce is_mthp_order helper
  khugepaged: generalize __collapse_huge_page_* for mTHP support
  khugepaged: introduce collapse_max_ptes_none helper function
  khugepaged: generalize collapse_huge_page for mTHP collapse
  khugepaged: skip collapsing mTHP to smaller orders
  khugepaged: add per-order mTHP collapse failure statistics
  khugepaged: improve tracepoints for mTHP orders
  khugepaged: introduce collapse_allowable_orders helper function
  khugepaged: Introduce mTHP collapse support
  khugepaged: avoid unnecessary mTHP collapse attempts
  Documentation: mm: update the admin guide for mTHP collapse

 Documentation/admin-guide/mm/transhuge.rst |  80 ++-
 include/linux/huge_mm.h                    |   5 +
 include/trace/events/huge_memory.h         |  34 +-
 mm/huge_memory.c                           |  11 +
 mm/khugepaged.c                            | 696 +++++++++++++++------
 mm/mremap.c                                |   2 +-
 6 files changed, 618 insertions(+), 210 deletions(-)

-- 
2.51.1

[PATCH v13 mm-new 01/16] khugepaged: rename hpage_collapse_* to collapse_*

[Nico Pache]

The hpage_collapse functions describe functions used by madvise_collapse
and khugepaged. remove the unnecessary hpage prefix to shorten the
function name.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 73 ++++++++++++++++++++++++-------------------------
 mm/mremap.c     |  2 +-
 2 files changed, 37 insertions(+), 38 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 97d1b2824386..959be77f2e65 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -394,14 +394,14 @@ void __init khugepaged_destroy(void)
 	kmem_cache_destroy(mm_slot_cache);
 }
 
-static inline int hpage_collapse_test_exit(struct mm_struct *mm)
+static inline int collapse_test_exit(struct mm_struct *mm)
 {
 	return atomic_read(&mm->mm_users) == 0;
 }
 
-static inline int hpage_collapse_test_exit_or_disable(struct mm_struct *mm)
+static inline int collapse_test_exit_or_disable(struct mm_struct *mm)
 {
-	return hpage_collapse_test_exit(mm) ||
+	return collapse_test_exit(mm) ||
 		mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm);
 }
 
@@ -435,7 +435,7 @@ void __khugepaged_enter(struct mm_struct *mm)
 	int wakeup;
 
 	/* __khugepaged_exit() must not run from under us */
-	VM_BUG_ON_MM(hpage_collapse_test_exit(mm), mm);
+	VM_BUG_ON_MM(collapse_test_exit(mm), mm);
 	if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm)))
 		return;
 
@@ -489,7 +489,7 @@ void __khugepaged_exit(struct mm_struct *mm)
 	} else if (slot) {
 		/*
 		 * This is required to serialize against
-		 * hpage_collapse_test_exit() (which is guaranteed to run
+		 * collapse_test_exit() (which is guaranteed to run
 		 * under mmap sem read mode). Stop here (after we return all
 		 * pagetables will be destroyed) until khugepaged has finished
 		 * working on the pagetables under the mmap_lock.
@@ -579,7 +579,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		folio = page_folio(page);
 		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
 
-		/* See hpage_collapse_scan_pmd(). */
+		/* See collapse_scan_pmd(). */
 		if (folio_maybe_mapped_shared(folio)) {
 			++shared;
 			if (cc->is_khugepaged &&
@@ -830,7 +830,7 @@ struct collapse_control khugepaged_collapse_control = {
 	.is_khugepaged = true,
 };
 
-static bool hpage_collapse_scan_abort(int nid, struct collapse_control *cc)
+static bool collapse_scan_abort(int nid, struct collapse_control *cc)
 {
 	int i;
 
@@ -865,7 +865,7 @@ static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void)
 }
 
 #ifdef CONFIG_NUMA
-static int hpage_collapse_find_target_node(struct collapse_control *cc)
+static int collapse_find_target_node(struct collapse_control *cc)
 {
 	int nid, target_node = 0, max_value = 0;
 
@@ -884,7 +884,7 @@ static int hpage_collapse_find_target_node(struct collapse_control *cc)
 	return target_node;
 }
 #else
-static int hpage_collapse_find_target_node(struct collapse_control *cc)
+static int collapse_find_target_node(struct collapse_control *cc)
 {
 	return 0;
 }
@@ -905,7 +905,7 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 	enum tva_type type = cc->is_khugepaged ? TVA_KHUGEPAGED :
 				 TVA_FORCED_COLLAPSE;
 
-	if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
+	if (unlikely(collapse_test_exit_or_disable(mm)))
 		return SCAN_ANY_PROCESS;
 
 	*vmap = vma = find_vma(mm, address);
@@ -978,7 +978,7 @@ static int check_pmd_still_valid(struct mm_struct *mm,
 
 /*
  * Bring missing pages in from swap, to complete THP collapse.
- * Only done if hpage_collapse_scan_pmd believes it is worthwhile.
+ * Only done if khugepaged_scan_pmd believes it is worthwhile.
  *
  * Called and returns without pte mapped or spinlocks held.
  * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
@@ -1065,7 +1065,7 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
 {
 	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
 		     GFP_TRANSHUGE);
-	int node = hpage_collapse_find_target_node(cc);
+	int node = collapse_find_target_node(cc);
 	struct folio *folio;
 
 	folio = __folio_alloc(gfp, HPAGE_PMD_ORDER, node, &cc->alloc_nmask);
@@ -1244,10 +1244,10 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	return result;
 }
 
-static int hpage_collapse_scan_pmd(struct mm_struct *mm,
-				   struct vm_area_struct *vma,
-				   unsigned long start_addr, bool *mmap_locked,
-				   struct collapse_control *cc)
+static int collapse_scan_pmd(struct mm_struct *mm,
+			     struct vm_area_struct *vma,
+			     unsigned long start_addr, bool *mmap_locked,
+			     struct collapse_control *cc)
 {
 	pmd_t *pmd;
 	pte_t *pte, *_pte;
@@ -1355,7 +1355,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 		 * hit record.
 		 */
 		node = folio_nid(folio);
-		if (hpage_collapse_scan_abort(node, cc)) {
+		if (collapse_scan_abort(node, cc)) {
 			result = SCAN_SCAN_ABORT;
 			goto out_unmap;
 		}
@@ -1421,7 +1421,7 @@ static void collect_mm_slot(struct mm_slot *slot)
 
 	lockdep_assert_held(&khugepaged_mm_lock);
 
-	if (hpage_collapse_test_exit(mm)) {
+	if (collapse_test_exit(mm)) {
 		/* free mm_slot */
 		hash_del(&slot->hash);
 		list_del(&slot->mm_node);
@@ -1769,7 +1769,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 		if (find_pmd_or_thp_or_none(mm, addr, &pmd) != SCAN_SUCCEED)
 			continue;
 
-		if (hpage_collapse_test_exit(mm))
+		if (collapse_test_exit(mm))
 			continue;
 
 		if (!file_backed_vma_is_retractable(vma))
@@ -2285,9 +2285,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	return result;
 }
 
-static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
-				    struct file *file, pgoff_t start,
-				    struct collapse_control *cc)
+static int collapse_scan_file(struct mm_struct *mm, unsigned long addr,
+			      struct file *file, pgoff_t start,
+			      struct collapse_control *cc)
 {
 	struct folio *folio = NULL;
 	struct address_space *mapping = file->f_mapping;
@@ -2342,7 +2342,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 		}
 
 		node = folio_nid(folio);
-		if (hpage_collapse_scan_abort(node, cc)) {
+		if (collapse_scan_abort(node, cc)) {
 			result = SCAN_SCAN_ABORT;
 			folio_put(folio);
 			break;
@@ -2392,7 +2392,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 	return result;
 }
 
-static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
+static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
 					    struct collapse_control *cc)
 	__releases(&khugepaged_mm_lock)
 	__acquires(&khugepaged_mm_lock)
@@ -2427,7 +2427,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 		goto breakouterloop_mmap_lock;
 
 	progress++;
-	if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
+	if (unlikely(collapse_test_exit_or_disable(mm)))
 		goto breakouterloop;
 
 	vma_iter_init(&vmi, mm, khugepaged_scan.address);
@@ -2435,7 +2435,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 		unsigned long hstart, hend;
 
 		cond_resched();
-		if (unlikely(hpage_collapse_test_exit_or_disable(mm))) {
+		if (unlikely(collapse_test_exit_or_disable(mm))) {
 			progress++;
 			break;
 		}
@@ -2456,7 +2456,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 			bool mmap_locked = true;
 
 			cond_resched();
-			if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
+			if (unlikely(collapse_test_exit_or_disable(mm)))
 				goto breakouterloop;
 
 			VM_BUG_ON(khugepaged_scan.address < hstart ||
@@ -2469,12 +2469,12 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 
 				mmap_read_unlock(mm);
 				mmap_locked = false;
-				*result = hpage_collapse_scan_file(mm,
+				*result = collapse_scan_file(mm,
 					khugepaged_scan.address, file, pgoff, cc);
 				fput(file);
 				if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
 					mmap_read_lock(mm);
-					if (hpage_collapse_test_exit_or_disable(mm))
+					if (collapse_test_exit_or_disable(mm))
 						goto breakouterloop;
 					*result = collapse_pte_mapped_thp(mm,
 						khugepaged_scan.address, false);
@@ -2483,7 +2483,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 					mmap_read_unlock(mm);
 				}
 			} else {
-				*result = hpage_collapse_scan_pmd(mm, vma,
+				*result = collapse_scan_pmd(mm, vma,
 					khugepaged_scan.address, &mmap_locked, cc);
 			}
 
@@ -2516,7 +2516,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 	 * Release the current mm_slot if this mm is about to die, or
 	 * if we scanned all vmas of this mm.
 	 */
-	if (hpage_collapse_test_exit(mm) || !vma) {
+	if (collapse_test_exit(mm) || !vma) {
 		/*
 		 * Make sure that if mm_users is reaching zero while
 		 * khugepaged runs here, khugepaged_exit will find
@@ -2567,8 +2567,8 @@ static void khugepaged_do_scan(struct collapse_control *cc)
 			pass_through_head++;
 		if (khugepaged_has_work() &&
 		    pass_through_head < 2)
-			progress += khugepaged_scan_mm_slot(pages - progress,
-							    &result, cc);
+			progress += collapse_scan_mm_slot(pages - progress,
+							  &result, cc);
 		else
 			progress = pages;
 		spin_unlock(&khugepaged_mm_lock);
@@ -2807,12 +2807,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 
 			mmap_read_unlock(mm);
 			mmap_locked = false;
-			result = hpage_collapse_scan_file(mm, addr, file, pgoff,
-							  cc);
+			result = collapse_scan_file(mm, addr, file, pgoff, cc);
 			fput(file);
 		} else {
-			result = hpage_collapse_scan_pmd(mm, vma, addr,
-							 &mmap_locked, cc);
+			result = collapse_scan_pmd(mm, vma, addr,
+						   &mmap_locked, cc);
 		}
 		if (!mmap_locked)
 			*lock_dropped = true;
diff --git a/mm/mremap.c b/mm/mremap.c
index 672264807db6..db31d0231e05 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -244,7 +244,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
 		goto out;
 	}
 	/*
-	 * Now new_pte is none, so hpage_collapse_scan_file() path can not find
+	 * Now new_pte is none, so collapse_scan_file() path can not find
 	 * this by traversing file->f_mapping, so there is no concurrency with
 	 * retract_page_tables(). In addition, we already hold the exclusive
 	 * mmap_lock, so this new_pte page is stable, so there is no need to get
-- 
2.51.1

[PATCH v13 mm-new 02/16] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

[Nico Pache]

The khugepaged daemon and madvise_collapse have two different
implementations that do almost the same thing.

Create collapse_single_pmd to increase code reuse and create an entry
point to these two users.

Refactor madvise_collapse and collapse_scan_mm_slot to use the new
collapse_single_pmd function. This introduces a minor behavioral change
that is most likely an undiscovered bug. The current implementation of
khugepaged tests collapse_test_exit_or_disable before calling
collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
case. By unifying these two callers madvise_collapse now also performs
this check. We also modify the return value to be SCAN_ANY_PROCESS which
properly indicates that this process is no longer valid to operate on.

We also guard the khugepaged_pages_collapsed variable to ensure its only
incremented for khugepaged.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 97 ++++++++++++++++++++++++++-----------------------
 1 file changed, 52 insertions(+), 45 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 959be77f2e65..433ea7283488 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2392,6 +2392,53 @@ static int collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 	return result;
 }
 
+/*
+ * Try to collapse a single PMD starting at a PMD aligned addr, and return
+ * the results.
+ */
+static int collapse_single_pmd(unsigned long addr,
+		struct vm_area_struct *vma, bool *mmap_locked,
+		struct collapse_control *cc)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	int result;
+	struct file *file;
+	pgoff_t pgoff;
+
+	if (vma_is_anonymous(vma)) {
+		result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
+		goto end;
+	}
+
+	file = get_file(vma->vm_file);
+	pgoff = linear_page_index(vma, addr);
+
+	mmap_read_unlock(mm);
+	*mmap_locked = false;
+	result = collapse_scan_file(mm, addr, file, pgoff, cc);
+	fput(file);
+	if (result != SCAN_PTE_MAPPED_HUGEPAGE)
+		goto end;
+
+	mmap_read_lock(mm);
+	*mmap_locked = true;
+	if (collapse_test_exit_or_disable(mm)) {
+		mmap_read_unlock(mm);
+		*mmap_locked = false;
+		return SCAN_ANY_PROCESS;
+	}
+	result = collapse_pte_mapped_thp(mm, addr, !cc->is_khugepaged);
+	if (result == SCAN_PMD_MAPPED)
+		result = SCAN_SUCCEED;
+	mmap_read_unlock(mm);
+	*mmap_locked = false;
+
+end:
+	if (cc->is_khugepaged && result == SCAN_SUCCEED)
+		++khugepaged_pages_collapsed;
+	return result;
+}
+
 static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
 					    struct collapse_control *cc)
 	__releases(&khugepaged_mm_lock)
@@ -2462,34 +2509,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
 			VM_BUG_ON(khugepaged_scan.address < hstart ||
 				  khugepaged_scan.address + HPAGE_PMD_SIZE >
 				  hend);
-			if (!vma_is_anonymous(vma)) {
-				struct file *file = get_file(vma->vm_file);
-				pgoff_t pgoff = linear_page_index(vma,
-						khugepaged_scan.address);
-
-				mmap_read_unlock(mm);
-				mmap_locked = false;
-				*result = collapse_scan_file(mm,
-					khugepaged_scan.address, file, pgoff, cc);
-				fput(file);
-				if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
-					mmap_read_lock(mm);
-					if (collapse_test_exit_or_disable(mm))
-						goto breakouterloop;
-					*result = collapse_pte_mapped_thp(mm,
-						khugepaged_scan.address, false);
-					if (*result == SCAN_PMD_MAPPED)
-						*result = SCAN_SUCCEED;
-					mmap_read_unlock(mm);
-				}
-			} else {
-				*result = collapse_scan_pmd(mm, vma,
-					khugepaged_scan.address, &mmap_locked, cc);
-			}
-
-			if (*result == SCAN_SUCCEED)
-				++khugepaged_pages_collapsed;
 
+			*result = collapse_single_pmd(khugepaged_scan.address,
+						      vma, &mmap_locked, cc);
 			/* move to next address */
 			khugepaged_scan.address += HPAGE_PMD_SIZE;
 			progress += HPAGE_PMD_NR;
@@ -2801,35 +2823,20 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
 		}
 		mmap_assert_locked(mm);
-		if (!vma_is_anonymous(vma)) {
-			struct file *file = get_file(vma->vm_file);
-			pgoff_t pgoff = linear_page_index(vma, addr);
 
-			mmap_read_unlock(mm);
-			mmap_locked = false;
-			result = collapse_scan_file(mm, addr, file, pgoff, cc);
-			fput(file);
-		} else {
-			result = collapse_scan_pmd(mm, vma, addr,
-						   &mmap_locked, cc);
-		}
+		result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
+
 		if (!mmap_locked)
 			*lock_dropped = true;
 
-handle_result:
 		switch (result) {
 		case SCAN_SUCCEED:
 		case SCAN_PMD_MAPPED:
 			++thps;
 			break;
-		case SCAN_PTE_MAPPED_HUGEPAGE:
-			BUG_ON(mmap_locked);
-			mmap_read_lock(mm);
-			result = collapse_pte_mapped_thp(mm, addr, true);
-			mmap_read_unlock(mm);
-			goto handle_result;
 		/* Whitelisted set of results where continuing OK */
 		case SCAN_NO_PTE_TABLE:
+		case SCAN_PTE_MAPPED_HUGEPAGE:
 		case SCAN_PTE_NON_PRESENT:
 		case SCAN_PTE_UFFD_WP:
 		case SCAN_LACK_REFERENCED_PAGE:
-- 
2.51.1

Re: [PATCH v13 mm-new 02/16] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

[Zi Yan]

On 1 Dec 2025, at 12:46, Nico Pache wrote:

> The khugepaged daemon and madvise_collapse have two different
> implementations that do almost the same thing.
>
> Create collapse_single_pmd to increase code reuse and create an entry
> point to these two users.
>
> Refactor madvise_collapse and collapse_scan_mm_slot to use the new
> collapse_single_pmd function. This introduces a minor behavioral change
> that is most likely an undiscovered bug. The current implementation of
> khugepaged tests collapse_test_exit_or_disable before calling
> collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
> case. By unifying these two callers madvise_collapse now also performs
> this check. We also modify the return value to be SCAN_ANY_PROCESS which
> properly indicates that this process is no longer valid to operate on.
>
> We also guard the khugepaged_pages_collapsed variable to ensure its only
> incremented for khugepaged.
>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 97 ++++++++++++++++++++++++++-----------------------
>  1 file changed, 52 insertions(+), 45 deletions(-)
>
LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi

[PATCH v13 mm-new 03/16] 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.

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 <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 433ea7283488..69fc6b41f010 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -892,14 +892,13 @@ 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 int 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 :
@@ -912,15 +911,16 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 	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)))
@@ -1117,7 +1117,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		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;
@@ -1151,7 +1152,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 * 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 */
@@ -2814,7 +2816,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			mmap_read_lock(mm);
 			mmap_locked = true;
 			result = hugepage_vma_revalidate(mm, addr, false, &vma,
-							 cc);
+							 cc, HPAGE_PMD_ORDER);
 			if (result  != SCAN_SUCCEED) {
 				last_fail = result;
 				goto out_nolock;
-- 
2.51.1

Re: [PATCH v13 mm-new 03/16] khugepaged: generalize hugepage_vma_revalidate for mTHP support

[Zi Yan]

On 1 Dec 2025, at 12:46, Nico Pache wrote:

> 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.
>
> 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 <lorenzo.stoakes@oracle.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Co-developed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi

[PATCH v13 mm-new 04/16] 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 <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Co-developed-by: Nico Pache <npache@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
Signed-off-by: Dev Jain <dev.jain@arm.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 21162493a0a0..f93365e182b4 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 8db0d81fca40..c1e1e91b0e61 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -621,6 +621,8 @@ 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);
@@ -686,6 +688,8 @@ 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 69fc6b41f010..8599c7fa112e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1061,21 +1061,26 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
 }
 
 static int 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 (order == HPAGE_PMD_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 (order == HPAGE_PMD_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;
@@ -1112,7 +1117,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 */
 	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;
 
@@ -1872,7 +1877,7 @@ static int 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;
 
-- 
2.51.1

Re: [PATCH v13 mm-new 04/16] khugepaged: generalize alloc_charge_folio()

[Zi Yan]

On 1 Dec 2025, at 12:46, Nico Pache wrote:

> 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 <lorenzo.stoakes@oracle.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Co-developed-by: Nico Pache <npache@redhat.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> Signed-off-by: Dev Jain <dev.jain@arm.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(-)
>
LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi

[PATCH v13 mm-new 05/16] khugepaged: introduce is_mthp_order helper

[Nico Pache]

In order to add mTHP support, we will often be checking if a given order
is a mTHP or PMD order. Lets create a simple helper function to keep the
code clean and readable.

Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 8599c7fa112e..9c041141b2e3 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -338,6 +338,11 @@ static bool pte_none_or_zero(pte_t pte)
 	return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
 }
 
+static bool is_mthp_order(unsigned int order)
+{
+	return order != HPAGE_PMD_ORDER;
+}
+
 int hugepage_madvise(struct vm_area_struct *vma,
 		     vm_flags_t *vm_flags, int advice)
 {
@@ -1071,13 +1076,13 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
 	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask);
 	if (!folio) {
 		*foliop = NULL;
-		if (order == HPAGE_PMD_ORDER)
+		if (!is_mthp_order(order))
 			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
 		count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC_FAILED);
 		return SCAN_ALLOC_HUGE_PAGE_FAIL;
 	}
 
-	if (order == HPAGE_PMD_ORDER)
+	if (!is_mthp_order(order))
 		count_vm_event(THP_COLLAPSE_ALLOC);
 	count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC);
 
-- 
2.51.1

Re: [PATCH v13 mm-new 05/16] khugepaged: introduce is_mthp_order helper

[Zi Yan]

On 1 Dec 2025, at 12:46, Nico Pache wrote:

> In order to add mTHP support, we will often be checking if a given order
> is a mTHP or PMD order. Lets create a simple helper function to keep the
> code clean and readable.
>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 8599c7fa112e..9c041141b2e3 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -338,6 +338,11 @@ static bool pte_none_or_zero(pte_t pte)
>  	return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
>  }
>
> +static bool is_mthp_order(unsigned int order)
> +{
> +	return order != HPAGE_PMD_ORDER;
> +}
> +

The code at the bottom of __folio_split() in mm/huge_memory.c
can use this helper.

The code in deferred_split_folio() uses folio_test_pmd_mappable()
instead of order == HPAGE_PMD_ORDER, but the code can be changed
to use the helper.

>  int hugepage_madvise(struct vm_area_struct *vma,
>  		     vm_flags_t *vm_flags, int advice)
>  {
> @@ -1071,13 +1076,13 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
>  	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask);
>  	if (!folio) {
>  		*foliop = NULL;
> -		if (order == HPAGE_PMD_ORDER)
> +		if (!is_mthp_order(order))
>  			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
>  		count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC_FAILED);
>  		return SCAN_ALLOC_HUGE_PAGE_FAIL;
>  	}
>
> -	if (order == HPAGE_PMD_ORDER)
> +	if (!is_mthp_order(order))
>  		count_vm_event(THP_COLLAPSE_ALLOC);
>  	count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC);
>
> -- 
> 2.51.1


Best Regards,
Yan, Zi

[PATCH v13 mm-new 06/16] khugepaged: generalize __collapse_huge_page_* for mTHP support

[Nico Pache]

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

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

No functional changes in this patch.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 78 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 48 insertions(+), 30 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9c041141b2e3..8dab49c53128 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -541,25 +541,25 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
 }
 
 static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
-					unsigned long start_addr,
-					pte_t *pte,
-					struct collapse_control *cc,
-					struct list_head *compound_pagelist)
+		unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
+		unsigned int order, struct list_head *compound_pagelist)
 {
 	struct page *page = NULL;
 	struct folio *folio = NULL;
 	unsigned long addr = start_addr;
 	pte_t *_pte;
 	int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
+	const unsigned long nr_pages = 1UL << order;
+	int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
 
-	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
+	for (_pte = pte; _pte < pte + nr_pages;
 	     _pte++, addr += PAGE_SIZE) {
 		pte_t pteval = ptep_get(_pte);
 		if (pte_none_or_zero(pteval)) {
 			++none_or_zero;
 			if (!userfaultfd_armed(vma) &&
 			    (!cc->is_khugepaged ||
-			     none_or_zero <= khugepaged_max_ptes_none)) {
+			     none_or_zero <= max_ptes_none)) {
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -587,8 +587,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		/* See collapse_scan_pmd(). */
 		if (folio_maybe_mapped_shared(folio)) {
 			++shared;
-			if (cc->is_khugepaged &&
-			    shared > khugepaged_max_ptes_shared) {
+			/*
+			 * TODO: Support shared pages without leading to further
+			 * mTHP collapses. Currently bringing in new pages via
+			 * shared may cause a future higher order collapse on a
+			 * rescan of the same range.
+			 */
+			if (is_mthp_order(order) || (cc->is_khugepaged &&
+			    shared > khugepaged_max_ptes_shared)) {
 				result = SCAN_EXCEED_SHARED_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 				goto out;
@@ -681,18 +687,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 }
 
 static void __collapse_huge_page_copy_succeeded(pte_t *pte,
-						struct vm_area_struct *vma,
-						unsigned long address,
-						spinlock_t *ptl,
-						struct list_head *compound_pagelist)
+		struct vm_area_struct *vma, unsigned long address,
+		spinlock_t *ptl, unsigned int order,
+		struct list_head *compound_pagelist)
 {
-	unsigned long end = address + HPAGE_PMD_SIZE;
+	unsigned long end = address + (PAGE_SIZE << order);
 	struct folio *src, *tmp;
 	pte_t pteval;
 	pte_t *_pte;
 	unsigned int nr_ptes;
+	const unsigned long nr_pages = 1UL << order;
 
-	for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
+	for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes,
 	     address += nr_ptes * PAGE_SIZE) {
 		nr_ptes = 1;
 		pteval = ptep_get(_pte);
@@ -745,13 +751,11 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
 }
 
 static void __collapse_huge_page_copy_failed(pte_t *pte,
-					     pmd_t *pmd,
-					     pmd_t orig_pmd,
-					     struct vm_area_struct *vma,
-					     struct list_head *compound_pagelist)
+		pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
+		unsigned int order, struct list_head *compound_pagelist)
 {
 	spinlock_t *pmd_ptl;
-
+	const unsigned long nr_pages = 1UL << order;
 	/*
 	 * Re-establish the PMD to point to the original page table
 	 * entry. Restoring PMD needs to be done prior to releasing
@@ -765,7 +769,7 @@ 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);
 }
 
 /*
@@ -785,16 +789,16 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
  */
 static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
 		pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
-		unsigned long address, spinlock_t *ptl,
+		unsigned long address, spinlock_t *ptl, unsigned int order,
 		struct list_head *compound_pagelist)
 {
 	unsigned int i;
 	int result = SCAN_SUCCEED;
-
+	const unsigned long nr_pages = 1UL << order;
 	/*
 	 * Copying pages' contents is subject to memory poison at any iteration.
 	 */
-	for (i = 0; i < HPAGE_PMD_NR; i++) {
+	for (i = 0; i < nr_pages; i++) {
 		pte_t pteval = ptep_get(pte + i);
 		struct page *page = folio_page(folio, i);
 		unsigned long src_addr = address + i * PAGE_SIZE;
@@ -813,10 +817,10 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
 
 	if (likely(result == SCAN_SUCCEED))
 		__collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
-						    compound_pagelist);
+						    order, compound_pagelist);
 	else
 		__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
-						 compound_pagelist);
+						 order, compound_pagelist);
 
 	return result;
 }
@@ -989,13 +993,12 @@ static int check_pmd_still_valid(struct mm_struct *mm,
  * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
  */
 static int __collapse_huge_page_swapin(struct mm_struct *mm,
-				       struct vm_area_struct *vma,
-				       unsigned long start_addr, pmd_t *pmd,
-				       int referenced)
+		struct vm_area_struct *vma, unsigned long start_addr,
+		pmd_t *pmd, int referenced, unsigned int order)
 {
 	int swapped_in = 0;
 	vm_fault_t ret = 0;
-	unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE);
+	unsigned long addr, end = start_addr + (PAGE_SIZE << order);
 	int result;
 	pte_t *pte = NULL;
 	spinlock_t *ptl;
@@ -1027,6 +1030,19 @@ static int __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_mthp_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);
@@ -1147,7 +1163,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		 * that case.  Continuing to collapse causes inconsistency.
 		 */
 		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
-						     referenced);
+						     referenced, HPAGE_PMD_ORDER);
 		if (result != SCAN_SUCCEED)
 			goto out_nolock;
 	}
@@ -1195,6 +1211,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
 	if (pte) {
 		result = __collapse_huge_page_isolate(vma, address, pte, cc,
+						      HPAGE_PMD_ORDER,
 						      &compound_pagelist);
 		spin_unlock(pte_ptl);
 	} else {
@@ -1225,6 +1242,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 
 	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
 					   vma, address, pte_ptl,
+					   HPAGE_PMD_ORDER,
 					   &compound_pagelist);
 	pte_unmap(pte);
 	if (unlikely(result != SCAN_SUCCEED))
-- 
2.51.1

[PATCH v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function

[Nico Pache]

The current mechanism for determining mTHP collapse scales the
khugepaged_max_ptes_none value based on the target order. This
introduces an undesirable feedback loop, or "creep", when max_ptes_none
is set to a value greater than HPAGE_PMD_NR / 2.

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

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

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

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

The limits can be ignored by passing full_scan=true, this is useful for
madvise_collapse (which ignores limits), or in the case of
collapse_scan_pmd(), allows the full PMD to be scanned when mTHP
collapse is available.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 8dab49c53128..f425238d5d4f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -463,6 +463,44 @@ void __khugepaged_enter(struct mm_struct *mm)
 		wake_up_interruptible(&khugepaged_wait);
 }
 
+/**
+ * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
+ * @order: The folio order being collapsed to
+ * @full_scan: Whether this is a full scan (ignore limits)
+ *
+ * For madvise-triggered collapses (full_scan=true), all limits are bypassed
+ * and allow up to HPAGE_PMD_NR - 1 empty PTEs.
+ *
+ * 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 (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP
+ * collapse will be attempted
+ *
+ * Return: Maximum number of empty PTEs allowed for the collapse operation
+ */
+static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
+{
+	/* ignore max_ptes_none limits */
+	if (full_scan)
+		return HPAGE_PMD_NR - 1;
+
+	if (!is_mthp_order(order))
+		return khugepaged_max_ptes_none;
+
+	/* Zero/non-present collapse disabled. */
+	if (!khugepaged_max_ptes_none)
+		return 0;
+
+	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
+		return (1 << order) - 1;
+
+	pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n",
+		      HPAGE_PMD_NR - 1);
+	return -EINVAL;
+}
+
 void khugepaged_enter_vma(struct vm_area_struct *vma,
 			  vm_flags_t vm_flags)
 {
@@ -550,7 +588,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	pte_t *_pte;
 	int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
 	const unsigned long nr_pages = 1UL << order;
-	int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
+	int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);
+
+	if (max_ptes_none == -EINVAL)
+		goto out;
 
 	for (_pte = pte; _pte < pte + nr_pages;
 	     _pte++, addr += PAGE_SIZE) {
-- 
2.51.1

Re: [PATCH v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function

[Baolin Wang]

On 2025/12/2 01:46, Nico Pache wrote:
> The current mechanism for determining mTHP collapse scales the
> khugepaged_max_ptes_none value based on the target order. This
> introduces an undesirable feedback loop, or "creep", when max_ptes_none
> is set to a value greater than HPAGE_PMD_NR / 2.
> 
> With this configuration, a successful collapse to order N will populate
> enough pages to satisfy the collapse condition on order N+1 on the next
> scan. This leads to unnecessary work and memory churn.
> 
> To fix this issue introduce a helper function that will limit mTHP
> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
> This effectively supports two modes:
> 
> - max_ptes_none=0: never introduce new none-pages for mTHP collapse.
> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
>    available mTHP order.
> 
> This removes the possiblilty of "creep", while not modifying any uAPI
> expectations. A warning will be emitted if any non-supported
> max_ptes_none value is configured with mTHP enabled.
> 
> The limits can be ignored by passing full_scan=true, this is useful for
> madvise_collapse (which ignores limits), or in the case of
> collapse_scan_pmd(), allows the full PMD to be scanned when mTHP
> collapse is available.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>   mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 8dab49c53128..f425238d5d4f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -463,6 +463,44 @@ void __khugepaged_enter(struct mm_struct *mm)
>   		wake_up_interruptible(&khugepaged_wait);
>   }
>   
> +/**
> + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
> + * @order: The folio order being collapsed to
> + * @full_scan: Whether this is a full scan (ignore limits)
> + *
> + * For madvise-triggered collapses (full_scan=true), all limits are bypassed
> + * and allow up to HPAGE_PMD_NR - 1 empty PTEs.
> + *
> + * 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 (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP
> + * collapse will be attempted
> + *
> + * Return: Maximum number of empty PTEs allowed for the collapse operation
> + */
> +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
> +{
> +	/* ignore max_ptes_none limits */
> +	if (full_scan)
> +		return HPAGE_PMD_NR - 1;
> +
> +	if (!is_mthp_order(order))
> +		return khugepaged_max_ptes_none;
> +
> +	/* Zero/non-present collapse disabled. */
> +	if (!khugepaged_max_ptes_none)
> +		return 0;
> +
> +	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
> +		return (1 << order) - 1;
> +
> +	pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n",
> +		      HPAGE_PMD_NR - 1);
> +	return -EINVAL;
> +}

Thanks. That aligns with what we talked about previously. So
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Re: [PATCH v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function

[Nico Pache]

Hi Andrew,

The bot has reported a potential uninitialized use of a variable.

Can you please squash the following fixup to this commit.

Thank you,
Nico

----8<----

From 846f79d91a25ebad76cbab3690ae315cfe3cf278 Mon Sep 17 00:00:00 2001
From: Nico Pache <npache@redhat.com>
Date: Wed, 3 Dec 2025 13:42:18 -0700
Subject: [PATCH] khugepaged: fixup unintialized _pte variable

There is a potential use of an uninitialized variable after
`khugepaged: introduce collapse_max_ptes_none helper function`

Andrew can you please append this to Patch 7 of my series

as reported by the kernel test robot
>> mm/khugepaged.c:593:6: warning: variable '_pte' is used uninitialized
whenever 'if' condition is true [-Wsometimes-uninitialized]
593 | if (max_ptes_none == -EINVAL)
| ^~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:724:25: note: uninitialized use occurs here
724 | release_pte_pages(pte, _pte, compound_pagelist);
| ^~~~
mm/khugepaged.c:593:2: note: remove the 'if' if its condition is always false
593 | if (max_ptes_none == -EINVAL)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
594 | goto out;
| ~~~~~~~~
mm/khugepaged.c:588:13: note: initialize the variable '_pte' to silence this warning
588 | pte_t *_pte;
| ^
| = NULL
1 warning generated.

Signed-off-by: Nico Pache <npache@redhat.com>
---
mm/khugepaged.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f425238d5d4f..7c7d04d6737e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -585,7 +585,7 @@ static int __collapse_huge_page_isolate(struct
vm_area_struct *vma,
struct page *page = NULL;
struct folio *folio = NULL;
unsigned long addr = start_addr;
- pte_t *_pte;
+ pte_t *_pte = pte;
int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
const unsigned long nr_pages = 1UL << order;
int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);
@@ -593,8 +593,7 @@ static int __collapse_huge_page_isolate(struct
vm_area_struct *vma,
if (max_ptes_none == -EINVAL)
goto out;
- for (_pte = pte; _pte < pte + nr_pages;
- _pte++, addr += PAGE_SIZE) {
+ for (; _pte < pte + nr_pages; _pte++, addr += PAGE_SIZE) {
pte_t pteval = ptep_get(_pte);
if (pte_none_or_zero(pteval)) {
++none_or_zero;

--
2.52.0

On 12/1/25 10:46 AM, Nico Pache wrote:
> The current mechanism for determining mTHP collapse scales the
> khugepaged_max_ptes_none value based on the target order. This
> introduces an undesirable feedback loop, or "creep", when max_ptes_none
> is set to a value greater than HPAGE_PMD_NR / 2.
> 
> With this configuration, a successful collapse to order N will populate
> enough pages to satisfy the collapse condition on order N+1 on the next
> scan. This leads to unnecessary work and memory churn.
> 
> To fix this issue introduce a helper function that will limit mTHP
> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
> This effectively supports two modes:
> 
> - max_ptes_none=0: never introduce new none-pages for mTHP collapse.
> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
>   available mTHP order.
> 
> This removes the possiblilty of "creep", while not modifying any uAPI
> expectations. A warning will be emitted if any non-supported
> max_ptes_none value is configured with mTHP enabled.
> 
> The limits can be ignored by passing full_scan=true, this is useful for
> madvise_collapse (which ignores limits), or in the case of
> collapse_scan_pmd(), allows the full PMD to be scanned when mTHP
> collapse is available.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 8dab49c53128..f425238d5d4f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -463,6 +463,44 @@ void __khugepaged_enter(struct mm_struct *mm)
>  		wake_up_interruptible(&khugepaged_wait);
>  }
>  
> +/**
> + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
> + * @order: The folio order being collapsed to
> + * @full_scan: Whether this is a full scan (ignore limits)
> + *
> + * For madvise-triggered collapses (full_scan=true), all limits are bypassed
> + * and allow up to HPAGE_PMD_NR - 1 empty PTEs.
> + *
> + * 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 (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP
> + * collapse will be attempted
> + *
> + * Return: Maximum number of empty PTEs allowed for the collapse operation
> + */
> +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
> +{
> +	/* ignore max_ptes_none limits */
> +	if (full_scan)
> +		return HPAGE_PMD_NR - 1;
> +
> +	if (!is_mthp_order(order))
> +		return khugepaged_max_ptes_none;
> +
> +	/* Zero/non-present collapse disabled. */
> +	if (!khugepaged_max_ptes_none)
> +		return 0;
> +
> +	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
> +		return (1 << order) - 1;
> +
> +	pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n",
> +		      HPAGE_PMD_NR - 1);
> +	return -EINVAL;
> +}
> +
>  void khugepaged_enter_vma(struct vm_area_struct *vma,
>  			  vm_flags_t vm_flags)
>  {
> @@ -550,7 +588,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  	pte_t *_pte;
>  	int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
>  	const unsigned long nr_pages = 1UL << order;
> -	int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
> +	int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);
> +
> +	if (max_ptes_none == -EINVAL)
> +		goto out;
>  
>  	for (_pte = pte; _pte < pte + nr_pages;
>  	     _pte++, addr += PAGE_SIZE) {

[PATCH v13 mm-new 08/16] 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
changes to the VMA from occurring.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 112 ++++++++++++++++++++++++++++++------------------
 1 file changed, 71 insertions(+), 41 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f425238d5d4f..b41f68bbac80 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1155,45 +1155,54 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
 	return SCAN_SUCCEED;
 }
 
-static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
-			      int referenced, int unmapped,
-			      struct collapse_control *cc)
+static int collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
+		int referenced, int unmapped, struct collapse_control *cc,
+		bool *mmap_locked, unsigned int 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;
 	int result = SCAN_FAIL;
 	struct vm_area_struct *vma;
 	struct mmu_notifier_range range;
+	bool anon_vma_locked = false;
+	const unsigned long nr_pages = 1UL << order;
+	const unsigned long pmd_address = start_addr & HPAGE_PMD_MASK;
 
-	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
+	VM_WARN_ON_ONCE(pmd_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.
+	 * If collapsing mTHPs we may have already released the read_lock.
 	 */
-	mmap_read_unlock(mm);
+	if (*mmap_locked) {
+		mmap_read_unlock(mm);
+		*mmap_locked = false;
+	}
 
-	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);
+	*mmap_locked = true;
+	result = hugepage_vma_revalidate(mm, pmd_address, true, &vma, cc, order);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
+		*mmap_locked = false;
 		goto out_nolock;
 	}
 
-	result = find_pmd_or_thp_or_none(mm, address, &pmd);
+	result = find_pmd_or_thp_or_none(mm, pmd_address, &pmd);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
+		*mmap_locked = false;
 		goto out_nolock;
 	}
 
@@ -1203,13 +1212,16 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		 * 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);
-		if (result != SCAN_SUCCEED)
+		result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd,
+						     referenced, order);
+		if (result != SCAN_SUCCEED) {
+			*mmap_locked = false;
 			goto out_nolock;
+		}
 	}
 
 	mmap_read_unlock(mm);
+	*mmap_locked = false;
 	/*
 	 * Prevent all access to pagetables with the exception of
 	 * gup_fast later handled by the ptep_clear_flush and the VM
@@ -1219,20 +1231,20 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 * mmap_lock.
 	 */
 	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
-					 HPAGE_PMD_ORDER);
+	result = hugepage_vma_revalidate(mm, pmd_address, 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_address, 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,
+				start_addr + (PAGE_SIZE << order));
 	mmu_notifier_invalidate_range_start(&range);
 
 	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
@@ -1244,24 +1256,21 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 * 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_address, 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));
 		/*
@@ -1271,21 +1280,21 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		 */
 		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 we must hold the lock
 	 */
-	anon_vma_unlock_write(vma->anon_vma);
+	if (!is_mthp_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;
 
@@ -1295,20 +1304,42 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 * write.
 	 */
 	__folio_mark_uptodate(folio);
-	pgtable = pmd_pgtable(_pmd);
+	if (is_mthp_order(order)) { /* mTHP collapse */
+		pte_t mthp_pte = mk_pte(folio_page(folio, 0), vma->vm_page_prot);
 
-	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);
+		mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
+		spin_lock(pmd_ptl);
+		WARN_ON_ONCE(!pmd_none(*pmd));
+		folio_ref_add(folio, nr_pages - 1);
+		folio_add_new_anon_rmap(folio, vma, start_addr, RMAP_EXCLUSIVE);
+		folio_add_lru_vma(folio, vma);
+		set_ptes(vma->vm_mm, start_addr, pte, mthp_pte, nr_pages);
+		update_mmu_cache_range(NULL, vma, start_addr, pte, nr_pages);
+
+		smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
+		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
+	} else { /* PMD collapse */
+		pgtable = pmd_pgtable(_pmd);
+
+		spin_lock(pmd_ptl);
+		WARN_ON_ONCE(!pmd_none(*pmd));
+		pgtable_trans_huge_deposit(mm, pmd, pgtable);
+		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_address);
+	}
 	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);
+	*mmap_locked = false;
 out_nolock:
+	WARN_ON_ONCE(*mmap_locked);
 	if (folio)
 		folio_put(folio);
 	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
@@ -1476,9 +1507,8 @@ static int collapse_scan_pmd(struct mm_struct *mm,
 	pte_unmap_unlock(pte, ptl);
 	if (result == SCAN_SUCCEED) {
 		result = collapse_huge_page(mm, start_addr, referenced,
-					    unmapped, cc);
-		/* collapse_huge_page will return with the mmap_lock released */
-		*mmap_locked = false;
+					    unmapped, cc, mmap_locked,
+					    HPAGE_PMD_ORDER);
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
-- 
2.51.1

[PATCH v13 mm-new 09/16] 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 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 <lorenzo.stoakes@oracle.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b41f68bbac80..b2ea56c9bb42 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -641,6 +641,14 @@ static int __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_mthp_order(order) && folio_order(folio) >= order) {
+			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			goto out;
+		}
 
 		if (folio_test_large(folio)) {
 			struct folio *f;
-- 
2.51.1

[PATCH v13 mm-new 10/16] 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 swap
	PTEs

- 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 shared
  	PTEs

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 dont 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.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 24 ++++++++++++++++++++++
 include/linux/huge_mm.h                    |  3 +++
 mm/huge_memory.c                           |  7 +++++++
 mm/khugepaged.c                            | 16 ++++++++++++---
 4 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index c51932e6275d..d396d1bfb274 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -714,6 +714,30 @@ 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. 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.
+
+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.
+
+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.
+
 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 f93365e182b4..1082b78e794d 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 c1e1e91b0e61..b4d9b3ac9a7c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -639,6 +639,10 @@ 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,
@@ -655,6 +659,9 @@ 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 b2ea56c9bb42..efb8a47af65a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -604,7 +604,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
-				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				if (!is_mthp_order(order))
+					count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_NONE);
 				goto out;
 			}
 		}
@@ -634,10 +636,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			 * shared may cause a future higher order collapse on a
 			 * rescan of the same range.
 			 */
-			if (is_mthp_order(order) || (cc->is_khugepaged &&
-			    shared > khugepaged_max_ptes_shared)) {
+			if (is_mthp_order(order)) {
+				result = SCAN_EXCEED_SHARED_PTE;
+				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
+				goto out;
+			}
+
+			if (cc->is_khugepaged &&
+			    shared > khugepaged_max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
 				goto out;
 			}
 		}
@@ -1086,6 +1095,7 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
 		 * range.
 		 */
 		if (is_mthp_order(order)) {
+			count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
 			pte_unmap(pte);
 			mmap_read_unlock(mm);
 			result = SCAN_EXCEED_SWAP_PTE;
-- 
2.51.1

Re: [PATCH v13 mm-new 10/16] khugepaged: add per-order mTHP collapse failure statistics

[Randy Dunlap]

On 12/1/25 9:46 AM, Nico Pache wrote:
> 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 swap
> 	PTEs
> 
> - 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 shared
>   	PTEs
> 
> 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 dont 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.
> 
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 24 ++++++++++++++++++++++
>  include/linux/huge_mm.h                    |  3 +++
>  mm/huge_memory.c                           |  7 +++++++
>  mm/khugepaged.c                            | 16 ++++++++++++---
>  4 files changed, 47 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index c51932e6275d..d396d1bfb274 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -714,6 +714,30 @@ 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. For mTHP collapse, Currently only max_ptes_none

	                                              currently

> +       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

	                                                size. If
or
	                                                size; if

> +       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.
> +
> +collapse_exceed_swap_pte
> +       The number of anonymous mTHP pte ranges which were unable to collapse due

	                               PTE

> +       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.
> +
> +collapse_exceed_shared_pte
> +       The number of anonymous mTHP pte ranges which were unable to collapse due

	                               PTE

> +       to containing at least one shared PTE. Currently khugepaged does not
> +       support collapsing mTHP pte ranges that contain a shared PTE. This

	                          PTE

> +       counter can be used to monitor the number of khugepaged mTHP collapses
> +       that failed due to the presence of a 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


-- 
~Randy

[PATCH v13 mm-new 11/16] 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.

Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
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 4cde53b45a85..0b8c26035aea 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -87,40 +87,44 @@ 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(
@@ -128,26 +132,30 @@ 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(
@@ -155,13 +163,15 @@ 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 efb8a47af65a..049da0305440 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -734,13 +734,13 @@ static int __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;
 }
 
@@ -1136,7 +1136,8 @@ static int __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;
 }
 
@@ -1360,7 +1361,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
 	WARN_ON_ONCE(*mmap_locked);
 	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;
 }
 
-- 
2.51.1

[PATCH v13 mm-new 12/16] 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 | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 049da0305440..33b70ca070b4 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -501,12 +501,22 @@ static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
 	return -EINVAL;
 }
 
+/* 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, bool is_khugepaged)
+{
+	enum tva_type tva_flags = is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
+	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, /*is_khugepaged=*/true))
 			__khugepaged_enter(vma->vm_mm);
 	}
 }
@@ -2606,7 +2616,7 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
 			progress++;
 			break;
 		}
-		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
+		if (!collapse_allowable_orders(vma, vma->vm_flags, /*is_khugepaged=*/true)) {
 skip:
 			progress++;
 			continue;
@@ -2912,7 +2922,7 @@ 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, /*is_khugepaged=*/false))
 		return -EINVAL;
 
 	cc = kmalloc(sizeof(*cc), GFP_KERNEL);
-- 
2.51.1

[PATCH v13 mm-new 13/16] 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, a 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. 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

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.

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 (m)THP. 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.

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.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 183 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 175 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 33b70ca070b4..fcb927cd5456 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -92,6 +92,32 @@ 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
+/*
+ * 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.
+ *
+ * ilog2(MAX_PTRS_PER_PTE) is log2 of the maximum number of PTE entries.
+ * This gives you the PMD_ORDER, and is needed in place of HPAGE_PMD_ORDER due
+ * to restrictions of some architectures (ie ppc64le).
+ *
+ * At most there will be 1 << (PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER) mTHP ranges
+ */
+#define MTHP_STACK_SIZE	(1UL << (ilog2(MAX_PTRS_PER_PTE) - KHUGEPAGED_MIN_MTHP_ORDER))
+
+/*
+ * Defines a range of PTE entries in a PTE page table which are being
+ * considered for (m)THP 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;
 
@@ -100,6 +126,11 @@ struct collapse_control {
 
 	/* nodemask for allocation fallback */
 	nodemask_t alloc_nmask;
+
+	/* bitmap used for mTHP collapse */
+	DECLARE_BITMAP(mthp_bitmap, MAX_PTRS_PER_PTE);
+	DECLARE_BITMAP(mthp_bitmap_mask, MAX_PTRS_PER_PTE);
+	struct mthp_range mthp_bitmap_stack[MTHP_STACK_SIZE];
 };
 
 /**
@@ -1375,6 +1406,121 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
 	return result;
 }
 
+static void 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 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 mthp_nr_occupied_pte_entries(struct collapse_control *cc,
+						 u16 offset, unsigned long nr_pte_entries)
+{
+	bitmap_zero(cc->mthp_bitmap_mask, HPAGE_PMD_NR);
+	bitmap_set(cc->mthp_bitmap_mask, offset, nr_pte_entries);
+	return bitmap_weight_and(cc->mthp_bitmap, cc->mthp_bitmap_mask, HPAGE_PMD_NR);
+}
+
+/*
+ * 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,
+		bool *mmap_locked, unsigned long enabled_orders)
+{
+	unsigned int max_ptes_none, nr_occupied_ptes;
+	struct mthp_range range;
+	unsigned long collapse_address;
+	int collapsed = 0, stack_size = 0;
+	unsigned long nr_pte_entries;
+	u16 offset;
+	u8 order;
+
+	mthp_stack_push(cc, &stack_size, 0, HPAGE_PMD_ORDER);
+
+	while (stack_size > 0) {
+		range = mthp_stack_pop(cc, &stack_size);
+		order = range.order;
+		offset = range.offset;
+		nr_pte_entries = 1UL << order;
+
+		if (!test_bit(order, &enabled_orders))
+			goto next_order;
+
+		max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);
+
+		if (max_ptes_none == -EINVAL)
+			return collapsed;
+
+		nr_occupied_ptes = mthp_nr_occupied_pte_entries(cc, offset, nr_pte_entries);
+
+		if (nr_occupied_ptes >= nr_pte_entries - max_ptes_none) {
+			int ret;
+
+			collapse_address = address + offset * PAGE_SIZE;
+			ret = collapse_huge_page(mm, collapse_address, referenced,
+						 unmapped, cc, mmap_locked,
+						 order);
+			if (ret == SCAN_SUCCEED) {
+				collapsed += nr_pte_entries;
+				continue;
+			}
+		}
+
+next_order:
+		if (order > KHUGEPAGED_MIN_MTHP_ORDER) {
+			const u8 next_order = order - 1;
+			const u16 mid_offset = offset + (nr_pte_entries / 2);
+
+			mthp_stack_push(cc, &stack_size, mid_offset, next_order);
+			mthp_stack_push(cc, &stack_size, offset, next_order);
+		}
+	}
+	return collapsed;
+}
+
 static int collapse_scan_pmd(struct mm_struct *mm,
 			     struct vm_area_struct *vma,
 			     unsigned long start_addr, bool *mmap_locked,
@@ -1382,11 +1528,15 @@ static int collapse_scan_pmd(struct mm_struct *mm,
 {
 	pmd_t *pmd;
 	pte_t *pte, *_pte;
+	int i;
 	int result = SCAN_FAIL, referenced = 0;
-	int none_or_zero = 0, shared = 0;
+	int none_or_zero = 0, shared = 0, nr_collapsed = 0;
 	struct page *page = NULL;
+	unsigned int max_ptes_none;
 	struct folio *folio = NULL;
 	unsigned long addr;
+	unsigned long enabled_orders;
+	bool full_scan = true;
 	spinlock_t *ptl;
 	int node = NUMA_NO_NODE, unmapped = 0;
 
@@ -1396,22 +1546,34 @@ static int collapse_scan_pmd(struct mm_struct *mm,
 	if (result != SCAN_SUCCEED)
 		goto out;
 
+	bitmap_zero(cc->mthp_bitmap, HPAGE_PMD_NR);
 	memset(cc->node_load, 0, sizeof(cc->node_load));
 	nodes_clear(cc->alloc_nmask);
+
+	enabled_orders = collapse_allowable_orders(vma, vma->vm_flags, cc->is_khugepaged);
+
+	/*
+	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
+	 * scan all pages to populate the bitmap for mTHP collapse.
+	 */
+	if (cc->is_khugepaged && enabled_orders == BIT(HPAGE_PMD_ORDER))
+		full_scan = false;
+	max_ptes_none = collapse_max_ptes_none(HPAGE_PMD_ORDER, full_scan);
+
 	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
 	if (!pte) {
 		result = SCAN_NO_PTE_TABLE;
 		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;
 		pte_t pteval = ptep_get(_pte);
 		if (pte_none_or_zero(pteval)) {
 			++none_or_zero;
 			if (!userfaultfd_armed(vma) &&
-			    (!cc->is_khugepaged ||
-			     none_or_zero <= khugepaged_max_ptes_none)) {
+			    none_or_zero <= max_ptes_none) {
 				continue;
 			} else {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -1479,6 +1641,8 @@ static int collapse_scan_pmd(struct mm_struct *mm,
 			}
 		}
 
+		/* Set bit for occupied pages */
+		bitmap_set(cc->mthp_bitmap, i, 1);
 		/*
 		 * Record which node the original page is from and save this
 		 * information to cc->node_load[].
@@ -1535,9 +1699,12 @@ static int collapse_scan_pmd(struct mm_struct *mm,
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
 	if (result == SCAN_SUCCEED) {
-		result = collapse_huge_page(mm, start_addr, referenced,
-					    unmapped, cc, mmap_locked,
-					    HPAGE_PMD_ORDER);
+		nr_collapsed = mthp_collapse(mm, start_addr, referenced, unmapped,
+					      cc, mmap_locked, enabled_orders);
+		if (nr_collapsed > 0)
+			result = SCAN_SUCCEED;
+		else
+			result = SCAN_FAIL;
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
-- 
2.51.1

[PATCH v13 mm-new 14/16] 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.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index fcb927cd5456..daf39aab7e86 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1503,9 +1503,42 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address,
 			ret = collapse_huge_page(mm, collapse_address, referenced,
 						 unmapped, cc, mmap_locked,
 						 order);
-			if (ret == SCAN_SUCCEED) {
+
+			switch (ret) {
+			/* Cases were we continue to next collapse candidate */
+			case SCAN_SUCCEED:
 				collapsed += nr_pte_entries;
+				fallthrough;
+			case SCAN_PTE_MAPPED_HUGEPAGE:
 				continue;
+			/* Cases were 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_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:
+				goto next_order;
+			/* Cases were no further collapse is possible */
+			case SCAN_CGROUP_CHARGE_FAIL:
+			case SCAN_COPY_MC:
+			case SCAN_ADDRESS_RANGE:
+			case SCAN_NO_PTE_TABLE:
+			case SCAN_ANY_PROCESS:
+			case SCAN_VMA_NULL:
+			case SCAN_VMA_CHECK:
+			case SCAN_SCAN_ABORT:
+			case SCAN_PAGE_ANON:
+			case SCAN_PMD_MAPPED:
+			case SCAN_FAIL:
+			default:
+				return collapsed;
 			}
 		}
 
-- 
2.51.1

[PATCH v13 mm-new 15/16] 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.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index daf39aab7e86..ae524220304a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -441,23 +441,23 @@ 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())
@@ -536,8 +536,14 @@ static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
 static unsigned long collapse_allowable_orders(struct vm_area_struct *vma,
 			vm_flags_t vm_flags, bool is_khugepaged)
 {
+	unsigned long orders;
 	enum tva_type tva_flags = is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
-	unsigned long orders = BIT(HPAGE_PMD_ORDER);
+
+	/* If khugepaged is scanning an anonymous vma, allow mTHP collapse */
+	if (is_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);
 }
@@ -546,7 +552,7 @@ 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()) {
+	    hugepage_enabled()) {
 		if (collapse_allowable_orders(vma, vm_flags, /*is_khugepaged=*/true))
 			__khugepaged_enter(vma->vm_mm);
 	}
@@ -2890,7 +2896,7 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
 
 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)
@@ -2963,7 +2969,7 @@ static void khugepaged_wait_work(void)
 		return;
 	}
 
-	if (hugepage_pmd_enabled())
+	if (hugepage_enabled())
 		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
 }
 
@@ -2994,7 +3000,7 @@ static 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;
 	}
@@ -3044,7 +3050,7 @@ 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");
@@ -3070,7 +3076,7 @@ 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);
 }
-- 
2.51.1

[PATCH v13 mm-new 16/16] 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: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 48 +++++++++++++---------
 1 file changed, 28 insertions(+), 20 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index d396d1bfb274..87bcfa80886a 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 +304,19 @@ 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 +336,15 @@ 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
 ===============
 
-- 
2.51.1

Re: [PATCH v13 mm-new 16/16] Documentation: mm: update the admin guide for mTHP collapse

[Randy Dunlap]

On 12/1/25 9:46 AM, Nico Pache wrote:
> 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: Bagas Sanjaya <bagasdotme@gmail.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 48 +++++++++++++---------
>  1 file changed, 28 insertions(+), 20 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index d396d1bfb274..87bcfa80886a 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

                                                    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")

                         "never").

>  
>  process THP controls

[snip]


-- 
~Randy