+ mm-collapse-declare-the-collapse-interface-in-collapseh.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/collapse: declare the collapse interface in collapse.h
has been added to the -mm mm-new branch.  Its filename is
     mm-collapse-declare-the-collapse-interface-in-collapseh.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-collapse-declare-the-collapse-interface-in-collapseh.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
Subject: mm/collapse: declare the collapse interface in collapse.h
Date: Thu, 10 Sep 2026 13:02:31 +0100

A collapse takes four calls:

  - collapse_control_init() - set up the control a caller carries;
  - collapse_scan_pmd() - scan one PTE table, under mmap_lock;
  - collapse_run_pmd() - collapse what the scan found, no mmap_lock;
  - collapse_control_release() - done with the control.

All four are static in khugepaged.c, as are collapse_possible_orders(),
which says what a VMA allows, and the revalidate a caller needs once a
collapse has given the mmap_lock up.  No other file can ask for a collapse
without them.

Declare them in collapse.h, with a comment stating the order they are
called in and who holds the lock over each step.

hugepage_vma_revalidate() becomes collapse_vma_revalidate(): it is part of
what a collapse offers now, not a helper of the daemon.

Preparation for implementing MADV_COLLAPSE in madvise.c.

No functional change.

Assisted-by: LLM
Link: https://lore.kernel.org/20260910120238.2529819-12-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/collapse.h   |   44 ++++++++++++++++++++++++++++++++++++++++++++
 mm/khugepaged.c |   18 +++++++++---------
 2 files changed, 53 insertions(+), 9 deletions(-)

--- a/mm/collapse.h~mm-collapse-declare-the-collapse-interface-in-collapseh
+++ a/mm/collapse.h
@@ -106,4 +106,48 @@ struct collapse_control {
 	bool scan_retract_only;
 };
 
+/* Which orders a VMA may collapse to, zero when it may not collapse at all */
+unsigned long collapse_possible_orders(struct vm_area_struct *vma,
+		vm_flags_t vm_flags, enum tva_type tva_flags);
+
+/*
+ * A caller states what it allows in cc->policy and then hands over one PTE
+ * table's worth of a VMA at a time:
+ *
+ *     collapse_control_init(cc)         once, before the first table
+ *     collapse_scan_pmd(vma, addr, ...) per table
+ *     collapse_run_pmd(mm, addr, cc)    when a scan found work
+ *     collapse_control_release(cc)      once, when done with the control
+ *
+ * The caller holds mmap_lock for reading over the scan and passes an address
+ * within @vma, aligned to the PTE table the scan is to judge.
+ *
+ * The scan returns with that lock still held.  It only reads, and almost every
+ * table it is offered has nothing to collapse, so a caller walks a whole VMA
+ * under the one lock it took to get there.  SCAN_SUCCEED means there is
+ * something to collapse; anything else is why there is not.
+ *
+ * The run is called without the lock and returns without it, taking what it
+ * needs in between: what it does -- allocate, isolate, copy, flush -- is slow
+ * enough that a writer would wait behind it.  The caller gives the lock up
+ * first, and with it @vma and anything derived under it, so a caller carrying
+ * on has to look up again with collapse_vma_revalidate().  The run revalidates
+ * for itself rather than trusting what the scan saw.
+ *
+ * A scan that found something has to be run: the file side takes a reference on
+ * the file while it still has the VMA to take it from, and the run is what
+ * gives it back.
+ */
+void collapse_control_init(struct collapse_control *cc);
+void collapse_control_release(struct collapse_control *cc);
+enum scan_result collapse_scan_pmd(struct vm_area_struct *vma,
+		unsigned long addr, struct collapse_control *cc,
+		unsigned long orders);
+enum scan_result collapse_run_pmd(struct mm_struct *mm, unsigned long addr,
+		struct collapse_control *cc);
+enum scan_result collapse_vma_revalidate(struct mm_struct *mm,
+		unsigned long address, bool expect_anon,
+		struct vm_area_struct **vmap, struct collapse_control *cc,
+		unsigned int order);
+
 #endif	/* __MM_COLLAPSE_H */
--- a/mm/khugepaged.c~mm-collapse-declare-the-collapse-interface-in-collapseh
+++ a/mm/khugepaged.c
@@ -498,7 +498,7 @@ void __khugepaged_enter(struct mm_struct
  * 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,
+unsigned long collapse_possible_orders(struct vm_area_struct *vma,
 		vm_flags_t vm_flags, enum tva_type tva_flags)
 {
 	unsigned long orders;
@@ -1016,7 +1016,7 @@ static int collapse_find_target_node(str
  * Returns enum scan_result value.
  */
 
-static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
+enum scan_result collapse_vma_revalidate(struct mm_struct *mm, unsigned long address,
 		bool expect_anon, struct vm_area_struct **vmap,
 		struct collapse_control *cc, unsigned int order)
 {
@@ -1264,7 +1264,7 @@ static enum scan_result collapse_huge_pa
 	}
 
 	mmap_read_lock(mm);
-	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
+	result = collapse_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
 					 &vma, cc, order);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
@@ -1299,7 +1299,7 @@ static enum scan_result collapse_huge_pa
 	 * mmap_lock.
 	 */
 	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
+	result = collapse_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true,
 					 &vma, cc, order);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
@@ -2743,13 +2743,13 @@ static enum scan_result collapse_scan_fi
 	return result;
 }
 
-static void collapse_control_init(struct collapse_control *cc)
+void collapse_control_init(struct collapse_control *cc)
 {
 	cc->progress = 0;
 	cc->scan_file = NULL;
 }
 
-static void collapse_control_release(struct collapse_control *cc)
+void collapse_control_release(struct collapse_control *cc)
 {
 	/* A scan that took a file reference should have been run */
 	if (WARN_ON_ONCE(cc->scan_file)) {
@@ -2758,7 +2758,7 @@ static void collapse_control_release(str
 	}
 }
 
-static enum scan_result collapse_scan_pmd(struct vm_area_struct *vma,
+enum scan_result collapse_scan_pmd(struct vm_area_struct *vma,
 		unsigned long addr, struct collapse_control *cc,
 		unsigned long orders)
 {
@@ -2802,7 +2802,7 @@ static enum scan_result collapse_scan_pm
 	return result;
 }
 
-static enum scan_result collapse_run_pmd(struct mm_struct *mm,
+enum scan_result collapse_run_pmd(struct mm_struct *mm,
 		unsigned long addr, struct collapse_control *cc)
 {
 	struct file *file = cc->scan_file;
@@ -3256,7 +3256,7 @@ int madvise_collapse(struct vm_area_stru
 		if (!vma) {
 			cond_resched();
 			mmap_read_lock(mm);
-			result = hugepage_vma_revalidate(mm, addr, false, &found,
+			result = collapse_vma_revalidate(mm, addr, false, &found,
 							 cc, HPAGE_PMD_ORDER);
 			if (result != SCAN_SUCCEED) {
 				last_fail = result;
_

Patches currently in -mm which might be from kas@kernel.org are

mm-huge_memory-do-not-touch-frozen-folios-in-deferred_split_isolate.patch
mm-huge_memory-dequeue-the-deferred-split-after-the-split-freeze.patch
mm-huge_memory-add-folio_reset_partially_mapped.patch
mm-khugepaged-drop-redundant-mm_struct-pin-in-madvise_collapse.patch
mm-khugepaged-count-collapses-where-khugepaged-makes-them.patch
mm-khugepaged-rename-mthp_present_ptes-bitmap-to-eligible_ptes.patch
mm-collapse-add-collapseh-for-the-collapse-interface.patch
mm-collapse-state-what-a-collapse-may-do-in-the-policy.patch
mm-collapse-drop-the-collapse_possible-wrapper.patch
mm-collapse-name-the-per-table-scan-reset-for-what-it-resets.patch
mm-collapse-separate-scanning-a-pte-table-from-collapsing-it.patch
mm-collapse-open-code-collapse_single_pmd-in-its-two-callers.patch
mm-collapse-work-out-the-orders-a-vma-allows-once-per-vma.patch
mm-collapse-declare-the-collapse-interface-in-collapseh.patch
mm-collapse-implement-madv_collapse-in-madvisec.patch