[PATCH v3 0/7] mm/khugepaged: several cleanups

[PATCH v3 0/7] mm/khugepaged: several cleanups

[Nico Pache (Red Hat)]

The following changes stem from a number of reviews during my khugepaged
mTHP support series [1]. Some of these are minor code cleanups, issues or
reviews that we decided to deferred to a followup series, or in the case
of the more major patch of the series, changes [2] Lance Yang attempted
while my series was in-flight and we decided to wait till later to try.

The first 3 patches introduce helper functions to increase code reuse and
readability. This includes a per-scan state clearing function, extracting
the young page check into a helper, and a count_collapse_event() function
to reduce a repetative pattern used across mTHP collapse.

The 4th patch was the byproduct of me throwing Claude at all the
comments in khugepaged verifying and looking for any outdated info.

The 5th patch is based on Lance Yang's commit series [2] trying to extract
the PTE state checking into a helper function. This required a bit of
rewriting due to differences after mTHP collapse was introduced. I also
took into account the changes requested during his patches review cycle.

The remaining 2 patches were review points during my mTHP series that we
agreed can be deferred to a later series.

Thank you to those whos reviews and work I leveraged to achieve these
cleanups.

V3 Changes:
 - Add Acks/RB tags
 - rename collapse_is_referenced() to pte_is_referenced()
 - fix count_collapse_event() call-site indentation to match kernel style
 - fold in the follow-up fix so collapse_scan_pmd() always assigns the
   local folio from collapse_check_pte()
 - normalize khugepaged comment references to use parentheses

V2 Changes:
 - Add Acks/RB tags
 - rename collapse_is_young() to collapse_is_referenced()
 - delete unncessary comment for collapse_control_init_scan()
 - merge fixup from V1 (fixes stale folio reference) into patch 5
 - keep original ordering for checks in patch 5
 - merge patch 7 into patch 4
 - conflict resolution from uffd_rwp changes

V2: https://lore.kernel.org/all/20260715025941.1571316-1-npache@redhat.com/
V1: https://lore.kernel.org/all/20260706154500.39178-1-npache@redhat.com/

[1] - https://lore.kernel.org/all/20260605161422.213817-1-npache@redhat.com/
[2] - https://lore.kernel.org/all/20251008043748.45554-1-lance.yang@linux.dev/

Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
Nico Pache (Red Hat) (7):
      mm/khugepaged: refactor per-scan state clearing into collapse_control_init_scan()
      mm/khugepaged: extract young page check into pte_is_referenced() helper
      mm/khugepaged: introduce a count_collapse_event() helper
      mm/khugepaged: fix outdated comments
      mm/khugepaged: Refactor the PTE state checks into a helper
      mm/khugepaged: unmap pte before releasing vma write lock
      mm: Documentation: clarify where the mTHP stats live

 Documentation/admin-guide/mm/transhuge.rst |   6 +-
 mm/khugepaged.c                            | 417 +++++++++++++++--------------
 2 files changed, 223 insertions(+), 200 deletions(-)
---
base-commit: 486855239e38a91d3eae26618ccb7845c1764625
change-id: 20260804-khugepaged_pte_refactor-5eb76e14b93a

Best regards,
--  
Nico Pache (Red Hat) <nico.pache@linux.dev>

[PATCH v3 1/7] mm/khugepaged: refactor per-scan state clearing into collapse_control_init_scan()

[Nico Pache (Red Hat)]

Extract the repeated clearing of node_load, alloc_nmask, and
mthp_present_ptes into a helper to reduce duplication in
collapse_scan_pmd() and collapse_scan_file(). Althought file scans do not
current use the bitmap, they will in the future, and clearing it now is
harmless.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 mm/khugepaged.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b237f6e7662a..1e26ea97381a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -629,6 +629,13 @@ void __khugepaged_exit(struct mm_struct *mm)
 	}
 }
 
+static void collapse_control_init_scan(struct collapse_control *cc)
+{
+	memset(cc->node_load, 0, sizeof(cc->node_load));
+	nodes_clear(cc->alloc_nmask);
+	bitmap_zero(cc->mthp_present_ptes, MAX_PTRS_PER_PTE);
+}
+
 static void release_pte_folio(struct folio *folio)
 {
 	node_stat_mod_folio(folio,
@@ -1617,9 +1624,7 @@ 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);
+	collapse_control_init_scan(cc);
 
 	enabled_orders = collapse_possible_orders(vma, vma->vm_flags, tva_flags);
 
@@ -2691,8 +2696,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 
 	present = 0;
 	swap = 0;
-	memset(cc->node_load, 0, sizeof(cc->node_load));
-	nodes_clear(cc->alloc_nmask);
+	collapse_control_init_scan(cc);
 	rcu_read_lock();
 	xas_for_each(&xas, folio, start + HPAGE_PMD_NR - 1) {
 		if (xas_retry(&xas, folio))

-- 
2.55.0

[PATCH v3 2/7] mm/khugepaged: extract young page check into pte_is_referenced() helper

[Nico Pache (Red Hat)]

This change deduplicates the "is this PTE/folio referenced enough to be
considered for a collapse" condition that was repeated in both
__collapse_huge_page_isolate() and collapse_scan_pmd(), extracting it into
a single inline helper function.

Also move the comment and use it as the function header. While we are at
it, updated the comment to clarify that a young pte is a recently accessed
one.

Acked-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 mm/khugepaged.c | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1e26ea97381a..9b15c1740943 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -672,6 +672,23 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
 	}
 }
 
+/*
+ * pte_is_referenced() - Check for enough referenced PTEs to justify collapsing
+ *
+ * If collapse was initiated by khugepaged, check that the page has been
+ * recently accessed (young pte) to justify collapsing the page.
+ *
+ * Return: true if the page has been recently accessed.
+ */
+static inline bool pte_is_referenced(struct collapse_control *cc, pte_t pteval,
+	struct folio *folio, struct vm_area_struct *vma, unsigned long addr)
+{
+	return cc->is_khugepaged &&
+	       (pte_young(pteval) || folio_test_young(folio) ||
+		folio_test_referenced(folio) ||
+		mmu_notifier_test_young(vma->vm_mm, addr));
+}
+
 static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
 		unsigned int order, struct list_head *compound_pagelist)
@@ -810,14 +827,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		if (folio_test_large(folio))
 			list_add_tail(&folio->lru, compound_pagelist);
 next:
-		/*
-		 * If collapse was initiated by khugepaged, check that there is
-		 * enough young pte to justify collapsing the page
-		 */
-		if (cc->is_khugepaged &&
-		    (pte_young(pteval) || folio_test_young(folio) ||
-		     folio_test_referenced(folio) ||
-		     mmu_notifier_test_young(vma->vm_mm, addr)))
+		if (pte_is_referenced(cc, pteval, folio, vma, addr))
 			referenced++;
 	}
 
@@ -1766,14 +1776,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 			goto out_unmap;
 		}
 
-		/*
-		 * If collapse was initiated by khugepaged, check that there is
-		 * enough young pte to justify collapsing the page
-		 */
-		if (cc->is_khugepaged &&
-		    (pte_young(pteval) || folio_test_young(folio) ||
-		     folio_test_referenced(folio) ||
-		     mmu_notifier_test_young(vma->vm_mm, addr)))
+		if (pte_is_referenced(cc, pteval, folio, vma, addr))
 			referenced++;
 	}
 	if (cc->is_khugepaged &&

-- 
2.55.0

[PATCH v3 3/7] mm/khugepaged: introduce a count_collapse_event() helper

[Nico Pache (Red Hat)]

Provide a simple helper function to help reduce a often used, and
duplicate pattern across the khugepaged code.

When collapsing to a PMD we need to record a vm_event and the mTHP_stat
event. When doing mTHP collapse we only update the mTHP stat.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 mm/khugepaged.c | 42 +++++++++++++++++++++---------------------
 1 file changed, 21 insertions(+), 21 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9b15c1740943..a2691587a510 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -689,6 +689,14 @@ static inline bool pte_is_referenced(struct collapse_control *cc, pte_t pteval,
 		mmu_notifier_test_young(vma->vm_mm, addr));
 }
 
+static void count_collapse_event(unsigned int order, enum vm_event_item vm_event,
+		enum mthp_stat_item mthp_event)
+{
+	if (is_pmd_order(order))
+		count_vm_event(vm_event);
+	count_mthp_stat(order, mthp_event);
+}
+
 static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
 		unsigned int order, struct list_head *compound_pagelist)
@@ -709,9 +717,8 @@ 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;
-				if (is_pmd_order(order))
-					count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
-				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_NONE);
+				count_collapse_event(order, THP_SCAN_EXCEED_NONE_PTE,
+						     MTHP_STAT_COLLAPSE_EXCEED_NONE);
 				goto out;
 			}
 			continue;
@@ -753,9 +760,8 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			 */
 			if (++shared > max_ptes_shared) {
 				result = 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);
+				count_collapse_event(order, THP_SCAN_EXCEED_SHARED_PTE,
+						     MTHP_STAT_COLLAPSE_EXCEED_SHARED);
 				goto out;
 			}
 		}
@@ -1264,15 +1270,12 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
 	folio = __folio_alloc(gfp, order, node, &cc->alloc_nmask);
 	if (!folio) {
 		*foliop = NULL;
-		if (is_pmd_order(order))
-			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
-		count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC_FAILED);
+		count_collapse_event(order, THP_COLLAPSE_ALLOC_FAILED,
+				     MTHP_STAT_COLLAPSE_ALLOC_FAILED);
 		return SCAN_ALLOC_HUGE_PAGE_FAIL;
 	}
 
-	if (is_pmd_order(order))
-		count_vm_event(THP_COLLAPSE_ALLOC);
-	count_mthp_stat(order, MTHP_STAT_COLLAPSE_ALLOC);
+	count_collapse_event(order, THP_COLLAPSE_ALLOC, MTHP_STAT_COLLAPSE_ALLOC);
 
 	if (unlikely(mem_cgroup_charge(folio, mm, gfp))) {
 		folio_put(folio);
@@ -1662,9 +1665,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		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);
-				count_mthp_stat(HPAGE_PMD_ORDER,
-						MTHP_STAT_COLLAPSE_EXCEED_NONE);
+				count_collapse_event(HPAGE_PMD_ORDER, THP_SCAN_EXCEED_NONE_PTE,
+						     MTHP_STAT_COLLAPSE_EXCEED_NONE);
 				goto out_unmap;
 			}
 			continue;
@@ -1672,9 +1674,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		if (!pte_present(pteval)) {
 			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);
+				count_collapse_event(HPAGE_PMD_ORDER, THP_SCAN_EXCEED_SWAP_PTE,
+						     MTHP_STAT_COLLAPSE_EXCEED_SWAP);
 				goto out_unmap;
 			}
 			/*
@@ -1731,9 +1732,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		if (folio_maybe_mapped_shared(folio)) {
 			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);
+				count_collapse_event(HPAGE_PMD_ORDER, THP_SCAN_EXCEED_SHARED_PTE,
+						     MTHP_STAT_COLLAPSE_EXCEED_SHARED);
 				goto out_unmap;
 			}
 		}

-- 
2.55.0

[PATCH v3 4/7] mm/khugepaged: fix outdated comments

[Nico Pache (Red Hat)]

Fix comment in collapse_scan_pmd() that still described the old
folio_mapcount() > folio_ref_count() check and a "512" false-positive
scenario. The code now uses folio_expected_ref_count() != folio_ref_count()
which doesn't suffer from the same limitation.

Fix comment in collapse_huge_page() that referenced ptep_clear_flush,
when the code actually uses pmdp_collapse_flush.

Fix comment in __collapse_huge_page_swapin() that referenced the old
function name khugepaged_scan_pmd, now collapse_scan_pmd.

Also clean up some simple typos and stale terminology (mmap_sem ->
mmap_lock, PG_lock -> folio lock, page -> folio, grammar).

We also clarify a comment regarding where the max_ptes_none check is
deferred to in mthp_collapse() from the original collapse_scan_pmd check.

Update all comments that references a function to include parentheses.

Acked-by: Usama Arif <usama.arif@linux.dev>
Assisted-by: Cursor(claude-sonnet-4):4.6
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 mm/khugepaged.c | 44 +++++++++++++++++++++-----------------------
 1 file changed, 21 insertions(+), 23 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index a2691587a510..dade6e68aa64 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -620,7 +620,7 @@ void __khugepaged_exit(struct mm_struct *mm)
 		/*
 		 * This is required to serialize against
 		 * collapse_test_exit() (which is guaranteed to run
-		 * under mmap sem read mode). Stop here (after we return all
+		 * under mmap_lock read mode). Stop here (after we return all
 		 * pagetables will be destroyed) until khugepaged has finished
 		 * working on the pagetables under the mmap_lock.
 		 */
@@ -789,8 +789,8 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		/*
 		 * We can do it before folio_isolate_lru because the
-		 * folio can't be freed from under us. NOTE: PG_lock
-		 * is needed to serialize against split_huge_page
+		 * folio can't be freed from under us. NOTE: folio lock
+		 * is needed to serialize against split_huge_page()
 		 * when invoked from the VM.
 		 */
 		if (!folio_trylock(folio)) {
@@ -816,7 +816,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		}
 
 		/*
-		 * Isolate the page to avoid collapsing an hugepage
+		 * Isolate the folio to avoid collapsing a hugepage
 		 * currently in use by the VM.
 		 */
 		if (!folio_isolate_lru(folio)) {
@@ -927,7 +927,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
 	 * Re-establish the PMD to point to the original page table
 	 * entry. Restoring PMD needs to be done prior to releasing
 	 * pages. Since pages are still isolated and locked here,
-	 * acquiring anon_vma_lock_write is unnecessary.
+	 * acquiring anon_vma_lock_write() is unnecessary.
 	 */
 	pmd_ptl = pmd_lock(vma->vm_mm, pmd);
 	pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
@@ -1101,9 +1101,9 @@ static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned l
 		return SCAN_VMA_CHECK;
 	/*
 	 * Anon VMA expected, the address may be unmapped then
-	 * remapped to file after khugepaged reaquired the mmap_lock.
+	 * remapped to file after khugepaged reacquired the mmap_lock.
 	 *
-	 * thp_vma_allowable_orders 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)))
@@ -1159,7 +1159,7 @@ 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.
+ * Only done if collapse_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
@@ -1227,7 +1227,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
 		pte = NULL;
 
 		/*
-		 * do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
+		 * do_swap_page() returns VM_FAULT_RETRY with released mmap_lock.
 		 * Note we treat VM_FAULT_RETRY as VM_FAULT_ERROR here because
 		 * we do not retry here and swap entry will remain in pagetable
 		 * resulting in later failure.
@@ -1291,7 +1291,7 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
 }
 
 /*
- * collapse_huge_page expects the mmap_lock to be unlocked before entering and
+ * 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.
@@ -1338,7 +1338,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 
 	if (unmapped) {
 		/*
-		 * __collapse_huge_page_swapin will return with mmap_lock
+		 * __collapse_huge_page_swapin() will return with mmap_lock
 		 * released when it fails. So we jump out_nolock directly in
 		 * that case.  Continuing to collapse causes inconsistency.
 		 */
@@ -1351,8 +1351,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 	mmap_read_unlock(mm);
 	/*
 	 * Prevent all access to pagetables with the exception of
-	 * gup_fast later handled by the ptep_clear_flush and the VM
-	 * handled by the anon_vma lock + PG_lock.
+	 * gup_fast later handled by the pmdp_collapse_flush() and the VM
+	 * handled by the anon_vma lock + folio lock.
 	 *
 	 * UFFDIO_MOVE is prevented to race as well thanks to the
 	 * mmap_lock.
@@ -1409,9 +1409,9 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 		spin_lock(pmd_ptl);
 		VM_WARN_ON_ONCE(!pmd_none(*pmd));
 		/*
-		 * We can only use set_pmd_at when establishing
+		 * We can only use set_pmd_at() when establishing
 		 * hugepmds and never for establishing regular pmds that
-		 * points to regular pagetables. Use pmd_populate for that
+		 * points to regular pagetables. Use pmd_populate() for that
 		 */
 		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
 		spin_unlock(pmd_ptl);
@@ -1643,7 +1643,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 
 	/*
 	 * If PMD is the only enabled order, enforce max_ptes_none, otherwise
-	 * scan all pages to populate the bitmap for mTHP collapse.
+	 * scan all pages to populate the bitmap for mTHP collapse. The bitmap
+	 * is then checked again in mthp_collapse() for each attempted order.
 	 */
 	if (enabled_orders != BIT(HPAGE_PMD_ORDER))
 		max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
@@ -1764,12 +1765,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 		/*
 		 * Check if the page has any GUP (or other external) pins.
 		 *
-		 * Here the check may be racy:
-		 * it may see folio_mapcount() > folio_ref_count().
-		 * But such case is ephemeral we could always retry collapse
-		 * later.  However it may report false positive if the page
-		 * has excessive GUP pins (i.e. 512).  Anyway the same check
-		 * will be done again later the risk seems low.
+		 * Here the check is racy, but such case is ephemeral and
+		 * we could always retry collapse later. Anyway the same
+		 * check will be done again later the risk seems low.
 		 */
 		if (folio_expected_ref_count(folio) != folio_ref_count(folio)) {
 			result = SCAN_PAGE_COUNT;
@@ -1789,7 +1787,7 @@ 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 */
+		/* collapse_huge_page() expects the lock to be dropped before calling */
 		mmap_read_unlock(mm);
 		result = mthp_collapse(mm, start_addr, referenced,
 				       unmapped, cc, enabled_orders);

-- 
2.55.0

[PATCH v3 5/7] mm/khugepaged: Refactor the PTE state checks into a helper

[Nico Pache (Red Hat)]

For anonymous collapse, the collapse_scan_pmd() and
__collapse_huge_page_isolate() functions share a large portion of their
logic. These functions both check the state of the PTEs and verify the
following:
	- max_pte_* values are not exceeded
	- uffd is not active
	- lazyfree properties
	- non-anonymous

Merge these checks into a helper collapse_check_pte() to reduce code
duplication. We also add a helper struct for this function called
pte_check_context which allows us to pass the required parameters in a
clean and elegant manner.

A helper function is also introduced pte_check_fail() to provide a clean
interface to set the pte_check_context failure results and return
PTE_CHECK_FAIL state. This helps reduce code duplications across the new
collapse_check_pte function.

Two slight modifications are done to the original functionality. We now
warn (instead of crash) if the anon test fails, and we leverage the
vm_normal_folio function instead of page->folio, this should be
functionally equivalent.

No other functional changes intended.

This patch is heavily based off work done by Lance Yang, but modified to
deal with conflicts and feedback received during the review cycle [1].

[1] https://lore.kernel.org/linux-mm/20251008043748.45554-1-lance.yang@linux.dev/
Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 mm/khugepaged.c | 298 +++++++++++++++++++++++++++++---------------------------
 1 file changed, 157 insertions(+), 141 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index dade6e68aa64..068368d27c2f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -65,6 +65,12 @@ enum scan_result {
 	SCAN_PAGE_DIRTY_OR_WRITEBACK,
 };
 
+enum pte_check_result {
+	PTE_CHECK_SUCCEED,
+	PTE_CHECK_FAIL,
+	PTE_CHECK_CONTINUE,
+};
+
 #define CREATE_TRACE_POINTS
 #include <trace/events/huge_memory.h>
 
@@ -119,6 +125,20 @@ struct collapse_control {
 	DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE);
 };
 
+struct pte_check_context {
+	struct collapse_control *cc;
+	struct vm_area_struct *vma;
+	unsigned int order;
+	struct folio *folio;
+	int none_or_zero;
+	int shared;
+	int unmapped;
+	enum scan_result result;
+	unsigned int max_ptes_none;
+	unsigned int max_ptes_swap;
+	unsigned int max_ptes_shared;
+};
+
 /**
  * struct khugepaged_scan - cursor for scanning
  * @mm_head: the head of the mm list to scan
@@ -697,74 +717,131 @@ static void count_collapse_event(unsigned int order, enum vm_event_item vm_event
 	count_mthp_stat(order, mthp_event);
 }
 
+/*
+ * pte_check_fail() - A simple helper to set the pte_check_context result and
+ * return PTE_CHECK_FAIL.
+ */
+static enum pte_check_result pte_check_fail(struct pte_check_context *ctx,
+		enum scan_result result)
+{
+	ctx->result = result;
+	return PTE_CHECK_FAIL;
+}
+
+/*
+ * collapse_check_pte() - Check if a PTE is suitable for collapse
+ *
+ * Check if a PTE is suitable for collapse based on the following criteria:
+ * - max_pte_* values are not exceeded
+ * - uffd is not active
+ * - lazyfree properties are not present
+ * - only anonymous pages are present
+ *
+ * a helper struct pte_check_context is used to pass and store relevant
+ * information between the collapse_check_pte() function and the caller.
+ *
+ * Return: PTE_CHECK_SUCCEED if the PTE is suitable for collapse,
+ *         PTE_CHECK_FAIL if the PTE is not suitable for collapse,
+ *         PTE_CHECK_CONTINUE if the scan should continue to check the next PTE.
+ */
+static enum pte_check_result collapse_check_pte(pte_t pteval,
+		unsigned long addr, struct pte_check_context *ctx)
+{
+	if (pte_none_or_zero(pteval)) {
+		if (++ctx->none_or_zero > ctx->max_ptes_none) {
+			count_collapse_event(ctx->order, THP_SCAN_EXCEED_NONE_PTE,
+					     MTHP_STAT_COLLAPSE_EXCEED_NONE);
+			return pte_check_fail(ctx, SCAN_EXCEED_NONE_PTE);
+		}
+		return PTE_CHECK_CONTINUE;
+	}
+	if (!pte_present(pteval)) {
+		if (ctx->unmapped == -1)
+			return pte_check_fail(ctx, SCAN_PTE_NON_PRESENT);
+		if (++ctx->unmapped > ctx->max_ptes_swap) {
+			count_collapse_event(ctx->order, THP_SCAN_EXCEED_SWAP_PTE,
+					     MTHP_STAT_COLLAPSE_EXCEED_SWAP);
+			return pte_check_fail(ctx, SCAN_EXCEED_SWAP_PTE);
+		}
+		if (pte_swp_uffd_any(pteval))
+			return pte_check_fail(ctx, SCAN_PTE_UFFD);
+		return PTE_CHECK_CONTINUE;
+	}
+	/*
+	 * Don't collapse if any of the small PTEs are armed with uffd
+	 * write protection. Marking the new huge pmd as write protected
+	 * could bring userfault messages that fall outside of the
+	 * registered range.
+	 */
+	if (pte_uffd(pteval))
+		return pte_check_fail(ctx, SCAN_PTE_UFFD);
+
+	ctx->folio = vm_normal_folio(ctx->vma, addr, pteval);
+	if (unlikely(!ctx->folio) || unlikely(folio_is_zone_device(ctx->folio)))
+		return pte_check_fail(ctx, SCAN_PAGE_NULL);
+
+	/*
+	 * If the vma has the VM_DROPPABLE flag, the collapse will
+	 * preserve the lazyfree property without needing to skip.
+	 */
+	if (ctx->cc->is_khugepaged && !(ctx->vma->vm_flags & VM_DROPPABLE) &&
+	    folio_test_lazyfree(ctx->folio) && !pte_dirty(pteval))
+		return pte_check_fail(ctx, SCAN_PAGE_LAZYFREE);
+
+	if (!folio_test_anon(ctx->folio)) {
+		VM_WARN_ON_FOLIO(!folio_test_anon(ctx->folio), ctx->folio);
+		return pte_check_fail(ctx, SCAN_PAGE_ANON);
+	}
+
+	if (folio_maybe_mapped_shared(ctx->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 (++ctx->shared > ctx->max_ptes_shared) {
+			count_collapse_event(ctx->order, THP_SCAN_EXCEED_SHARED_PTE,
+					     MTHP_STAT_COLLAPSE_EXCEED_SHARED);
+			return pte_check_fail(ctx, SCAN_EXCEED_SHARED_PTE);
+		}
+	}
+
+	return PTE_CHECK_SUCCEED;
+}
+
 static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
 		unsigned int order, struct list_head *compound_pagelist)
 {
-	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;
-	pte_t *_pte;
-	int none_or_zero = 0, shared = 0, referenced = 0;
+	pte_t *_pte, pteval;
+	int referenced = 0;
 	enum scan_result result = SCAN_FAIL;
+	enum pte_check_result pte_check;
+	struct pte_check_context ctx = {
+		.cc = cc,
+		.vma = vma,
+		.order = order,
+		.unmapped = -1, /* don't check swap PTEs */
+		.max_ptes_none = collapse_max_ptes_none(cc, vma, order),
+		.max_ptes_shared = collapse_max_ptes_shared(cc, order),
+	};
 
 	for (_pte = pte; _pte < pte + nr_pages;
 	     _pte++, addr += PAGE_SIZE) {
-		pte_t pteval = ptep_get(_pte);
-		if (pte_none_or_zero(pteval)) {
-			if (++none_or_zero > max_ptes_none) {
-				result = SCAN_EXCEED_NONE_PTE;
-				count_collapse_event(order, THP_SCAN_EXCEED_NONE_PTE,
-						     MTHP_STAT_COLLAPSE_EXCEED_NONE);
-				goto out;
-			}
-			continue;
-		}
-		if (!pte_present(pteval)) {
-			result = SCAN_PTE_NON_PRESENT;
-			goto out;
-		}
-		if (pte_uffd(pteval)) {
-			result = SCAN_PTE_UFFD;
-			goto out;
-		}
-		page = vm_normal_page(vma, addr, pteval);
-		if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
-			result = SCAN_PAGE_NULL;
-			goto out;
-		}
-
-		folio = page_folio(page);
-		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
-
-		/*
-		 * If the vma has the VM_DROPPABLE flag, the collapse will
-		 * preserve the lazyfree property without needing to skip.
-		 */
-		if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
-		    folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
-			result = SCAN_PAGE_LAZYFREE;
+		pteval = ptep_get(_pte);
+		pte_check = collapse_check_pte(pteval, addr, &ctx);
+		folio = ctx.folio;
+		if (pte_check == PTE_CHECK_FAIL) {
+			result = ctx.result;
 			goto out;
 		}
+		if (pte_check == PTE_CHECK_CONTINUE)
+			continue;
 
-		/* 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_collapse_event(order, THP_SCAN_EXCEED_SHARED_PTE,
-						     MTHP_STAT_COLLAPSE_EXCEED_SHARED);
-				goto out;
-			}
-		}
 		/*
 		 * TODO: In some cases of partially-mapped folios, we'd actually
 		 * want to collapse.
@@ -841,13 +918,13 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		result = SCAN_LACK_REFERENCED_PAGE;
 	} else {
 		result = SCAN_SUCCEED;
-		trace_mm_collapse_huge_page_isolate(folio, none_or_zero,
+		trace_mm_collapse_huge_page_isolate(folio, ctx.none_or_zero,
 						    referenced, result, order);
 		return result;
 	}
 out:
 	release_pte_pages(pte, _pte, compound_pagelist);
-	trace_mm_collapse_huge_page_isolate(folio, none_or_zero,
+	trace_mm_collapse_huge_page_isolate(folio, ctx.none_or_zero,
 					    referenced, result, order);
 	return result;
 }
@@ -1459,7 +1536,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 			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);
+					  /*uffd=*/ false);
 		if (pte_ptl != pmd_ptl)
 			spin_unlock(pte_ptl);
 	}
@@ -1613,24 +1690,30 @@ 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_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, 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;
+	int referenced = 0;
+	enum scan_result result = SCAN_FAIL;
 	unsigned long addr;
 	unsigned long enabled_orders;
 	spinlock_t *ptl;
-	int node = NUMA_NO_NODE, unmapped = 0;
+	int node = NUMA_NO_NODE;
+	enum pte_check_result pte_check;
 
 	VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
 
+	struct pte_check_context ctx = {
+		.cc = cc,
+		.vma = vma,
+		.order = HPAGE_PMD_ORDER,
+		.max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER),
+		.max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER),
+		.max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER),
+	};
+
 	result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
 	if (result != SCAN_SUCCEED) {
 		cc->progress++;
@@ -1647,7 +1730,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 	 * is then checked again in mthp_collapse() for each attempted order.
 	 */
 	if (enabled_orders != BIT(HPAGE_PMD_ORDER))
-		max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
+		ctx.max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
 
 	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
 	if (!pte) {
@@ -1663,81 +1746,14 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 
 		cc->progress++;
 
-		if (pte_none_or_zero(pteval)) {
-			if (++none_or_zero > max_ptes_none) {
-				result = SCAN_EXCEED_NONE_PTE;
-				count_collapse_event(HPAGE_PMD_ORDER, THP_SCAN_EXCEED_NONE_PTE,
-						     MTHP_STAT_COLLAPSE_EXCEED_NONE);
-				goto out_unmap;
-			}
-			continue;
-		}
-		if (!pte_present(pteval)) {
-			if (++unmapped > max_ptes_swap) {
-				result = SCAN_EXCEED_SWAP_PTE;
-				count_collapse_event(HPAGE_PMD_ORDER, THP_SCAN_EXCEED_SWAP_PTE,
-						     MTHP_STAT_COLLAPSE_EXCEED_SWAP);
-				goto out_unmap;
-			}
-			/*
-			 * Always be strict with uffd-wp
-			 * enabled swap entries.  Please see
-			 * comment below for pte_uffd().
-			 */
-			if (pte_swp_uffd_any(pteval)) {
-				result = SCAN_PTE_UFFD;
-				goto out_unmap;
-			}
-			continue;
-		}
-		if (pte_uffd(pteval)) {
-			/*
-			 * Don't collapse the page if any of the small
-			 * PTEs are armed with uffd write protection.
-			 * Here we can also mark the new huge pmd as
-			 * write protected if any of the small ones is
-			 * marked but that could bring unknown
-			 * userfault messages that falls outside of
-			 * the registered range.  So, just be simple.
-			 */
-			result = SCAN_PTE_UFFD;
-			goto out_unmap;
-		}
-
-		page = vm_normal_page(vma, addr, pteval);
-		if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
-			result = SCAN_PAGE_NULL;
-			goto out_unmap;
-		}
-		folio = page_folio(page);
-
-		/*
-		 * If the vma has the VM_DROPPABLE flag, the collapse will
-		 * preserve the lazyfree property without needing to skip.
-		 */
-		if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
-		    folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
-			result = SCAN_PAGE_LAZYFREE;
-			goto out_unmap;
-		}
-
-		if (!folio_test_anon(folio)) {
-			result = SCAN_PAGE_ANON;
+		pte_check = collapse_check_pte(pteval, addr, &ctx);
+		folio = ctx.folio;
+		if (pte_check == PTE_CHECK_FAIL) {
+			result = ctx.result;
 			goto out_unmap;
 		}
-
-		/*
-		 * We treat a single page as shared if any part of the THP
-		 * is shared.
-		 */
-		if (folio_maybe_mapped_shared(folio)) {
-			if (++shared > max_ptes_shared) {
-				result = SCAN_EXCEED_SHARED_PTE;
-				count_collapse_event(HPAGE_PMD_ORDER, THP_SCAN_EXCEED_SHARED_PTE,
-						     MTHP_STAT_COLLAPSE_EXCEED_SHARED);
-				goto out_unmap;
-			}
-		}
+		if (pte_check == PTE_CHECK_CONTINUE)
+			continue;
 
 		/* Set bit for occupied pages */
 		__set_bit(i, cc->mthp_present_ptes);
@@ -1779,7 +1795,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 	}
 	if (cc->is_khugepaged &&
 		   (!referenced ||
-		    (unmapped && referenced < HPAGE_PMD_NR / 2))) {
+		    (ctx.unmapped && referenced < HPAGE_PMD_NR / 2))) {
 		result = SCAN_LACK_REFERENCED_PAGE;
 	} else {
 		result = SCAN_SUCCEED;
@@ -1790,13 +1806,13 @@ 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 = mthp_collapse(mm, start_addr, referenced,
-				       unmapped, cc, enabled_orders);
+				       ctx.unmapped, cc, enabled_orders);
 		/* mmap_lock was released above, set lock_dropped */
 		*lock_dropped = true;
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
-				     none_or_zero, result, unmapped);
+				     ctx.none_or_zero, result, ctx.unmapped);
 	return result;
 }
 

-- 
2.55.0

[PATCH v3 6/7] mm/khugepaged: unmap pte before releasing vma write lock

[Nico Pache (Red Hat)]

We are currently dropping the anon_vma write lock before unmapping the
PTE. Although this is safe, due to us still holding the mmap_write_lock,
its safer and less confusing to switch the order of these two operations.

Suggested-by: David Hildenbrand <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 mm/khugepaged.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 068368d27c2f..3338e9dc11dd 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1546,10 +1546,10 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
 
 	result = SCAN_SUCCEED;
 out_up_write:
-	if (anon_vma_locked)
-		anon_vma_unlock_write(vma->anon_vma);
 	if (pte)
 		pte_unmap(pte);
+	if (anon_vma_locked)
+		anon_vma_unlock_write(vma->anon_vma);
 	mmap_write_unlock(mm);
 out_nolock:
 	if (folio)

-- 
2.55.0

[PATCH v3 7/7] mm: Documentation: clarify where the mTHP stats live

[Nico Pache (Red Hat)]

The note about khugepaged counters references /proc/vmstat for the PMD
case, but never mentions where the mTHPs stats can be found
(i.e.: /sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats/)
Add a small addition to this section for clarity.

Also fix a missing period while we are at it.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Suggested-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
---
 Documentation/admin-guide/mm/transhuge.rst | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 16f37135ed80..b187d618452f 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -224,7 +224,7 @@ 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
 all THP sizes are disabled (when both the per-size anon control and the
-top-level control are "never")
+top-level control are "never").
 
 process THP controls
 --------------------
@@ -301,7 +301,9 @@ 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)::
+and counters in /proc/vmstat consulted for more accurate accounting.
+Per-order mTHP collapse statistics are also available under
+/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats/)::
 
 	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
 

-- 
2.55.0

Re: [PATCH v3 0/7] mm/khugepaged: several cleanups

[Andrew Morton]

On Tue, 04 Aug 2026 13:24:58 -0600 "Nico Pache (Red Hat)" <nico.pache@linux.dev> wrote:

> The following changes stem from a number of reviews during my khugepaged
> mTHP support series [1]. Some of these are minor code cleanups, issues or
> reviews that we decided to deferred to a followup series, or in the case
> of the more major patch of the series, changes [2] Lance Yang attempted
> while my series was in-flight and we decided to wait till later to try.
> 
> The first 3 patches introduce helper functions to increase code reuse and
> readability. This includes a per-scan state clearing function, extracting
> the young page check into a helper, and a count_collapse_event() function
> to reduce a repetative pattern used across mTHP collapse.
> 
> The 4th patch was the byproduct of me throwing Claude at all the
> comments in khugepaged verifying and looking for any outdated info.
> 
> The 5th patch is based on Lance Yang's commit series [2] trying to extract
> the PTE state checking into a helper function. This required a bit of
> rewriting due to differences after mTHP collapse was introduced. I also
> took into account the changes requested during his patches review cycle.
> 
> The remaining 2 patches were review points during my mTHP series that we
> agreed can be deferred to a later series.

So "no functional change intended"?

Sashiko had a little doc niggle:
	https://sashiko.dev/#/patchset/20260804-khugepaged_pte_refactor-v3-0-0364cad642a0@linux.dev

Re: [PATCH v3 2/7] mm/khugepaged: extract young page check into pte_is_referenced() helper

[David Hildenbrand (Arm)]

On 8/4/26 21:25, Nico Pache (Red Hat) wrote:
> This change deduplicates the "is this PTE/folio referenced enough to be
> considered for a collapse" condition that was repeated in both
> __collapse_huge_page_isolate() and collapse_scan_pmd(), extracting it into
> a single inline helper function.
> 
> Also move the comment and use it as the function header. While we are at
> it, updated the comment to clarify that a young pte is a recently accessed
> one.
> 
> Acked-by: Usama Arif <usama.arif@linux.dev>
> Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
> ---
>  mm/khugepaged.c | 35 +++++++++++++++++++----------------
>  1 file changed, 19 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1e26ea97381a..9b15c1740943 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -672,6 +672,23 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>  	}
>  }
>  
> +/*
> + * pte_is_referenced() - Check for enough referenced PTEs to justify collapsing
> + *
> + * If collapse was initiated by khugepaged, check that the page has been
> + * recently accessed (young pte) to justify collapsing the page.

That's rather imprecise.

> + *
> + * Return: true if the page has been recently accessed.
> + */

Ideally we'd find a helper that doesn't need any of this documentation :

(1) Just keep the cc->is_khugepaged out of this helper. It over-complicates the
    semantic.

(2) Find name that incorporates that we are dealing with a PTE mapping of a
    folio

> +static inline bool pte_is_referenced(struct collapse_control *cc, pte_t pteval,
> +	struct folio *folio, struct vm_area_struct *vma, unsigned long addr)
> +{
> +	return cc->is_khugepaged &&
> +	       (pte_young(pteval) || folio_test_young(folio) ||
> +		folio_test_referenced(folio) ||
> +		mmu_notifier_test_young(vma->vm_mm, addr));
> +}

static inline bool folio_pte_referenced(struct folio *folio,
		struct vm_area_struct *vma, unsigned long addr, pte_t pteval)
{
	/* The folio was referenced previously ... */
	if (folio_test_young(folio) ||  folio_test_referenced(folio))
		return true;
	/* ... or just now through the (given) PTE. */
	return pte_young(pteval) || mmu_notifier_test_young(vma->vm_mm, addr)
}

?

-- 
Cheers,

David

Re: [PATCH v3 3/7] mm/khugepaged: introduce a count_collapse_event() helper

[David Hildenbrand (Arm)]

On 8/4/26 21:25, Nico Pache (Red Hat) wrote:
> Provide a simple helper function to help reduce a often used, and
> duplicate pattern across the khugepaged code.
> 
> When collapsing to a PMD we need to record a vm_event and the mTHP_stat
> event. When doing mTHP collapse we only update the mTHP stat.
> 
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Acked-by: Usama Arif <usama.arif@linux.dev>
> Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
> ---

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

-- 
Cheers,

David

Re: [PATCH v3 4/7] mm/khugepaged: fix outdated comments

[David Hildenbrand (Arm)]

On 8/4/26 21:25, Nico Pache (Red Hat) wrote:
> Fix comment in collapse_scan_pmd() that still described the old
> folio_mapcount() > folio_ref_count() check and a "512" false-positive
> scenario. The code now uses folio_expected_ref_count() != folio_ref_count()
> which doesn't suffer from the same limitation.
> 
> Fix comment in collapse_huge_page() that referenced ptep_clear_flush,
> when the code actually uses pmdp_collapse_flush.
> 
> Fix comment in __collapse_huge_page_swapin() that referenced the old
> function name khugepaged_scan_pmd, now collapse_scan_pmd.
> 
> Also clean up some simple typos and stale terminology (mmap_sem ->
> mmap_lock, PG_lock -> folio lock, page -> folio, grammar).
> 
> We also clarify a comment regarding where the max_ptes_none check is
> deferred to in mthp_collapse() from the original collapse_scan_pmd check.
> 
> Update all comments that references a function to include parentheses.
> 
> Acked-by: Usama Arif <usama.arif@linux.dev>
> Assisted-by: Cursor(claude-sonnet-4):4.6
> Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>

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

-- 
Cheers,

David

Re: [PATCH v3 1/7] mm/khugepaged: refactor per-scan state clearing into collapse_control_init_scan()

[Zi Yan]

On 4 Aug 2026, at 15:24, Nico Pache (Red Hat) wrote:

> Extract the repeated clearing of node_load, alloc_nmask, and
> mthp_present_ptes into a helper to reduce duplication in
> collapse_scan_pmd() and collapse_scan_file(). Althought file scans do not
> current use the bitmap, they will in the future, and clearing it now is
> harmless.
>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Acked-by: Usama Arif <usama.arif@linux.dev>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
> ---
>  mm/khugepaged.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
LGTM.

Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi