[PATCH mm-unstable v19 00/14] khugepaged: add mTHP collapse support

[PATCH mm-unstable v19 00/14] khugepaged: add mTHP collapse 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 mTHP collapse will default to max_ptes_none=0. If a mTHP collapse is
attempted, but contains swapped out, or shared pages, we don't perform
the collapse.
It is now also possible to collapse to mTHPs without requiring the PMD THP
size to be enabled. These limitations are to prevent collapse "creep"
behavior. This prevents constantly promoting mTHPs to the next available
size, which would occur because a collapse introduces more non-zero pages
that would satisfy the promotion condition on subsequent scans.

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

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

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

V19 Changes:
- Rebased onto mm-unstable (base: 9282f9bdbdf2, where v18 was based)
- Added RBs/Acks
- [patch 04] Make max_ptes_none const; guard pr_warn_once with
  check so we only warn for non-zero intermediate values and use a single
  "return 0" path (David)
- [patch 06] Add comment explaining the mmu_notifier_range covers only
  the modified PTE range (David); change BUG_ON to VM_WARN_ON_ONCE
  (David); fix update_mmu_cache_range() arch safety issue: reinstall
  PMD via pmd_populate() before calling map_anon_folio_pte_nopf() and
  hold the PTE lock nested inside pmd_ptl during the operation (David,
  Lance); drop the now-unnecessary smp_wmb() as __folio_mark_uptodate()
  provides the required ordering (David, Lance)
- [patch 07] Clarify commit message: "mTHP to a folio of equal or
  smaller size, possibly resulting in a partially mapped source folio"
  (David)
- [patch 08] Add Lorenzo's RB and David's Ack; move TODO comment about
  shared pages to patch 4 (David)
- [patch 10] Rename collapse_allowable_orders() to
  collapse_possible_orders() and add collapse_possible() boolean wrapper
  for callers that only need a yes/no answer (David)
- [patch 11] Major rework: replace the DFS stack-based algorithm with a
  simpler linear forward-walking approach using offset + order (David);
  remove mthp_range struct, mthp_bitmap_stack[], stack push/pop
  functions, and MTHP_STACK_SIZE; add max_order_from_offset() helper
  using __ffs(offset) for natural alignment; rename mthp_bitmap to
  mthp_present_ptes (David); remove temporary mthp_bitmap_mask and use
  bitmap_weight_from() directly (David); fix result propagation so
  mthp_collapse() returns enum scan_result and properly propagates
  SCAN_ALLOC_HUGE_PAGE_FAIL and SCAN_PTE_MAPPED_HUGEPAGE to callers
  (Lance); fold in v18 fixup for potential use-after-free of vma in
  mthp_collapse() by passing NULL to collapse_max_ptes_none()
- [patch 12] Rework to match new linear algorithm; expand the set of
  failures that allow retrying at a lower order
- [patch 14] Re-add note about file/shmem still only collapsing to
  PMD-sized THPs (David)

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

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

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

 Documentation/admin-guide/mm/transhuge.rst |  71 ++-
 include/linux/huge_mm.h                    |   5 +
 include/trace/events/huge_memory.h         |  34 +-
 mm/huge_memory.c                           |  11 +
 mm/khugepaged.c                            | 616 ++++++++++++++++-----
 5 files changed, 569 insertions(+), 168 deletions(-)


base-commit: 9282f9bdbdf22a1914f0b085f2fe5e8141cbc3de
-- 
2.54.0

[PATCH mm-unstable v19 01/14] mm/khugepaged: generalize hugepage_vma_revalidate for mTHP support

[Nico Pache]

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

We cannot collapse VMA regions that do not span the full PMD. This is due
to the potential of the PMD being shared by another VMA which leaves us
vulnerable to race conditions if neighboring VMAs are resized. Always
check the PMD order here to ensure its not shared by another VMA. We'd
need to lock all VMAs in the PMD range to support this which may lead to
increased lock contention and code complexity.

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

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index a4b97ec8ce56..b3910042bbf7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -905,12 +905,13 @@ static int collapse_find_target_node(struct collapse_control *cc)
 
 /*
  * If mmap_lock temporarily dropped, revalidate vma
- * before taking mmap_lock.
+ * after taking the mmap_lock again.
  * Returns enum scan_result value.
  */
 
 static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
-		bool expect_anon, struct vm_area_struct **vmap, struct collapse_control *cc)
+		bool expect_anon, struct vm_area_struct **vmap,
+		struct collapse_control *cc, unsigned int order)
 {
 	struct vm_area_struct *vma;
 	enum tva_type type = cc->is_khugepaged ? TVA_KHUGEPAGED :
@@ -923,15 +924,22 @@ static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned l
 	if (!vma)
 		return SCAN_VMA_NULL;
 
+	/*
+	 * We cannot collapse VMA regions that do not span the full PMD. This is
+	 * due to the potential of the PMD being shared by another VMA leaving
+	 * us vulnerable to a race condition. Always check the PMD order here to
+	 * ensure its not shared by another VMA. We'd need to lock all VMAs in
+	 * the PMD range to support this.
+	 */
 	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)))
@@ -1124,7 +1132,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 		goto out_nolock;
 
 	mmap_read_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
+	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
+					 HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
 		goto out_nolock;
@@ -1158,7 +1167,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * mmap_lock.
 	 */
 	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
+	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
+					 HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 	/* check if the pmd is still valid */
@@ -2861,8 +2871,8 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			mmap_unlocked = false;
 			*lock_dropped = true;
 			result = hugepage_vma_revalidate(mm, addr, false, &vma,
-							 cc);
-			if (result  != SCAN_SUCCEED) {
+							 cc, HPAGE_PMD_ORDER);
+			if (result != SCAN_SUCCEED) {
 				last_fail = result;
 				goto out_nolock;
 			}
-- 
2.54.0

[PATCH mm-unstable v19 02/14] mm/khugepaged: generalize alloc_charge_folio()

[Nico Pache]

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

Pass order to alloc_charge_folio() and update mTHP statistics.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Co-developed-by: Nico Pache <npache@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst |  8 ++++++++
 include/linux/huge_mm.h                    |  2 ++
 mm/huge_memory.c                           |  4 ++++
 mm/khugepaged.c                            | 20 +++++++++++++-------
 4 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 76f4eb14e262..a74844e01f1e 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 58382e97a66d..443852423790 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 1f14c5c48b4a..eea83da9114a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -699,6 +699,8 @@ static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
 DEFINE_MTHP_STAT_ATTR(anon_fault_alloc, MTHP_STAT_ANON_FAULT_ALLOC);
 DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
 DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
+DEFINE_MTHP_STAT_ATTR(collapse_alloc, MTHP_STAT_COLLAPSE_ALLOC);
+DEFINE_MTHP_STAT_ATTR(collapse_alloc_failed, MTHP_STAT_COLLAPSE_ALLOC_FAILED);
 DEFINE_MTHP_STAT_ATTR(zswpout, MTHP_STAT_ZSWPOUT);
 DEFINE_MTHP_STAT_ATTR(swpin, MTHP_STAT_SWPIN);
 DEFINE_MTHP_STAT_ATTR(swpin_fallback, MTHP_STAT_SWPIN_FALLBACK);
@@ -764,6 +766,8 @@ static struct attribute *any_stats_attrs[] = {
 #endif
 	&split_attr.attr,
 	&split_failed_attr.attr,
+	&collapse_alloc_attr.attr,
+	&collapse_alloc_failed_attr.attr,
 	NULL,
 };
 
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b3910042bbf7..44564c179636 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1077,28 +1077,34 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 }
 
 static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
-		struct collapse_control *cc)
+		struct collapse_control *cc, unsigned int order)
 {
 	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
 		     GFP_TRANSHUGE);
 	int node = collapse_find_target_node(cc);
 	struct folio *folio;
 
-	folio = __folio_alloc(gfp, HPAGE_PMD_ORDER, node, &cc->alloc_nmask);
+	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask);
 	if (!folio) {
 		*foliop = NULL;
-		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
+		if (is_pmd_order(order))
+			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
+		count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC_FAILED);
 		return SCAN_ALLOC_HUGE_PAGE_FAIL;
 	}
 
-	count_vm_event(THP_COLLAPSE_ALLOC);
+	if (is_pmd_order(order))
+		count_vm_event(THP_COLLAPSE_ALLOC);
+	count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC);
+
 	if (unlikely(mem_cgroup_charge(folio, mm, gfp))) {
 		folio_put(folio);
 		*foliop = NULL;
 		return SCAN_CGROUP_CHARGE_FAIL;
 	}
 
-	count_memcg_folio_events(folio, THP_COLLAPSE_ALLOC, 1);
+	if (is_pmd_order(order))
+		count_memcg_folio_events(folio, THP_COLLAPSE_ALLOC, 1);
 
 	*foliop = folio;
 	return SCAN_SUCCEED;
@@ -1127,7 +1133,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 */
 	mmap_read_unlock(mm);
 
-	result = alloc_charge_folio(&folio, mm, cc);
+	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out_nolock;
 
@@ -1908,7 +1914,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
 	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
 
-	result = alloc_charge_folio(&new_folio, mm, cc);
+	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out;
 
-- 
2.54.0

[PATCH mm-unstable v19 03/14] mm/khugepaged: rework max_ptes_* handling with helper functions

[Nico Pache]

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

With these changes we abstract all the madvise_collapse() special casing
(do not respect the sysctls) away from the functions that utilize them.
And will be used later in this series to cleanly restrict the mTHP
collapse behavior.

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

Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Suggested-by: David Hildenbrand <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 120 +++++++++++++++++++++++++++++++++---------------
 1 file changed, 84 insertions(+), 36 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 44564c179636..f56ab049a6c4 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -348,6 +348,64 @@ static bool pte_none_or_zero(pte_t pte)
 	return pte_present(pte) && is_zero_pfn(pte_pfn(pte));
 }
 
+/**
+ * collapse_max_ptes_none - Calculate maximum allowed empty PTEs or PTEs mapping
+ * the shared zeropage for the given collapse operation.
+ * @cc: The collapse control struct
+ * @vma: The vma to check for userfaultfd
+ *
+ * Return: Maximum number of empty/shared zeropage PTEs for the collapse operation
+ */
+static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
+		struct vm_area_struct *vma)
+{
+	if (vma && userfaultfd_armed(vma))
+		return 0;
+	/* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */
+	if (!cc->is_khugepaged)
+		return HPAGE_PMD_NR;
+	/* For all other cases respect the user defined maximum */
+	return khugepaged_max_ptes_none;
+}
+
+/**
+ * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
+ * anonymous pages for the given collapse operation.
+ * @cc: The collapse control struct
+ *
+ * Return: Maximum number of PTEs that map shared anonymous pages for the
+ * collapse operation
+ */
+static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
+{
+	/*
+	 * For MADV_COLLAPSE, do not restrict the number of PTEs that map shared
+	 * anonymous pages.
+	 */
+	if (!cc->is_khugepaged)
+		return HPAGE_PMD_NR;
+	return khugepaged_max_ptes_shared;
+}
+
+/**
+ * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
+ * maximum allowed non-present pagecache entries for the given collapse operation.
+ * @cc: The collapse control struct
+ *
+ * Return: Maximum number of non-present PTEs or the maximum allowed non-present
+ * pagecache entries for the collapse operation.
+ */
+static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
+{
+	/*
+	 * For MADV_COLLAPSE, do not restrict the number PTEs entries or
+	 * pagecache entries that are non-present.
+	 */
+	if (!cc->is_khugepaged)
+		return HPAGE_PMD_NR;
+	return khugepaged_max_ptes_swap;
+}
+
 int hugepage_madvise(struct vm_area_struct *vma,
 		     vm_flags_t *vm_flags, int advice)
 {
@@ -543,6 +601,8 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
 		struct list_head *compound_pagelist)
 {
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
 	struct page *page = NULL;
 	struct folio *folio = NULL;
 	unsigned long addr = start_addr;
@@ -554,16 +614,12 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	     _pte++, addr += PAGE_SIZE) {
 		pte_t pteval = ptep_get(_pte);
 		if (pte_none_or_zero(pteval)) {
-			++none_or_zero;
-			if (!userfaultfd_armed(vma) &&
-			    (!cc->is_khugepaged ||
-			     none_or_zero <= khugepaged_max_ptes_none)) {
-				continue;
-			} else {
+			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
 				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 				goto out;
 			}
+			continue;
 		}
 		if (!pte_present(pteval)) {
 			result = SCAN_PTE_NON_PRESENT;
@@ -594,9 +650,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		/* See collapse_scan_pmd(). */
 		if (folio_maybe_mapped_shared(folio)) {
-			++shared;
-			if (cc->is_khugepaged &&
-			    shared > khugepaged_max_ptes_shared) {
+			if (++shared > max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 				goto out;
@@ -1271,6 +1325,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		struct vm_area_struct *vma, unsigned long start_addr,
 		bool *lock_dropped, struct collapse_control *cc)
 {
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
 	pmd_t *pmd;
 	pte_t *pte, *_pte;
 	int none_or_zero = 0, shared = 0, referenced = 0;
@@ -1304,36 +1361,29 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 
 		pte_t pteval = ptep_get(_pte);
 		if (pte_none_or_zero(pteval)) {
-			++none_or_zero;
-			if (!userfaultfd_armed(vma) &&
-			    (!cc->is_khugepaged ||
-			     none_or_zero <= khugepaged_max_ptes_none)) {
-				continue;
-			} else {
+			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
 				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 				goto out_unmap;
 			}
+			continue;
 		}
 		if (!pte_present(pteval)) {
-			++unmapped;
-			if (!cc->is_khugepaged ||
-			    unmapped <= khugepaged_max_ptes_swap) {
-				/*
-				 * Always be strict with uffd-wp
-				 * enabled swap entries.  Please see
-				 * comment below for pte_uffd_wp().
-				 */
-				if (pte_swp_uffd_wp_any(pteval)) {
-					result = SCAN_PTE_UFFD_WP;
-					goto out_unmap;
-				}
-				continue;
-			} else {
+			if (++unmapped > max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				goto out_unmap;
 			}
+			/*
+			 * Always be strict with uffd-wp
+			 * enabled swap entries.  Please see
+			 * comment below for pte_uffd_wp().
+			 */
+			if (pte_swp_uffd_wp_any(pteval)) {
+				result = SCAN_PTE_UFFD_WP;
+				goto out_unmap;
+			}
+			continue;
 		}
 		if (pte_uffd_wp(pteval)) {
 			/*
@@ -1376,9 +1426,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		 * is shared.
 		 */
 		if (folio_maybe_mapped_shared(folio)) {
-			++shared;
-			if (cc->is_khugepaged &&
-			    shared > khugepaged_max_ptes_shared) {
+			if (++shared > max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
 				goto out_unmap;
@@ -2333,6 +2381,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 		unsigned long addr, struct file *file, pgoff_t start,
 		struct collapse_control *cc)
 {
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
 	struct folio *folio = NULL;
 	struct address_space *mapping = file->f_mapping;
 	XA_STATE(xas, &mapping->i_pages, start);
@@ -2351,8 +2401,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 
 		if (xa_is_value(folio)) {
 			swap += 1 << xas_get_order(&xas);
-			if (cc->is_khugepaged &&
-			    swap > khugepaged_max_ptes_swap) {
+			if (swap > max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
 				break;
@@ -2423,8 +2472,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 		cc->progress += HPAGE_PMD_NR;
 
 	if (result == SCAN_SUCCEED) {
-		if (cc->is_khugepaged &&
-		    present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
+		if (present < HPAGE_PMD_NR - max_ptes_none) {
 			result = SCAN_EXCEED_NONE_PTE;
 			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 		} else {
-- 
2.54.0

[PATCH mm-unstable v19 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Nico Pache]

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

The current mechanism for determining collapse with the
khugepaged_max_ptes_none value is not designed with mTHP in mind. This
raises a key design issue: if we support user defined max_pte_none values
(even those scaled by order), a collapse of a lower order can introduces
an feedback loop, or "creep", when max_ptes_none is set to a value greater
than HPAGE_PMD_NR / 2. [1]

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

To fix this issue introduce a helper function that will limit mTHP
collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
This effectively supports two modes: [2]

- max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
  that maps the shared zeropage. Consequently, no memory bloat.
- max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
  available mTHP order.

This removes the possibility of "creep", and a warning will be emitted if
any non-supported max_ptes_none value is configured with mTHP enabled.
Any intermediate value will default mTHP collapse to max_ptes_none=0.

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

No functional changes in this patch; however it defines future behavior
for mTHP collapse.

[1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com
[2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 126 +++++++++++++++++++++++++++++++++++-------------
 1 file changed, 93 insertions(+), 33 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f56ab049a6c4..474ee97c54ba 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -353,30 +353,51 @@ static bool pte_none_or_zero(pte_t pte)
  * the shared zeropage for the given collapse operation.
  * @cc: The collapse control struct
  * @vma: The vma to check for userfaultfd
+ * @order: The folio order being collapsed to
  *
  * Return: Maximum number of empty/shared zeropage PTEs for the collapse operation
  */
 static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
-		struct vm_area_struct *vma)
+		struct vm_area_struct *vma, unsigned int order)
 {
+	const unsigned int max_ptes_none = khugepaged_max_ptes_none;
+
 	if (vma && userfaultfd_armed(vma))
 		return 0;
 	/* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
-	/* For all other cases respect the user defined maximum */
-	return khugepaged_max_ptes_none;
+	/* for PMD collapse, respect the user defined maximum */
+	if (is_pmd_order(order))
+		return max_ptes_none;
+	/*
+	 * for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT,
+	 * scale the maximum number of PTEs to the order of the collapse.
+	 */
+	if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
+		return (1 << order) - 1;
+	/*
+	 * For mTHP collapse of values other than 0 or KHUGEPAGED_MAX_PTES_LIMIT,
+	 * emit a warning and return 0.
+	 */
+	if (max_ptes_none)
+		pr_warn_once("mTHP collapse does not support max_ptes_none"
+		     " values other than 0 or %u, defaulting to 0.\n",
+		     KHUGEPAGED_MAX_PTES_LIMIT);
+	return 0;
 }
 
 /**
  * collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
  * anonymous pages for the given collapse operation.
  * @cc: The collapse control struct
+ * @order: The folio order being collapsed to
  *
  * Return: Maximum number of PTEs that map shared anonymous pages for the
  * collapse operation
  */
-static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
+static unsigned int collapse_max_ptes_shared(struct collapse_control *cc,
+		unsigned int order)
 {
 	/*
 	 * For MADV_COLLAPSE, do not restrict the number of PTEs that map shared
@@ -384,6 +405,13 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
 	 */
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
+	/*
+	 * for mTHP collapse do not allow collapsing anonymous memory pages that
+	 * are shared between processes.
+	 */
+	if (!is_pmd_order(order))
+		return 0;
+	/* for PMD collapse, respect the user defined maximum */
 	return khugepaged_max_ptes_shared;
 }
 
@@ -391,11 +419,13 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
  * collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
  * maximum allowed non-present pagecache entries for the given collapse operation.
  * @cc: The collapse control struct
+ * @order: The folio order being collapsed to
  *
  * Return: Maximum number of non-present PTEs or the maximum allowed non-present
  * pagecache entries for the collapse operation.
  */
-static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
+static unsigned int collapse_max_ptes_swap(struct collapse_control *cc,
+		unsigned int order)
 {
 	/*
 	 * For MADV_COLLAPSE, do not restrict the number PTEs entries or
@@ -403,6 +433,10 @@ static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
 	 */
 	if (!cc->is_khugepaged)
 		return HPAGE_PMD_NR;
+	/* for mTHP collapse do not allow any non-present PTEs or pagecache entries */
+	if (!is_pmd_order(order))
+		return 0;
+	/* for PMD collapse, respect the user defined maximum */
 	return khugepaged_max_ptes_swap;
 }
 
@@ -599,10 +633,11 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
 
 static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
-		struct list_head *compound_pagelist)
+		unsigned int order, struct list_head *compound_pagelist)
 {
-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
-	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, order);
+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, order);
+	const unsigned long nr_pages = 1UL << order;
 	struct page *page = NULL;
 	struct folio *folio = NULL;
 	unsigned long addr = start_addr;
@@ -610,7 +645,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	int none_or_zero = 0, shared = 0, referenced = 0;
 	enum scan_result result = SCAN_FAIL;
 
-	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)) {
@@ -650,6 +685,12 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		/* See collapse_scan_pmd(). */
 		if (folio_maybe_mapped_shared(folio)) {
+			/*
+			 * TODO: Support shared pages without leading to further
+			 * mTHP collapses. Currently bringing in new pages via
+			 * shared may cause a future higher order collapse on a
+			 * rescan of the same range.
+			 */
 			if (++shared > max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
@@ -743,18 +784,18 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 }
 
 static void __collapse_huge_page_copy_succeeded(pte_t *pte,
-						struct vm_area_struct *vma,
-						unsigned long address,
-						spinlock_t *ptl,
-						struct list_head *compound_pagelist)
+		struct vm_area_struct *vma, unsigned long address,
+		spinlock_t *ptl, unsigned int order,
+		struct list_head *compound_pagelist)
 {
-	unsigned long end = address + HPAGE_PMD_SIZE;
+	const unsigned long nr_pages = 1UL << order;
+	unsigned long end = address + (PAGE_SIZE * nr_pages);
 	struct folio *src, *tmp;
 	pte_t pteval;
 	pte_t *_pte;
 	unsigned int nr_ptes;
 
-	for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
+	for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes,
 	     address += nr_ptes * PAGE_SIZE) {
 		nr_ptes = 1;
 		pteval = ptep_get(_pte);
@@ -807,11 +848,10 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
 }
 
 static void __collapse_huge_page_copy_failed(pte_t *pte,
-					     pmd_t *pmd,
-					     pmd_t orig_pmd,
-					     struct vm_area_struct *vma,
-					     struct list_head *compound_pagelist)
+		pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
+		unsigned int order, struct list_head *compound_pagelist)
 {
+	const unsigned long nr_pages = 1UL << order;
 	spinlock_t *pmd_ptl;
 
 	/*
@@ -827,7 +867,7 @@ 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);
 }
 
 /*
@@ -847,16 +887,17 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
  */
 static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
 		pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
-		unsigned long address, spinlock_t *ptl,
+		unsigned long address, spinlock_t *ptl, unsigned int order,
 		struct list_head *compound_pagelist)
 {
+	const unsigned long nr_pages = 1UL << order;
 	unsigned int i;
 	enum scan_result result = SCAN_SUCCEED;
 
 	/*
 	 * Copying pages' contents is subject to memory poison at any iteration.
 	 */
-	for (i = 0; i < HPAGE_PMD_NR; i++) {
+	for (i = 0; i < nr_pages; i++) {
 		pte_t pteval = ptep_get(pte + i);
 		struct page *page = folio_page(folio, i);
 		unsigned long src_addr = address + i * PAGE_SIZE;
@@ -875,10 +916,10 @@ static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *foli
 
 	if (likely(result == SCAN_SUCCEED))
 		__collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
-						    compound_pagelist);
+						    order, compound_pagelist);
 	else
 		__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
-						 compound_pagelist);
+						 order, compound_pagelist);
 
 	return result;
 }
@@ -1051,16 +1092,20 @@ static enum scan_result check_pmd_still_valid(struct mm_struct *mm,
  * Bring missing pages in from swap, to complete THP collapse.
  * Only done if khugepaged_scan_pmd believes it is worthwhile.
  *
+ * For mTHP orders the function bails on the first swap entry, because
+ * faulting pages back in during collapse could re-populate PTEs that
+ * push a later scan over the threshold for a higher-order collapse.
+ *
  * Called and returns without pte mapped or spinlocks held.
  * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
  */
 static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
-		struct vm_area_struct *vma, unsigned long start_addr, pmd_t *pmd,
-		int referenced)
+		struct vm_area_struct *vma, unsigned long start_addr,
+		pmd_t *pmd, int referenced, unsigned int order)
 {
 	int swapped_in = 0;
 	vm_fault_t ret = 0;
-	unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE);
+	unsigned long addr, end = start_addr + (PAGE_SIZE << order);
 	enum scan_result result;
 	pte_t *pte = NULL;
 	spinlock_t *ptl;
@@ -1092,6 +1137,19 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 		    pte_present(vmf.orig_pte))
 			continue;
 
+		/*
+		 * TODO: Support swapin without leading to further mTHP
+		 * collapses. Currently bringing in new pages via swapin may
+		 * cause a future higher order collapse on a rescan of the same
+		 * range.
+		 */
+		if (!is_pmd_order(order)) {
+			pte_unmap(pte);
+			mmap_read_unlock(mm);
+			result = SCAN_EXCEED_SWAP_PTE;
+			goto out;
+		}
+
 		vmf.pte = pte;
 		vmf.ptl = ptl;
 		ret = do_swap_page(&vmf);
@@ -1212,7 +1270,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 		 * that case.  Continuing to collapse causes inconsistency.
 		 */
 		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
-						     referenced);
+						     referenced, HPAGE_PMD_ORDER);
 		if (result != SCAN_SUCCEED)
 			goto out_nolock;
 	}
@@ -1260,6 +1318,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
 	if (pte) {
 		result = __collapse_huge_page_isolate(vma, address, pte, cc,
+						      HPAGE_PMD_ORDER,
 						      &compound_pagelist);
 		spin_unlock(pte_ptl);
 	} else {
@@ -1290,6 +1349,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 
 	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
 					   vma, address, pte_ptl,
+					   HPAGE_PMD_ORDER,
 					   &compound_pagelist);
 	pte_unmap(pte);
 	if (unlikely(result != SCAN_SUCCEED))
@@ -1325,9 +1385,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		struct vm_area_struct *vma, unsigned long start_addr,
 		bool *lock_dropped, struct collapse_control *cc)
 {
-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
-	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
-	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
 	pmd_t *pmd;
 	pte_t *pte, *_pte;
 	int none_or_zero = 0, shared = 0, referenced = 0;
@@ -2381,8 +2441,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 		unsigned long addr, struct file *file, pgoff_t start,
 		struct collapse_control *cc)
 {
-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
-	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
+	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER);
+	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
 	struct folio *folio = NULL;
 	struct address_space *mapping = file->f_mapping;
 	XA_STATE(xas, &mapping->i_pages, start);
-- 
2.54.0

[PATCH mm-unstable v19 05/14] mm/khugepaged: require collapse_huge_page to enter/exit with the lock dropped

[Nico Pache]

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

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

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

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 474ee97c54ba..e4b2ca77ecf6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1222,6 +1222,12 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
 	return SCAN_SUCCEED;
 }
 
+/*
+ * collapse_huge_page expects the mmap_lock to be unlocked before entering and
+ * will always return with the lock unlocked, to avoid holding the mmap_lock
+ * while allocating a THP, as that could trigger direct reclaim/compaction.
+ * Note that the VMA must be rechecked after grabbing the mmap_lock again.
+ */
 static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		int referenced, int unmapped, struct collapse_control *cc)
 {
@@ -1237,14 +1243,6 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
-	/*
-	 * Before allocating the hugepage, release the mmap_lock read lock.
-	 * The allocation can take potentially a long time if it involves
-	 * sync compaction, and we do not need to hold the mmap_lock during
-	 * that. We will recheck the vma after taking it again in write mode.
-	 */
-	mmap_read_unlock(mm);
-
 	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out_nolock;
@@ -1549,6 +1547,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
 	if (result == SCAN_SUCCEED) {
+		/* collapse_huge_page expects the lock to be dropped before calling */
+		mmap_read_unlock(mm);
 		result = collapse_huge_page(mm, start_addr, referenced,
 					    unmapped, cc);
 		/* collapse_huge_page will return with the mmap_lock released */
-- 
2.54.0

[PATCH mm-unstable v19 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[Nico Pache]

Pass an order 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.

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

To properly establish the page table hierarchy without violating any
expectations from certain architectures (e.g. MIPS), we must make sure to
have the PMD reinstalled before the PTEs, and hold both PTE/PMD locks
before calling update_mmu_cache_range() (if they are distinct locks).

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 105 ++++++++++++++++++++++++++++++------------------
 1 file changed, 67 insertions(+), 38 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e4b2ca77ecf6..c2769d82a719 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1228,34 +1228,36 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
  * while allocating a THP, as that could trigger direct reclaim/compaction.
  * Note that the VMA must be rechecked after grabbing the mmap_lock again.
  */
-static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
-		int referenced, int unmapped, struct collapse_control *cc)
+static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
+		int referenced, int unmapped, struct collapse_control *cc,
+		unsigned int order)
 {
+	const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK;
+	const unsigned long end_addr = start_addr + (PAGE_SIZE << order);
 	LIST_HEAD(compound_pagelist);
 	pmd_t *pmd, _pmd;
-	pte_t *pte;
+	pte_t *pte = NULL;
 	pgtable_t pgtable;
 	struct folio *folio;
 	spinlock_t *pmd_ptl, *pte_ptl;
 	enum scan_result result = SCAN_FAIL;
 	struct vm_area_struct *vma;
 	struct mmu_notifier_range range;
+	bool anon_vma_locked = false;
 
-	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
-
-	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
+	result = alloc_charge_folio(&folio, mm, cc, order);
 	if (result != SCAN_SUCCEED)
 		goto out_nolock;
 
 	mmap_read_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
-					 HPAGE_PMD_ORDER);
+	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
+					 &vma, cc, order);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
 		goto out_nolock;
 	}
 
-	result = find_pmd_or_thp_or_none(mm, address, &pmd);
+	result = find_pmd_or_thp_or_none(mm, pmd_addr, &pmd);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
 		goto out_nolock;
@@ -1267,8 +1269,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 		 * released when it fails. So we jump out_nolock directly in
 		 * that case.  Continuing to collapse causes inconsistency.
 		 */
-		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
-						     referenced, HPAGE_PMD_ORDER);
+		result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd,
+						     referenced, order);
 		if (result != SCAN_SUCCEED)
 			goto out_nolock;
 	}
@@ -1283,20 +1285,28 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * mmap_lock.
 	 */
 	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
-					 HPAGE_PMD_ORDER);
+	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
+					 &vma, cc, order);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 	/* check if the pmd is still valid */
 	vma_start_write(vma);
-	result = check_pmd_still_valid(mm, address, pmd);
+	result = check_pmd_still_valid(mm, pmd_addr, pmd);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 
 	anon_vma_lock_write(vma->anon_vma);
+	anon_vma_locked = true;
 
-	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
-				address + HPAGE_PMD_SIZE);
+	/*
+	 * Only notify about the PTE range we will actually modify. While we
+	 * temporary unmap the whole PTE table for mTHP collapse, we'll remap
+	 * it later, leaving other PTEs effectively unmodified. The locks we
+	 * hold prevent anybody from stumbling over such temporarily unmapped
+	 * PTE tables.
+	 */
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, start_addr,
+				end_addr);
 	mmu_notifier_invalidate_range_start(&range);
 
 	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
@@ -1308,26 +1318,23 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * Parallel GUP-fast is fine since GUP-fast will back off when
 	 * it detects PMD is changed.
 	 */
-	_pmd = pmdp_collapse_flush(vma, address, pmd);
+	_pmd = pmdp_collapse_flush(vma, pmd_addr, pmd);
 	spin_unlock(pmd_ptl);
 	mmu_notifier_invalidate_range_end(&range);
 	tlb_remove_table_sync_one();
 
-	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
+	pte = pte_offset_map_lock(mm, &_pmd, start_addr, &pte_ptl);
 	if (pte) {
-		result = __collapse_huge_page_isolate(vma, address, pte, cc,
-						      HPAGE_PMD_ORDER,
-						      &compound_pagelist);
+		result = __collapse_huge_page_isolate(vma, start_addr, pte, cc,
+						      order, &compound_pagelist);
 		spin_unlock(pte_ptl);
 	} else {
 		result = SCAN_NO_PTE_TABLE;
 	}
 
 	if (unlikely(result != SCAN_SUCCEED)) {
-		if (pte)
-			pte_unmap(pte);
 		spin_lock(pmd_ptl);
-		BUG_ON(!pmd_none(*pmd));
+		VM_WARN_ON_ONCE(!pmd_none(*pmd));
 		/*
 		 * We can only use set_pmd_at when establishing
 		 * hugepmds and never for establishing regular pmds that
@@ -1335,21 +1342,24 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 		 */
 		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
 		spin_unlock(pmd_ptl);
-		anon_vma_unlock_write(vma->anon_vma);
 		goto out_up_write;
 	}
 
 	/*
-	 * All pages are isolated and locked so anon_vma rmap
-	 * can't run anymore.
+	 * For PMD collapse all pages are isolated and locked so anon_vma
+	 * rmap can't run anymore. For mTHP collapse the PMD entry has been
+	 * removed and not all pages are isolated and locked, so we must hold
+	 * the lock to prevent neighboring folios from attempting to access
+	 * this PMD until its reinstalled.
 	 */
-	anon_vma_unlock_write(vma->anon_vma);
+	if (is_pmd_order(order)) {
+		anon_vma_unlock_write(vma->anon_vma);
+		anon_vma_locked = false;
+	}
 
 	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
-					   vma, address, pte_ptl,
-					   HPAGE_PMD_ORDER,
-					   &compound_pagelist);
-	pte_unmap(pte);
+					   vma, start_addr, pte_ptl,
+					   order, &compound_pagelist);
 	if (unlikely(result != SCAN_SUCCEED))
 		goto out_up_write;
 
@@ -1359,18 +1369,37 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
 	 * write.
 	 */
 	__folio_mark_uptodate(folio);
-	pgtable = pmd_pgtable(_pmd);
-
 	spin_lock(pmd_ptl);
-	BUG_ON(!pmd_none(*pmd));
-	pgtable_trans_huge_deposit(mm, pmd, pgtable);
-	map_anon_folio_pmd_nopf(folio, pmd, vma, address);
+	VM_WARN_ON_ONCE(!pmd_none(*pmd));
+	if (is_pmd_order(order)) {
+		pgtable = pmd_pgtable(_pmd);
+		pgtable_trans_huge_deposit(mm, pmd, pgtable);
+		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
+	} else {
+		/*
+		 * Some architectures (e.g. MIPS) walk the live page table in
+		 * their implementation. update_mmu_cache_range() must be called
+		 * with a valid page table hierarchy and the PTE lock held.
+		 * Acquire it nested inside pmd_ptl when they are distinct locks.
+		 */
+		if (pte_ptl != pmd_ptl)
+			spin_lock_nested(pte_ptl, SINGLE_DEPTH_NESTING);
+		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
+		map_anon_folio_pte_nopf(folio, pte, vma, start_addr,
+					  /*uffd_wp=*/ false);
+		if (pte_ptl != pmd_ptl)
+			spin_unlock(pte_ptl);
+	}
 	spin_unlock(pmd_ptl);
 
 	folio = NULL;
 
 	result = SCAN_SUCCEED;
 out_up_write:
+	if (anon_vma_locked)
+		anon_vma_unlock_write(vma->anon_vma);
+	if (pte)
+		pte_unmap(pte);
 	mmap_write_unlock(mm);
 out_nolock:
 	if (folio)
@@ -1550,7 +1579,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		/* collapse_huge_page expects the lock to be dropped before calling */
 		mmap_read_unlock(mm);
 		result = collapse_huge_page(mm, start_addr, referenced,
-					    unmapped, cc);
+					    unmapped, cc, HPAGE_PMD_ORDER);
 		/* collapse_huge_page will return with the mmap_lock released */
 		*lock_dropped = true;
 	}
-- 
2.54.0

[PATCH mm-unstable v19 07/14] mm/khugepaged: skip collapsing mTHP to smaller orders

[Nico Pache]

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

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

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

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Acked-by: Usama Arif <usama.arif@linux.dev>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c2769d82a719..191e529c185c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -697,6 +697,14 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 				goto out;
 			}
 		}
+		/*
+		 * TODO: In some cases of partially-mapped folios, we'd actually
+		 * want to collapse.
+		 */
+		if (!is_pmd_order(order) && folio_order(folio) >= order) {
+			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			goto out;
+		}
 
 		if (folio_test_large(folio)) {
 			struct folio *f;
-- 
2.54.0

[PATCH mm-unstable v19 08/14] mm/khugepaged: add per-order mTHP collapse failure statistics

[Nico Pache]

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

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

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

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

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

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

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

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 14 ++++++++++++++
 include/linux/huge_mm.h                    |  3 +++
 mm/huge_memory.c                           |  7 +++++++
 mm/khugepaged.c                            | 15 +++++++++++++--
 4 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index a74844e01f1e..b98e18c80185 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -714,6 +714,20 @@ nr_anon_partially_mapped
        an anonymous THP as "partially mapped" and count it here, even though it
        is not actually partially mapped anymore.
 
+collapse_exceed_none_pte
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_none threshold.
+
+collapse_exceed_swap_pte
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_swap threshold. For non-PMD orders this occurs if a mTHP range
+       contains at least one swap PTE.
+
+collapse_exceed_shared_pte
+       The number of collapse attempts that failed due to exceeding the
+       max_ptes_shared threshold. For non-PMD orders this occurs if a mTHP range
+       contains at least one shared PTE.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in ``/proc/vmstat`` to help
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 443852423790..148109ebd08a 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 eea83da9114a..222e421d9e8e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -717,6 +717,10 @@ DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
 DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
 DEFINE_MTHP_STAT_ATTR(nr_anon, MTHP_STAT_NR_ANON);
 DEFINE_MTHP_STAT_ATTR(nr_anon_partially_mapped, MTHP_STAT_NR_ANON_PARTIALLY_MAPPED);
+DEFINE_MTHP_STAT_ATTR(collapse_exceed_swap_pte, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
+DEFINE_MTHP_STAT_ATTR(collapse_exceed_none_pte, MTHP_STAT_COLLAPSE_EXCEED_NONE);
+DEFINE_MTHP_STAT_ATTR(collapse_exceed_shared_pte, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
+
 
 static struct attribute *anon_stats_attrs[] = {
 	&anon_fault_alloc_attr.attr,
@@ -733,6 +737,9 @@ static struct attribute *anon_stats_attrs[] = {
 	&split_deferred_attr.attr,
 	&nr_anon_attr.attr,
 	&nr_anon_partially_mapped_attr.attr,
+	&collapse_exceed_swap_pte_attr.attr,
+	&collapse_exceed_none_pte_attr.attr,
+	&collapse_exceed_shared_pte_attr.attr,
 	NULL,
 };
 
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 191e529c185c..ac4731addafa 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -651,7 +651,9 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		if (pte_none_or_zero(pteval)) {
 			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
-				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				if (is_pmd_order(order))
+					count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_NONE);
 				goto out;
 			}
 			continue;
@@ -693,7 +695,9 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			 */
 			if (++shared > max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
-				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				if (is_pmd_order(order))
+					count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
 				goto out;
 			}
 		}
@@ -1152,6 +1156,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 		 * range.
 		 */
 		if (!is_pmd_order(order)) {
+			count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
 			pte_unmap(pte);
 			mmap_read_unlock(mm);
 			result = SCAN_EXCEED_SWAP_PTE;
@@ -1459,6 +1464,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
 				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				count_mthp_stat(HPAGE_PMD_ORDER,
+						MTHP_STAT_COLLAPSE_EXCEED_NONE);
 				goto out_unmap;
 			}
 			continue;
@@ -1467,6 +1474,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			if (++unmapped > max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
+				count_mthp_stat(HPAGE_PMD_ORDER,
+						MTHP_STAT_COLLAPSE_EXCEED_SWAP);
 				goto out_unmap;
 			}
 			/*
@@ -1524,6 +1533,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			if (++shared > max_ptes_shared) {
 				result = SCAN_EXCEED_SHARED_PTE;
 				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				count_mthp_stat(HPAGE_PMD_ORDER,
+						MTHP_STAT_COLLAPSE_EXCEED_SHARED);
 				goto out_unmap;
 			}
 		}
-- 
2.54.0

[PATCH mm-unstable v19 09/14] mm/khugepaged: improve tracepoints for mTHP orders

[Nico Pache]

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

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

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index bcdc57eea270..291fae364c62 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -89,40 +89,44 @@ 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(
@@ -130,26 +134,30 @@ 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(
@@ -157,13 +165,15 @@ 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 ac4731addafa..26c343a6fa3d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -785,13 +785,13 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	} else {
 		result = SCAN_SUCCEED;
 		trace_mm_collapse_huge_page_isolate(folio, none_or_zero,
-						    referenced, result);
+						    referenced, result, order);
 		return result;
 	}
 out:
 	release_pte_pages(pte, _pte, compound_pagelist);
 	trace_mm_collapse_huge_page_isolate(folio, none_or_zero,
-					    referenced, result);
+					    referenced, result, order);
 	return result;
 }
 
@@ -1197,7 +1197,8 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 
 	result = SCAN_SUCCEED;
 out:
-	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, result);
+	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, result,
+					   order);
 	return result;
 }
 
@@ -1417,7 +1418,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 out_nolock:
 	if (folio)
 		folio_put(folio);
-	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
+	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result, order);
 	return result;
 }
 
-- 
2.54.0

[PATCH mm-unstable v19 10/14] mm/khugepaged: introduce collapse_possible_orders helper functions

[Nico Pache]

Add collapse_possible_orders() to generalize THP order eligibility. The
function determines which THP orders are permitted based on collapse
context (khugepaged vs madv_collapse). We also add collapse_possible()
as a thin wrapper around collapse_possible_orders() that returns a bool
rather than the whole bitmap.

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

Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 26c343a6fa3d..ec886a031952 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -554,12 +554,30 @@ void __khugepaged_enter(struct mm_struct *mm)
 		wake_up_interruptible(&khugepaged_wait);
 }
 
+/*
+ * Check what orders are possible based on the vma and collapse type.
+ * This is used to determine if mTHP collapse is a viable option.
+ */
+static unsigned long collapse_possible_orders(struct vm_area_struct *vma,
+		vm_flags_t vm_flags, enum tva_type tva_flags)
+{
+	const unsigned long orders = BIT(HPAGE_PMD_ORDER);
+
+	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
+}
+
+static bool collapse_possible(struct vm_area_struct *vma,
+		vm_flags_t vm_flags, enum tva_type tva_flags)
+{
+	return collapse_possible_orders(vma, vm_flags, tva_flags);
+}
+
 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_possible(vma, vm_flags, TVA_KHUGEPAGED))
 			__khugepaged_enter(vma->vm_mm);
 	}
 }
@@ -2700,7 +2718,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
 			cc->progress++;
 			break;
 		}
-		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
+		if (!collapse_possible(vma, vma->vm_flags, TVA_KHUGEPAGED)) {
 			cc->progress++;
 			continue;
 		}
@@ -3010,7 +3028,7 @@ 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_possible(vma, vma->vm_flags, TVA_FORCED_COLLAPSE))
 		return -EINVAL;
 
 	cc = kmalloc_obj(*cc);
-- 
2.54.0

[PATCH mm-unstable v19 11/14] mm/khugepaged: Introduce mTHP collapse support

[Nico Pache]

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

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

To enabled mTHP collapse we make the following changes:

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

Implement mthp_collapse() to walk forward through the bitmap and
determine the best eligible order for each naturally-aligned region. The
algorithm starts at the beginning of the PMD range and, for each offset,
tries the highest order that fits the alignment. If the number of
occupied PTEs in that region satisfies the max_ptes_none threshold for
that order, a collapse is attempted. On failure, the order is
decremented and the same offset is retried at the next smaller size. Once
the smallest enabled order is exhausted (or a collapse succeeds), the
offset advances past the region just processed, and the next attempt
starts at the highest order permitted by the new offset's natural
alignment.

The algorithm works as follows:
    1) set offset=0 and order=HPAGE_PMD_ORDER
    2) if the order is not enabled, go to step (5)
    3) count occupied PTEs in the (offset, order) range using
       bitmap_weight_from()
    4) if the count satisfies the max_ptes_none threshold, attempt
       collapse; on success, advance to step (6)
    5) if a smaller enabled order exists, decrement order and retry
       from step (2) at the same offset
    6) advance offset past the current region and compute the next
       order from the new offset's natural alignment via __ffs(offset),
       capped at HPAGE_PMD_ORDER
    7) repeat from step (2) until the full PMD range is covered

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

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

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

Any other max_ptes_none value will emit a warning and default mTHP
collapse to max_ptes_none=0. There should be no behavior change for PMD
collapse.

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

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

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

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 146 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 138 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ec886a031952..430047316f43 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -99,6 +99,8 @@ 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
+
 struct collapse_control {
 	bool is_khugepaged;
 
@@ -110,6 +112,9 @@ struct collapse_control {
 
 	/* nodemask for allocation fallback */
 	nodemask_t alloc_nmask;
+
+	/* Each bit represents a single occupied (!none/zero) page. */
+	DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE);
 };
 
 /**
@@ -1440,20 +1445,130 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 	return result;
 }
 
+/* Return the highest naturally aligned order that fits at @offset within a PMD. */
+static unsigned int max_order_from_offset(unsigned int offset)
+{
+	if (offset == 0)
+		return HPAGE_PMD_ORDER;
+
+	return min_t(unsigned int, __ffs(offset), HPAGE_PMD_ORDER);
+}
+
+/*
+ * 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_present_ptes represents a single occupied (!none/zero)
+ * page. We start at the PMD order and check if it is eligible for collapse;
+ * if not, we check the left and right halves of the PTE page table we are
+ * examining at a lower order.
+ *
+ * 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 enum scan_result mthp_collapse(struct mm_struct *mm,
+		unsigned long address, int referenced, int unmapped,
+		struct collapse_control *cc, unsigned long enabled_orders)
+{
+	unsigned int nr_occupied_ptes, nr_ptes, max_ptes_none;
+	enum scan_result last_result = SCAN_FAIL;
+	int collapsed = 0;
+	bool alloc_failed = false;
+	unsigned long collapse_address;
+	unsigned int offset = 0;
+	unsigned int order = HPAGE_PMD_ORDER;
+
+	while (offset < HPAGE_PMD_NR) {
+		nr_ptes = 1UL << order;
+
+		if (!test_bit(order, &enabled_orders))
+			goto next_order;
+
+		max_ptes_none = collapse_max_ptes_none(cc, NULL, order);
+		nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset,
+						      offset + nr_ptes);
+
+		if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
+			enum scan_result ret;
+
+			collapse_address = address + offset * PAGE_SIZE;
+			ret = collapse_huge_page(mm, collapse_address, referenced,
+						 unmapped, cc, order);
+			switch (ret) {
+			/* Cases where we continue to next collapse candidate */
+			case SCAN_SUCCEED:
+				collapsed += nr_ptes;
+				fallthrough;
+			case SCAN_PTE_MAPPED_HUGEPAGE:
+				goto next_offset;
+			/* Cases where lower orders might still succeed */
+			case SCAN_ALLOC_HUGE_PAGE_FAIL:
+				alloc_failed = true;
+				last_result = ret;
+				goto next_order;
+			/* Cases where no further collapse is possible */
+			case SCAN_PMD_MAPPED:
+				fallthrough;
+			default:
+				last_result = ret;
+				goto done;
+			}
+		}
+
+next_order:
+		/*
+		 * Continue with the next smaller order if there is still
+		 * any smaller order enabled. When at the smallest order
+		 * we must always move to the next offset.
+		 */
+		if (order > KHUGEPAGED_MIN_MTHP_ORDER &&
+			(enabled_orders & GENMASK(order - 1, 0))) {
+			order--;
+			continue;
+		}
+next_offset:
+		/*
+		 * Advance past the region we just processed and determine the
+		 * highest order we can attempt next. Since huge pages must be
+		 * naturally aligned, the max order we can attempt next is
+		 * limited by the alignment of the new offset.
+		 * E.g. if we collapsed a order-2 mTHP at offset 0, offset
+		 * becomes 4 and __ffs(4) == 2, so the next attempt starts at
+		 * order 2.
+		 */
+		offset += nr_ptes;
+		order = max_order_from_offset(offset);
+	}
+done:
+	if (collapsed)
+		return SCAN_SUCCEED;
+	if (alloc_failed)
+		return SCAN_ALLOC_HUGE_PAGE_FAIL;
+	return last_result;
+}
+
 static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		struct vm_area_struct *vma, unsigned long start_addr,
 		bool *lock_dropped, struct collapse_control *cc)
 {
-	const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
 	const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
 	const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
+	unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
+	enum tva_type tva_flags = cc->is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE;
 	pmd_t *pmd;
-	pte_t *pte, *_pte;
+	pte_t *pte, *_pte, pteval;
+	int i;
 	int none_or_zero = 0, shared = 0, referenced = 0;
 	enum scan_result result = SCAN_FAIL;
 	struct page *page = NULL;
 	struct folio *folio = NULL;
 	unsigned long addr;
+	unsigned long enabled_orders;
 	spinlock_t *ptl;
 	int node = NUMA_NO_NODE, unmapped = 0;
 
@@ -1465,8 +1580,19 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		goto out;
 	}
 
+	bitmap_zero(cc->mthp_present_ptes, MAX_PTRS_PER_PTE);
 	memset(cc->node_load, 0, sizeof(cc->node_load));
 	nodes_clear(cc->alloc_nmask);
+
+	enabled_orders = collapse_possible_orders(vma, vma->vm_flags, tva_flags);
+
+	/*
+	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
+	 * scan all pages to populate the bitmap for mTHP collapse.
+	 */
+	if (enabled_orders != BIT(HPAGE_PMD_ORDER))
+		max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
+
 	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
 	if (!pte) {
 		cc->progress++;
@@ -1474,11 +1600,13 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		goto out;
 	}
 
-	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
-	     _pte++, addr += PAGE_SIZE) {
+	for (i = 0; i < HPAGE_PMD_NR; i++) {
+		_pte = pte + i;
+		addr = start_addr + i * PAGE_SIZE;
+		pteval = ptep_get(_pte);
+
 		cc->progress++;
 
-		pte_t pteval = ptep_get(_pte);
 		if (pte_none_or_zero(pteval)) {
 			if (++none_or_zero > max_ptes_none) {
 				result = SCAN_EXCEED_NONE_PTE;
@@ -1558,6 +1686,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			}
 		}
 
+		/* Set bit for occupied pages */
+		__set_bit(i, cc->mthp_present_ptes);
 		/*
 		 * Record which node the original page is from and save this
 		 * information to cc->node_load[].
@@ -1616,9 +1746,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 	if (result == SCAN_SUCCEED) {
 		/* collapse_huge_page expects the lock to be dropped before calling */
 		mmap_read_unlock(mm);
-		result = collapse_huge_page(mm, start_addr, referenced,
-					    unmapped, cc, HPAGE_PMD_ORDER);
-		/* collapse_huge_page will return with the mmap_lock released */
+		result = mthp_collapse(mm, start_addr, referenced,
+				       unmapped, cc, enabled_orders);
+		/* mmap_lock was released above, set lock_dropped */
 		*lock_dropped = true;
 	}
 out:
-- 
2.54.0

[PATCH mm-unstable v19 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[Nico Pache]

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

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 430047316f43..7de92b28dd30 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1499,6 +1499,7 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
 			collapse_address = address + offset * PAGE_SIZE;
 			ret = collapse_huge_page(mm, collapse_address, referenced,
 						 unmapped, cc, order);
+
 			switch (ret) {
 			/* Cases where we continue to next collapse candidate */
 			case SCAN_SUCCEED:
@@ -1509,6 +1510,18 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
 			/* Cases where lower orders might still succeed */
 			case SCAN_ALLOC_HUGE_PAGE_FAIL:
 				alloc_failed = true;
+				fallthrough;
+			case SCAN_LACK_REFERENCED_PAGE:
+			case SCAN_EXCEED_NONE_PTE:
+			case SCAN_EXCEED_SWAP_PTE:
+			case SCAN_EXCEED_SHARED_PTE:
+			case SCAN_PAGE_LOCK:
+			case SCAN_PAGE_COUNT:
+			case SCAN_PAGE_NULL:
+			case SCAN_DEL_PAGE_LRU:
+			case SCAN_PTE_NON_PRESENT:
+			case SCAN_PTE_UFFD_WP:
+			case SCAN_PAGE_LAZYFREE:
 				last_result = ret;
 				goto next_order;
 			/* Cases where no further collapse is possible */
-- 
2.54.0

[PATCH mm-unstable v19 13/14] mm/khugepaged: run khugepaged for all orders

[Nico Pache]

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

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

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

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

After this patch khugepaged mTHP collapse is fully enabled.

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 7de92b28dd30..996e014a03d3 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -503,23 +503,23 @@ 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())
@@ -566,7 +566,13 @@ void __khugepaged_enter(struct mm_struct *mm)
 static unsigned long collapse_possible_orders(struct vm_area_struct *vma,
 		vm_flags_t vm_flags, enum tva_type tva_flags)
 {
-	const unsigned long orders = BIT(HPAGE_PMD_ORDER);
+	unsigned long orders;
+
+	/* If khugepaged is scanning an anonymous vma, allow mTHP collapse */
+	if ((tva_flags == TVA_KHUGEPAGED) && vma_is_anonymous(vma))
+		orders = THP_ORDERS_ALL_ANON;
+	else
+		orders = BIT(HPAGE_PMD_ORDER);
 
 	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
 }
@@ -580,11 +586,9 @@ static bool collapse_possible(struct vm_area_struct *vma,
 void khugepaged_enter_vma(struct vm_area_struct *vma,
 			  vm_flags_t vm_flags)
 {
-	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
-	    hugepage_pmd_enabled()) {
-		if (collapse_possible(vma, vm_flags, TVA_KHUGEPAGED))
-			__khugepaged_enter(vma->vm_mm);
-	}
+	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) && hugepage_enabled()
+	    && collapse_possible(vma, vm_flags, TVA_KHUGEPAGED))
+		__khugepaged_enter(vma->vm_mm);
 }
 
 void __khugepaged_exit(struct mm_struct *mm)
@@ -2936,7 +2940,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
 
 static int khugepaged_has_work(void)
 {
-	return !list_empty(&khugepaged_scan.mm_head) && hugepage_pmd_enabled();
+	return !list_empty(&khugepaged_scan.mm_head) && hugepage_enabled();
 }
 
 static int khugepaged_wait_event(void)
@@ -3009,7 +3013,7 @@ static void khugepaged_wait_work(void)
 		return;
 	}
 
-	if (hugepage_pmd_enabled())
+	if (hugepage_enabled())
 		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
 }
 
@@ -3040,7 +3044,7 @@ void set_recommended_min_free_kbytes(void)
 	int nr_zones = 0;
 	unsigned long recommended_min;
 
-	if (!hugepage_pmd_enabled()) {
+	if (!hugepage_enabled()) {
 		calculate_min_free_kbytes();
 		goto update_wmarks;
 	}
@@ -3090,7 +3094,7 @@ 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");
@@ -3116,7 +3120,7 @@ int start_stop_khugepaged(void)
 void khugepaged_min_free_kbytes_update(void)
 {
 	mutex_lock(&khugepaged_mutex);
-	if (hugepage_pmd_enabled() && khugepaged_thread)
+	if (hugepage_enabled() && khugepaged_thread)
 		set_recommended_min_free_kbytes();
 	mutex_unlock(&khugepaged_mutex);
 }
-- 
2.54.0

[PATCH mm-unstable v19 14/14] Documentation: mm: update the admin guide for mTHP collapse

[Nico Pache]

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

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 49 ++++++++++++++--------
 1 file changed, 32 insertions(+), 17 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index b98e18c80185..23f8d13c2629 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
@@ -265,8 +266,8 @@ 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
+   khugepaged currently only searches for opportunities to collapse file/shmem
+   to PMD-sized THP. Only anonymous memory will attempt to collapse to other THP
    sizes.
 
 khugepaged runs usually at low frequency so while one may not want to
@@ -296,11 +297,11 @@ 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 +309,21 @@ 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. At
+HPAGE_PMD_NR - 1, we collapse to the highest possible order. Any intermediate
+value will emit a warning and mTHP collapse will default to max_ptes_none=0.
+
+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 +343,15 @@ that THP is shared. Exceeding the number would block the collapse::
 
 A higher value may increase memory footprint for some workloads.
 
+.. note::
+   For mTHP collapse, khugepaged does not support collapsing regions that
+   contain shared or swapped out pages, as this could lead to continuous
+   promotion to higher orders. The collapse will fail if any shared or
+   swapped PTEs are encountered during the scan.
+
+   Currently, madvise_collapse only supports collapsing to PMD-sized THPs
+   and does not attempt mTHP collapses.
+
 Boot parameters
 ===============
 
-- 
2.54.0

Re: [PATCH mm-unstable v19 10/14] mm/khugepaged: introduce collapse_possible_orders helper functions

[Lorenzo Stoakes]

On Fri, Jun 05, 2026 at 10:14:17AM -0600, Nico Pache wrote:
> Add collapse_possible_orders() to generalize THP order eligibility. The
> function determines which THP orders are permitted based on collapse
> context (khugepaged vs madv_collapse). We also add collapse_possible()
> as a thin wrapper around collapse_possible_orders() that returns a bool
> rather than the whole bitmap.
>
> This consolidates collapse configuration logic and provides a clean
> interface for future mTHP collapse support where the orders may be
> different.
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: Nico Pache <npache@redhat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

> ---
>  mm/khugepaged.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 26c343a6fa3d..ec886a031952 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -554,12 +554,30 @@ void __khugepaged_enter(struct mm_struct *mm)
>  		wake_up_interruptible(&khugepaged_wait);
>  }
>
> +/*
> + * Check what orders are possible based on the vma and collapse type.
> + * This is used to determine if mTHP collapse is a viable option.
> + */
> +static unsigned long collapse_possible_orders(struct vm_area_struct *vma,
> +		vm_flags_t vm_flags, enum tva_type tva_flags)
> +{
> +	const unsigned long orders = BIT(HPAGE_PMD_ORDER);
> +
> +	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
> +}
> +
> +static bool collapse_possible(struct vm_area_struct *vma,
> +		vm_flags_t vm_flags, enum tva_type tva_flags)
> +{
> +	return collapse_possible_orders(vma, vm_flags, tva_flags);
> +}
> +
>  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_possible(vma, vm_flags, TVA_KHUGEPAGED))
>  			__khugepaged_enter(vma->vm_mm);
>  	}
>  }
> @@ -2700,7 +2718,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
>  			cc->progress++;
>  			break;
>  		}
> -		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
> +		if (!collapse_possible(vma, vma->vm_flags, TVA_KHUGEPAGED)) {
>  			cc->progress++;
>  			continue;
>  		}
> @@ -3010,7 +3028,7 @@ 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_possible(vma, vma->vm_flags, TVA_FORCED_COLLAPSE))
>  		return -EINVAL;
>
>  	cc = kmalloc_obj(*cc);
> --
> 2.54.0
>

Re: [PATCH mm-unstable v19 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[David Hildenbrand (Arm)]

On 6/5/26 18:14, Nico Pache wrote:
> Pass an order 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.
> 
> For non-PMD collapse we must leave the anon VMA write locked until after
> we collapse the mTHP-- in the PMD case all the pages are isolated, but in
> the mTHP case this is not true, and we must keep the lock to prevent
> access/changes to the page tables. This can happen if the rmap walkers hit
> a pmd_none while the PMD entry is currently unavailable due to being
> temporarily removed during the collapse phase.
> 
> To properly establish the page table hierarchy without violating any
> expectations from certain architectures (e.g. MIPS), we must make sure to
> have the PMD reinstalled before the PTEs, and hold both PTE/PMD locks
> before calling update_mmu_cache_range() (if they are distinct locks).
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---

[...]

>  	 */
>  	__folio_mark_uptodate(folio);
> -	pgtable = pmd_pgtable(_pmd);
> -
>  	spin_lock(pmd_ptl);
> -	BUG_ON(!pmd_none(*pmd));
> -	pgtable_trans_huge_deposit(mm, pmd, pgtable);
> -	map_anon_folio_pmd_nopf(folio, pmd, vma, address);
> +	VM_WARN_ON_ONCE(!pmd_none(*pmd));
> +	if (is_pmd_order(order)) {
> +		pgtable = pmd_pgtable(_pmd);
> +		pgtable_trans_huge_deposit(mm, pmd, pgtable);
> +		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
> +	} else {
> +		/*
> +		 * Some architectures (e.g. MIPS) walk the live page table in
> +		 * their implementation. update_mmu_cache_range() must be called
> +		 * with a valid page table hierarchy and the PTE lock held.
> +		 * Acquire it nested inside pmd_ptl when they are distinct locks.
> +		 */
> +		if (pte_ptl != pmd_ptl)
> +			spin_lock_nested(pte_ptl, SINGLE_DEPTH_NESTING);
> +		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> +		map_anon_folio_pte_nopf(folio, pte, vma, start_addr,
> +					  /*uffd_wp=*/ false);
> +		if (pte_ptl != pmd_ptl)
> +			spin_unlock(pte_ptl);
> +	}
>  	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);

We re-enable some page table walkers before we unmap the PTE.

We still hold the mmap lock in write mode, so nothing would currently try
reclaiming the page table concurrently.

So I guess this works right now, but we should likely rework that code later to
either revert both statements. Or maybe we can simply unmap like we did, and
simply remap before we call map_anon_folio_pte_nopf()? Remapping should not fail.

Alternatively to an unmap+remap, I think we could also unmap earlier for PMD

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6de935e76ceb..ba2a2508dda6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1378,6 +1378,8 @@ static enum scan_result collapse_huge_page(struct
mm_struct *mm, unsigned long s
        if (is_pmd_order(order)) {
                anon_vma_unlock_write(vma->anon_vma);
                anon_vma_locked = false;
+               pte_unmap(pte);
+               pte = NULL;
        }

        result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,

But this can also be handled later.

We now hold an anon_vma lock a bit longer for !pmd-collapse. But there is also
less to copy. If that bites us, we can try optimizing later.


So after another skim, I think this patch is ready for primetime. We can address
the things mentioned above later ... and any fallout can be fixed later, if any.

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


-- 
Cheers,

David

Re: [PATCH mm-unstable v19 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[David Hildenbrand (Arm)]

On 6/5/26 18:14, Nico Pache wrote:
> There are cases where, if an attempted collapse fails, all subsequent
> orders are guaranteed to also fail. Avoid these collapse attempts by
> bailing out early.
> 
> Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
> Acked-by: Usama Arif <usama.arif@linux.dev>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 430047316f43..7de92b28dd30 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1499,6 +1499,7 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
>  			collapse_address = address + offset * PAGE_SIZE;
>  			ret = collapse_huge_page(mm, collapse_address, referenced,
>  						 unmapped, cc, order);
> +

Unrelated to this patch, but not the end of the world :)

-- 
Cheers,

David

Re: [PATCH mm-unstable v19 14/14] Documentation: mm: update the admin guide for mTHP collapse

[David Hildenbrand (Arm)]

On 6/5/26 18:14, 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: Lorenzo Stoakes <ljs@kernel.org>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---

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

-- 
Cheers,

David

Re: [PATCH mm-unstable v19 11/14] mm/khugepaged: Introduce mTHP collapse support

[David Hildenbrand (Arm)]

On 6/5/26 18:14, Nico Pache wrote:
> Enable khugepaged to collapse to mTHP orders. This patch implements the
> main scanning logic using a bitmap to track occupied pages and the
> algorithm to find optimal collapse sizes.
> 
> Previous to this patch, PMD collapse had 3 main phases, a light weight
> scanning phase (mmap_read_lock) that determines a potential PMD
> collapse, an alloc phase (mmap unlocked), then finally heavier collapse
> phase (mmap_write_lock).
> 
> To enabled mTHP collapse we make the following changes:
> 
> During PMD scan phase, track occupied pages in a bitmap. When mTHP
> orders are enabled, we remove the restriction of max_ptes_none during the
> scan phase to avoid missing potential mTHP collapse candidates. Once we
> have scanned the full PMD range and updated the bitmap to track occupied
> pages, we use the bitmap to find the optimal mTHP size.
> 
> Implement mthp_collapse() to walk forward through the bitmap and
> determine the best eligible order for each naturally-aligned region. The
> algorithm starts at the beginning of the PMD range and, for each offset,
> tries the highest order that fits the alignment. If the number of
> occupied PTEs in that region satisfies the max_ptes_none threshold for
> that order, a collapse is attempted. On failure, the order is
> decremented and the same offset is retried at the next smaller size. Once
> the smallest enabled order is exhausted (or a collapse succeeds), the
> offset advances past the region just processed, and the next attempt
> starts at the highest order permitted by the new offset's natural
> alignment.
> 
> The algorithm works as follows:
>     1) set offset=0 and order=HPAGE_PMD_ORDER
>     2) if the order is not enabled, go to step (5)
>     3) count occupied PTEs in the (offset, order) range using
>        bitmap_weight_from()
>     4) if the count satisfies the max_ptes_none threshold, attempt
>        collapse; on success, advance to step (6)
>     5) if a smaller enabled order exists, decrement order and retry
>        from step (2) at the same offset
>     6) advance offset past the current region and compute the next
>        order from the new offset's natural alignment via __ffs(offset),
>        capped at HPAGE_PMD_ORDER
>     7) repeat from step (2) until the full PMD range is covered
> 
> mTHP collapses reject regions containing swapped out or shared pages.
> This is because adding new entries can lead to new none pages, and these
> may lead to constant promotion into a higher order mTHP. A similar
> issue can occur with "max_ptes_none > HPAGE_PMD_NR/2" due to a collapse
> introducing at least 2x the number of pages, and on a future scan will
> satisfy the promotion condition once again. This issue is prevented via
> the collapse_max_ptes_none() function which imposes the max_ptes_none
> restrictions above.
> 
> We currently only support mTHP collapse for max_ptes_none values of 0
> and HPAGE_PMD_NR - 1. resulting in the following behavior:
> 
>     - max_ptes_none=0: Never introduce new empty pages during collapse
>     - max_ptes_none=HPAGE_PMD_NR-1: Always try collapse to the highest
>       available mTHP order
> 
> Any other max_ptes_none value will emit a warning and default mTHP
> collapse to max_ptes_none=0. There should be no behavior change for PMD
> collapse.
> 
> Once we determine what mTHP sizes fits best in that PMD range a collapse
> is attempted. A minimum collapse order of 2 is used as this is the lowest
> order supported by anon memory as defined by THP_ORDERS_ALL_ANON.
> 
> Currently madv_collapse is not supported and will only attempt PMD
> collapse.
> 
> We can also remove the check for is_khugepaged inside the PMD scan as
> the collapse_max_ptes_none() function handles this logic now.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---

Yeah, overall much simpler and much easier to get. As discussed, we can optimize
this later to traverse enabled orders more efficiently.

> +	bitmap_zero(cc->mthp_present_ptes, MAX_PTRS_PER_PTE);
>  	memset(cc->node_load, 0, sizeof(cc->node_load));
>  	nodes_clear(cc->alloc_nmask);
> +
> +	enabled_orders = collapse_possible_orders(vma, vma->vm_flags, tva_flags);
> +
> +	/*
> +	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
> +	 * scan all pages to populate the bitmap for mTHP collapse.
> +	 */

I think it would have been good to mention where the check is performed for mTHP
collapse. Can be added later.


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

-- 
Cheers,

David

Re: [PATCH mm-unstable v19 00/14] khugepaged: add mTHP collapse support

[David Hildenbrand (Arm)]

On 6/5/26 18:14, Nico Pache wrote:
> 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 mTHP collapse will default to max_ptes_none=0. If a mTHP collapse is
> attempted, but contains swapped out, or shared pages, we don't perform
> the collapse.
> It is now also possible to collapse to mTHPs without requiring the PMD THP
> size to be enabled. These limitations are to prevent collapse "creep"
> behavior. This prevents constantly promoting mTHPs to the next available
> size, which would occur because a collapse introduces more non-zero pages
> that would satisfy the promotion condition on subsequent scans.
> 
> Patch 1-2:   Generalize hugepage_vma_revalidate and alloc_charge_folio
>              for arbitrary orders.
> Patch 3:     Rework max_ptes_* handling into helper functions
> Patch 4:     Generalize __collapse_huge_page_* for mTHP support
> Patch 5:     Require collapse_huge_page to enter/exit with the lock dropped
> Patch 6:     Generalize collapse_huge_page for mTHP collapse
> Patch 7:     Skip collapsing mTHP to smaller orders
> Patch 8-9:   Add per-order mTHP statistics and tracepoints
> Patch 10:    Introduce collapse_possible_orders helper functions
> Patch 11-13: Introduce bitmap and mTHP collapse support, fully enabled
> Patch 14:    Documentation
> 

Went through it and didn't find any blockers. Let's wait for Lorenzo's assessment.

If he also doesn't find anything major, I think we can move forward with merging
it and handle smaller things as follow-ups.

-- 
Cheers,

David

Re: [PATCH mm-unstable v19 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[Lorenzo Stoakes]

On Fri, Jun 05, 2026 at 07:48:17PM +0200, David Hildenbrand (Arm) wrote:
> On 6/5/26 18:14, Nico Pache wrote:
> > Pass an order 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.
> >
> > For non-PMD collapse we must leave the anon VMA write locked until after
> > we collapse the mTHP-- in the PMD case all the pages are isolated, but in
> > the mTHP case this is not true, and we must keep the lock to prevent
> > access/changes to the page tables. This can happen if the rmap walkers hit
> > a pmd_none while the PMD entry is currently unavailable due to being
> > temporarily removed during the collapse phase.
> >
> > To properly establish the page table hierarchy without violating any
> > expectations from certain architectures (e.g. MIPS), we must make sure to
> > have the PMD reinstalled before the PTEs, and hold both PTE/PMD locks
> > before calling update_mmu_cache_range() (if they are distinct locks).
> >
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
>
> [...]
>
> >  	 */
> >  	__folio_mark_uptodate(folio);
> > -	pgtable = pmd_pgtable(_pmd);
> > -
> >  	spin_lock(pmd_ptl);
> > -	BUG_ON(!pmd_none(*pmd));
> > -	pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > -	map_anon_folio_pmd_nopf(folio, pmd, vma, address);
> > +	VM_WARN_ON_ONCE(!pmd_none(*pmd));
> > +	if (is_pmd_order(order)) {
> > +		pgtable = pmd_pgtable(_pmd);
> > +		pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > +		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
> > +	} else {
> > +		/*
> > +		 * Some architectures (e.g. MIPS) walk the live page table in
> > +		 * their implementation. update_mmu_cache_range() must be called
> > +		 * with a valid page table hierarchy and the PTE lock held.
> > +		 * Acquire it nested inside pmd_ptl when they are distinct locks.
> > +		 */
> > +		if (pte_ptl != pmd_ptl)
> > +			spin_lock_nested(pte_ptl, SINGLE_DEPTH_NESTING);
> > +		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> > +		map_anon_folio_pte_nopf(folio, pte, vma, start_addr,
> > +					  /*uffd_wp=*/ false);
> > +		if (pte_ptl != pmd_ptl)
> > +			spin_unlock(pte_ptl);
> > +	}
> >  	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);
>
> We re-enable some page table walkers before we unmap the PTE.
>
> We still hold the mmap lock in write mode, so nothing would currently try
> reclaiming the page table concurrently.

Reclaim uses rmap walkers though?

Oh you mean as in page table teardown, we're safe from higher level page table
teardown, but we're not safe from zap PTE page table teardown, as
CONFIG_PT_RECLAIM makes this possible on zap now.

That is RCU safe, so the unmap would keep us safe here, but now we could lose
the PTE page table.

But, only MADV_DONTNEED sets reclaim_pt = true, and that holds the VMA read lock
so we're safe.

And anyway:

MADV_DONTNEED - VMA read lock (we hold VMA write lock)
zap_vma_for_reaping() - mmap read lock
process teardown, munmap - mmap read lock
fault - vma/mmap read lock

So the vma/mmap locks save us from those.

So rmap-wise, only the i_mmap walkers remain (truncate, hole punch, et al. and
also hugetlbfs truncate/hole-punch which does its own nonsense too), but none of
those allow for reclaim_pt to happen in any case.

So yeah we're safe but we should what, reorder these 2 statements?

But yes I agree that can be a follow-up, nothing's broken AFAICT.

>
> So I guess this works right now, but we should likely rework that code later to
> either revert both statements. Or maybe we can simply unmap like we did, and
> simply remap before we call map_anon_folio_pte_nopf()? Remapping should not fail.
>
> Alternatively to an unmap+remap, I think we could also unmap earlier for PMD
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6de935e76ceb..ba2a2508dda6 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1378,6 +1378,8 @@ static enum scan_result collapse_huge_page(struct
> mm_struct *mm, unsigned long s
>         if (is_pmd_order(order)) {
>                 anon_vma_unlock_write(vma->anon_vma);
>                 anon_vma_locked = false;
> +               pte_unmap(pte);
> +               pte = NULL;
>         }
>
>         result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
>
> But this can also be handled later.

Yup I mean it'd be nicer to do it in one place if we can (+ impact of holding
RCU lock longer not an issue), but all this code needs rewokr anyway.

>
> We now hold an anon_vma lock a bit longer for !pmd-collapse. But there is also
> less to copy. If that bites us, we can try optimizing later.

Yeah I do worry about holding these locks longer. But we'll see.

>
>
> So after another skim, I think this patch is ready for primetime. We can address
> the things mentioned above later ... and any fallout can be fixed later, if any.
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Yes, also from my side - after a git range-diff and looking into above, LGTM,
so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

Now to go look at the core algo patch :)

>
>
> --
> Cheers,
>
> David

Cheers, Lorenzo

Re: [PATCH mm-unstable v19 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[Lorenzo Stoakes]

On Fri, Jun 05, 2026 at 07:49:34PM +0200, David Hildenbrand (Arm) wrote:
> On 6/5/26 18:14, Nico Pache wrote:
> > There are cases where, if an attempted collapse fails, all subsequent
> > orders are guaranteed to also fail. Avoid these collapse attempts by
> > bailing out early.
> >
> > Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
> > Acked-by: Usama Arif <usama.arif@linux.dev>
> > Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >  mm/khugepaged.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 430047316f43..7de92b28dd30 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1499,6 +1499,7 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
> >  			collapse_address = address + offset * PAGE_SIZE;
> >  			ret = collapse_huge_page(mm, collapse_address, referenced,
> >  						 unmapped, cc, order);
> > +
>
> Unrelated to this patch, but not the end of the world :)

NA....h that's fine ;)

>
> --
> Cheers,
>
> David

Cheers, Lorenzo

Re: [PATCH mm-unstable v19 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[Lorenzo Stoakes]

On Fri, Jun 05, 2026 at 10:14:13AM -0600, Nico Pache wrote:
> Pass an order 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.
>
> For non-PMD collapse we must leave the anon VMA write locked until after
> we collapse the mTHP-- in the PMD case all the pages are isolated, but in
> the mTHP case this is not true, and we must keep the lock to prevent
> access/changes to the page tables. This can happen if the rmap walkers hit
> a pmd_none while the PMD entry is currently unavailable due to being
> temporarily removed during the collapse phase.
>
> To properly establish the page table hierarchy without violating any
> expectations from certain architectures (e.g. MIPS), we must make sure to
> have the PMD reinstalled before the PTEs, and hold both PTE/PMD locks
> before calling update_mmu_cache_range() (if they are distinct locks).
>
> Signed-off-by: Nico Pache <npache@redhat.com>

In case Andrew missing my tag in the lengthily reply in [0]:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

:)

(Though probably he has a script that does this for him!)

[0]:https://lore.kernel.org/linux-mm/aiMNT7fBUkZS1EJK@lucifer/

Cheers, Lorenzo

> ---
>  mm/khugepaged.c | 105 ++++++++++++++++++++++++++++++------------------
>  1 file changed, 67 insertions(+), 38 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index e4b2ca77ecf6..c2769d82a719 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1228,34 +1228,36 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
>   * while allocating a THP, as that could trigger direct reclaim/compaction.
>   * Note that the VMA must be rechecked after grabbing the mmap_lock again.
>   */
> -static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
> -		int referenced, int unmapped, struct collapse_control *cc)
> +static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
> +		int referenced, int unmapped, struct collapse_control *cc,
> +		unsigned int order)
>  {
> +	const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK;
> +	const unsigned long end_addr = start_addr + (PAGE_SIZE << order);
>  	LIST_HEAD(compound_pagelist);
>  	pmd_t *pmd, _pmd;
> -	pte_t *pte;
> +	pte_t *pte = NULL;
>  	pgtable_t pgtable;
>  	struct folio *folio;
>  	spinlock_t *pmd_ptl, *pte_ptl;
>  	enum scan_result result = SCAN_FAIL;
>  	struct vm_area_struct *vma;
>  	struct mmu_notifier_range range;
> +	bool anon_vma_locked = false;
>
> -	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> -
> -	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
> +	result = alloc_charge_folio(&folio, mm, cc, order);
>  	if (result != SCAN_SUCCEED)
>  		goto out_nolock;
>
>  	mmap_read_lock(mm);
> -	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
> -					 HPAGE_PMD_ORDER);
> +	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
> +					 &vma, cc, order);
>  	if (result != SCAN_SUCCEED) {
>  		mmap_read_unlock(mm);
>  		goto out_nolock;
>  	}
>
> -	result = find_pmd_or_thp_or_none(mm, address, &pmd);
> +	result = find_pmd_or_thp_or_none(mm, pmd_addr, &pmd);
>  	if (result != SCAN_SUCCEED) {
>  		mmap_read_unlock(mm);
>  		goto out_nolock;
> @@ -1267,8 +1269,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  		 * released when it fails. So we jump out_nolock directly in
>  		 * that case.  Continuing to collapse causes inconsistency.
>  		 */
> -		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> -						     referenced, HPAGE_PMD_ORDER);
> +		result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd,
> +						     referenced, order);
>  		if (result != SCAN_SUCCEED)
>  			goto out_nolock;
>  	}
> @@ -1283,20 +1285,28 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  	 * mmap_lock.
>  	 */
>  	mmap_write_lock(mm);
> -	result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
> -					 HPAGE_PMD_ORDER);
> +	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
> +					 &vma, cc, order);
>  	if (result != SCAN_SUCCEED)
>  		goto out_up_write;
>  	/* check if the pmd is still valid */
>  	vma_start_write(vma);
> -	result = check_pmd_still_valid(mm, address, pmd);
> +	result = check_pmd_still_valid(mm, pmd_addr, pmd);
>  	if (result != SCAN_SUCCEED)
>  		goto out_up_write;
>
>  	anon_vma_lock_write(vma->anon_vma);
> +	anon_vma_locked = true;
>
> -	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
> -				address + HPAGE_PMD_SIZE);
> +	/*
> +	 * Only notify about the PTE range we will actually modify. While we
> +	 * temporary unmap the whole PTE table for mTHP collapse, we'll remap
> +	 * it later, leaving other PTEs effectively unmodified. The locks we
> +	 * hold prevent anybody from stumbling over such temporarily unmapped
> +	 * PTE tables.
> +	 */
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, start_addr,
> +				end_addr);
>  	mmu_notifier_invalidate_range_start(&range);
>
>  	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> @@ -1308,26 +1318,23 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  	 * Parallel GUP-fast is fine since GUP-fast will back off when
>  	 * it detects PMD is changed.
>  	 */
> -	_pmd = pmdp_collapse_flush(vma, address, pmd);
> +	_pmd = pmdp_collapse_flush(vma, pmd_addr, pmd);
>  	spin_unlock(pmd_ptl);
>  	mmu_notifier_invalidate_range_end(&range);
>  	tlb_remove_table_sync_one();
>
> -	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> +	pte = pte_offset_map_lock(mm, &_pmd, start_addr, &pte_ptl);
>  	if (pte) {
> -		result = __collapse_huge_page_isolate(vma, address, pte, cc,
> -						      HPAGE_PMD_ORDER,
> -						      &compound_pagelist);
> +		result = __collapse_huge_page_isolate(vma, start_addr, pte, cc,
> +						      order, &compound_pagelist);
>  		spin_unlock(pte_ptl);
>  	} else {
>  		result = SCAN_NO_PTE_TABLE;
>  	}
>
>  	if (unlikely(result != SCAN_SUCCEED)) {
> -		if (pte)
> -			pte_unmap(pte);
>  		spin_lock(pmd_ptl);
> -		BUG_ON(!pmd_none(*pmd));
> +		VM_WARN_ON_ONCE(!pmd_none(*pmd));
>  		/*
>  		 * We can only use set_pmd_at when establishing
>  		 * hugepmds and never for establishing regular pmds that
> @@ -1335,21 +1342,24 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  		 */
>  		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
>  		spin_unlock(pmd_ptl);
> -		anon_vma_unlock_write(vma->anon_vma);
>  		goto out_up_write;
>  	}
>
>  	/*
> -	 * All pages are isolated and locked so anon_vma rmap
> -	 * can't run anymore.
> +	 * For PMD collapse all pages are isolated and locked so anon_vma
> +	 * rmap can't run anymore. For mTHP collapse the PMD entry has been
> +	 * removed and not all pages are isolated and locked, so we must hold
> +	 * the lock to prevent neighboring folios from attempting to access
> +	 * this PMD until its reinstalled.
>  	 */
> -	anon_vma_unlock_write(vma->anon_vma);
> +	if (is_pmd_order(order)) {
> +		anon_vma_unlock_write(vma->anon_vma);
> +		anon_vma_locked = false;
> +	}
>
>  	result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> -					   vma, address, pte_ptl,
> -					   HPAGE_PMD_ORDER,
> -					   &compound_pagelist);
> -	pte_unmap(pte);
> +					   vma, start_addr, pte_ptl,
> +					   order, &compound_pagelist);
>  	if (unlikely(result != SCAN_SUCCEED))
>  		goto out_up_write;
>
> @@ -1359,18 +1369,37 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>  	 * write.
>  	 */
>  	__folio_mark_uptodate(folio);
> -	pgtable = pmd_pgtable(_pmd);
> -
>  	spin_lock(pmd_ptl);
> -	BUG_ON(!pmd_none(*pmd));
> -	pgtable_trans_huge_deposit(mm, pmd, pgtable);
> -	map_anon_folio_pmd_nopf(folio, pmd, vma, address);
> +	VM_WARN_ON_ONCE(!pmd_none(*pmd));
> +	if (is_pmd_order(order)) {
> +		pgtable = pmd_pgtable(_pmd);
> +		pgtable_trans_huge_deposit(mm, pmd, pgtable);
> +		map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
> +	} else {
> +		/*
> +		 * Some architectures (e.g. MIPS) walk the live page table in
> +		 * their implementation. update_mmu_cache_range() must be called
> +		 * with a valid page table hierarchy and the PTE lock held.
> +		 * Acquire it nested inside pmd_ptl when they are distinct locks.
> +		 */
> +		if (pte_ptl != pmd_ptl)
> +			spin_lock_nested(pte_ptl, SINGLE_DEPTH_NESTING);
> +		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> +		map_anon_folio_pte_nopf(folio, pte, vma, start_addr,
> +					  /*uffd_wp=*/ false);
> +		if (pte_ptl != pmd_ptl)
> +			spin_unlock(pte_ptl);
> +	}
>  	spin_unlock(pmd_ptl);
>
>  	folio = NULL;
>
>  	result = SCAN_SUCCEED;
>  out_up_write:
> +	if (anon_vma_locked)
> +		anon_vma_unlock_write(vma->anon_vma);
> +	if (pte)
> +		pte_unmap(pte);
>  	mmap_write_unlock(mm);
>  out_nolock:
>  	if (folio)
> @@ -1550,7 +1579,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  		/* collapse_huge_page expects the lock to be dropped before calling */
>  		mmap_read_unlock(mm);
>  		result = collapse_huge_page(mm, start_addr, referenced,
> -					    unmapped, cc);
> +					    unmapped, cc, HPAGE_PMD_ORDER);
>  		/* collapse_huge_page will return with the mmap_lock released */
>  		*lock_dropped = true;
>  	}
> --
> 2.54.0
>

Re: [PATCH mm-unstable v19 14/14] Documentation: mm: update the admin guide for mTHP collapse

[Lorenzo Stoakes]

On Fri, Jun 05, 2026 at 10:14:21AM -0600, 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: Lorenzo Stoakes <ljs@kernel.org>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
> Signed-off-by: Nico Pache <npache@redhat.com>

This is completely fine, and no blockers, but just a couple tiny things
below Claude brought up for a possible trivial follow up.

> ---
>  Documentation/admin-guide/mm/transhuge.rst | 49 ++++++++++++++--------
>  1 file changed, 32 insertions(+), 17 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index b98e18c80185..23f8d13c2629 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")

Claude was very pedantic and said we need a full stop here :P

This is not a blocker, obviously...!

>
>  process THP controls
> @@ -265,8 +266,8 @@ 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
> +   khugepaged currently only searches for opportunities to collapse file/shmem
> +   to PMD-sized THP. Only anonymous memory will attempt to collapse to other THP
>     sizes.
>
>  khugepaged runs usually at low frequency so while one may not want to
> @@ -296,11 +297,11 @@ 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)::

So Claude said maybe it's worth mentioning that the per-mTHP counters are only
actually exposed through
/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats/ and maybe worth
mentioning here too?

>
>  	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
>
> @@ -308,16 +309,21 @@ 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. At
> +HPAGE_PMD_NR - 1, we collapse to the highest possible order. Any intermediate
> +value will emit a warning and mTHP collapse will default to max_ptes_none=0.
> +
> +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 +343,15 @@ that THP is shared. Exceeding the number would block the collapse::
>
>  A higher value may increase memory footprint for some workloads.
>
> +.. note::
> +   For mTHP collapse, khugepaged does not support collapsing regions that
> +   contain shared or swapped out pages, as this could lead to continuous
> +   promotion to higher orders. The collapse will fail if any shared or
> +   swapped PTEs are encountered during the scan.
> +
> +   Currently, madvise_collapse only supports collapsing to PMD-sized THPs
> +   and does not attempt mTHP collapses.
> +
>  Boot parameters
>  ===============
>
> --
> 2.54.0
>

Cheers, Lorenzo