Patch series in this pull request:

- The 2 patch series "powerpc/pseries/cmm: two smaller fixes" from David
   Hildenbrand fixes a couple of minor things in ppc land.
 
 - The 4 patch series "Improve folio split related functions" from Zi Yan
   provides some cleanups and minorish fixes in the folio splitting code.
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Merge tag 'mm-stable-2025-12-11-11-39' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Pull more MM updates from Andrew Morton:

 - "powerpc/pseries/cmm: two smaller fixes" (David Hildenbrand)
   fixes a couple of minor things in ppc land

 - "Improve folio split related functions" (Zi Yan)
   some cleanups and minorish fixes in the folio splitting code

* tag 'mm-stable-2025-12-11-11-39' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm:
  mm/damon/tests/core-kunit: avoid damos_test_commit stack warning
  mm: vmscan: correct nr_requested tracing in scan_folios
  MAINTAINERS: add idr core-api doc file to XARRAY
  mm/hugetlb: fix incorrect error return from hugetlb_reserve_pages()
  mm: fix CONFIG_STACK_GROWSUP typo in mm.h
  mm/huge_memory: fix folio split stats counting
  mm/huge_memory: make min_order_for_split() always return an order
  mm/huge_memory: replace can_split_folio() with direct refcount calculation
  mm/huge_memory: change folio_split_supported() to folio_check_splittable()
  mm/sparse: fix sparse_vmemmap_init_nid_early definition without CONFIG_SPARSEMEM
  powerpc/pseries/cmm: adjust BALLOON_MIGRATE when migrating pages
  powerpc/pseries/cmm: call balloon_devinfo_init() also without CONFIG_BALLOON_COMPACTION

MAINTAINERS

@@ -28321,6 +28321,7 @@ M:	Matthew Wilcox <willy@infradead.org>
 L:	linux-fsdevel@vger.kernel.org
 L:	linux-mm@kvack.org
 S:	Supported
+F:	Documentation/core-api/idr.rst
 F:	Documentation/core-api/xarray.rst
 F:	include/linux/idr.h
 F:	include/linux/xarray.h

arch/powerpc/platforms/pseries/cmm.c

@@ -532,6 +532,7 @@ static int cmm_migratepage(struct balloon_dev_info *b_dev_info,
 
 	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
 	balloon_page_insert(b_dev_info, newpage);
+	__count_vm_event(BALLOON_MIGRATE);
 	b_dev_info->isolated_pages--;
 	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
 
@@ -550,7 +551,6 @@ static int cmm_migratepage(struct balloon_dev_info *b_dev_info,
 
 static void cmm_balloon_compaction_init(void)
 {
-	balloon_devinfo_init(&b_dev_info);
 	b_dev_info.migratepage = cmm_migratepage;
 }
 #else /* CONFIG_BALLOON_COMPACTION */
@@ -572,6 +572,7 @@ static int cmm_init(void)
 	if (!firmware_has_feature(FW_FEATURE_CMO) && !simulate)
 		return -EOPNOTSUPP;
 
+	balloon_devinfo_init(&b_dev_info);
 	cmm_balloon_compaction_init();
 
 	rc = register_oom_notifier(&cmm_oom_nb);

include/linux/huge_mm.h

@@ -369,14 +369,13 @@ enum split_type {
 	SPLIT_TYPE_NON_UNIFORM,
 };
 
-bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
 int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		unsigned int new_order);
 int folio_split_unmapped(struct folio *folio, unsigned int new_order);
-int min_order_for_split(struct folio *folio);
+unsigned int min_order_for_split(struct folio *folio);
 int split_folio_to_list(struct folio *folio, struct list_head *list);
-bool folio_split_supported(struct folio *folio, unsigned int new_order,
+int folio_check_splittable(struct folio *folio, unsigned int new_order,
-		enum split_type split_type, bool warns);
+			   enum split_type split_type);
 int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
 		struct list_head *list);
 
@@ -407,7 +406,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
 static inline int try_folio_split_to_order(struct folio *folio,
 		struct page *page, unsigned int new_order)
 {
-	if (!folio_split_supported(folio, new_order, SPLIT_TYPE_NON_UNIFORM, /* warns= */ false))
+	if (folio_check_splittable(folio, new_order, SPLIT_TYPE_NON_UNIFORM))
 		return split_huge_page_to_order(&folio->page, new_order);
 	return folio_split(folio, new_order, page, NULL);
 }
@@ -631,10 +630,10 @@ static inline int split_huge_page(struct page *page)
 	return -EINVAL;
 }
 
-static inline int min_order_for_split(struct folio *folio)
+static inline unsigned int min_order_for_split(struct folio *folio)
 {
 	VM_WARN_ON_ONCE_FOLIO(1, folio);
-	return -EINVAL;
+	return 0;
 }
 
 static inline int split_folio_to_list(struct folio *folio, struct list_head *list)

include/linux/mm.h

@@ -438,7 +438,7 @@ enum {
 #define VM_NOHUGEPAGE	INIT_VM_FLAG(NOHUGEPAGE)
 #define VM_MERGEABLE	INIT_VM_FLAG(MERGEABLE)
 #define VM_STACK	INIT_VM_FLAG(STACK)
-#ifdef CONFIG_STACK_GROWS_UP
+#ifdef CONFIG_STACK_GROWSUP
 #define VM_STACK_EARLY	INIT_VM_FLAG(STACK_EARLY)
 #else
 #define VM_STACK_EARLY	VM_NONE

include/linux/mmzone.h

@@ -2289,7 +2289,7 @@ void sparse_init(void);
 #else
 #define sparse_init()	do {} while (0)
 #define sparse_index_init(_sec, _nid)  do {} while (0)
-#define sparse_vmemmap_init_nid_early(_nid, _use) do {} while (0)
+#define sparse_vmemmap_init_nid_early(_nid) do {} while (0)
 #define sparse_vmemmap_init_nid_late(_nid) do {} while (0)
 #define pfn_in_present_section pfn_valid
 #define subsection_map_init(_pfn, _nr_pages) do {} while (0)

mm/damon/tests/core-kunit.h

@@ -924,7 +924,7 @@ static void damos_test_commit_for(struct kunit *test, struct damos *dst,
 	}
 }
 
-static void damos_test_commit(struct kunit *test)
+static void damos_test_commit_pageout(struct kunit *test)
 {
 	damos_test_commit_for(test,
 			&(struct damos){
@@ -945,6 +945,10 @@ static void damos_test_commit(struct kunit *test)
 					DAMOS_WMARK_FREE_MEM_RATE,
 					800, 50, 30},
 			});
+}
+
+static void damos_test_commit_migrate_hot(struct kunit *test)
+{
 	damos_test_commit_for(test,
 			&(struct damos){
 				.pattern = (struct damos_access_pattern){
@@ -1230,7 +1234,8 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damos_test_commit_quota),
 	KUNIT_CASE(damos_test_commit_dests),
 	KUNIT_CASE(damos_test_commit_filter),
-	KUNIT_CASE(damos_test_commit),
+	KUNIT_CASE(damos_test_commit_pageout),
+	KUNIT_CASE(damos_test_commit_migrate_hot),
 	KUNIT_CASE(damon_test_commit_target_regions),
 	KUNIT_CASE(damos_test_filter_out),
 	KUNIT_CASE(damon_test_feed_loop_next_input),

mm/huge_memory.c

@@ -3464,23 +3464,6 @@ static void lru_add_split_folio(struct folio *folio, struct folio *new_folio,
 	}
 }
 
-/* Racy check whether the huge page can be split */
-bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
-{
-	int extra_pins;
-
-	/* Additional pins from page cache */
-	if (folio_test_anon(folio))
-		extra_pins = folio_test_swapcache(folio) ?
-				folio_nr_pages(folio) : 0;
-	else
-		extra_pins = folio_nr_pages(folio);
-	if (pextra_pins)
-		*pextra_pins = extra_pins;
-	return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins -
-					caller_pins;
-}
-
 static bool page_range_has_hwpoisoned(struct page *page, long nr_pages)
 {
 	for (; nr_pages; page++, nr_pages--)
@@ -3697,15 +3680,40 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 	return 0;
 }
 
-bool folio_split_supported(struct folio *folio, unsigned int new_order,
+/**
-		enum split_type split_type, bool warns)
+ * folio_check_splittable() - check if a folio can be split to a given order
+ * @folio: folio to be split
+ * @new_order: the smallest order of the after split folios (since buddy
+ *             allocator like split generates folios with orders from @folio's
+ *             order - 1 to new_order).
+ * @split_type: uniform or non-uniform split
+ *
+ * folio_check_splittable() checks if @folio can be split to @new_order using
+ * @split_type method. The truncated folio check must come first.
+ *
+ * Context: folio must be locked.
+ *
+ * Return: 0 - @folio can be split to @new_order, otherwise an error number is
+ * returned.
+ */
+int folio_check_splittable(struct folio *folio, unsigned int new_order,
+			   enum split_type split_type)
 {
+	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
+	/*
+	 * Folios that just got truncated cannot get split. Signal to the
+	 * caller that there was a race.
+	 *
+	 * TODO: this will also currently refuse folios without a mapping in the
+	 * swapcache (shmem or to-be-anon folios).
+	 */
+	if (!folio->mapping && !folio_test_anon(folio))
+		return -EBUSY;
+
 	if (folio_test_anon(folio)) {
 		/* order-1 is not supported for anonymous THP. */
-		VM_WARN_ONCE(warns && new_order == 1,
-				"Cannot split to order-1 folio");
 		if (new_order == 1)
-			return false;
+			return -EINVAL;
 	} else if (split_type == SPLIT_TYPE_NON_UNIFORM || new_order) {
 		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
 		    !mapping_large_folio_support(folio->mapping)) {
@@ -3726,9 +3734,7 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
 			 * case, the mapping does not actually support large
 			 * folios properly.
 			 */
-			VM_WARN_ONCE(warns,
+			return -EINVAL;
-				"Cannot split file folio to non-0 order");
-			return false;
 		}
 	}
 
@@ -3741,19 +3747,31 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
 	 * here.
 	 */
 	if ((split_type == SPLIT_TYPE_NON_UNIFORM || new_order) && folio_test_swapcache(folio)) {
-		VM_WARN_ONCE(warns,
+		return -EINVAL;
-			"Cannot split swapcache folio to non-0 order");
-		return false;
 	}
 
-	return true;
+	if (is_huge_zero_folio(folio))
+		return -EINVAL;
+
+	if (folio_test_writeback(folio))
+		return -EBUSY;
+
+	return 0;
+}
+
+/* Number of folio references from the pagecache or the swapcache. */
+static unsigned int folio_cache_ref_count(const struct folio *folio)
+{
+	if (folio_test_anon(folio) && !folio_test_swapcache(folio))
+		return 0;
+	return folio_nr_pages(folio);
 }
 
 static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
 					     struct page *split_at, struct xa_state *xas,
 					     struct address_space *mapping, bool do_lru,
 					     struct list_head *list, enum split_type split_type,
-					     pgoff_t end, int *nr_shmem_dropped, int extra_pins)
+					     pgoff_t end, int *nr_shmem_dropped)
 {
 	struct folio *end_folio = folio_next(folio);
 	struct folio *new_folio, *next;
@@ -3764,10 +3782,9 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 	VM_WARN_ON_ONCE(!mapping && end);
 	/* Prevent deferred_split_scan() touching ->_refcount */
 	ds_queue = folio_split_queue_lock(folio);
-	if (folio_ref_freeze(folio, 1 + extra_pins)) {
+	if (folio_ref_freeze(folio, folio_cache_ref_count(folio) + 1)) {
 		struct swap_cluster_info *ci = NULL;
 		struct lruvec *lruvec;
-		int expected_refs;
 
 		if (old_order > 1) {
 			if (!list_empty(&folio->_deferred_list)) {
@@ -3835,8 +3852,8 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 
 			zone_device_private_split_cb(folio, new_folio);
 
-			expected_refs = folio_expected_ref_count(new_folio) + 1;
+			folio_ref_unfreeze(new_folio,
-			folio_ref_unfreeze(new_folio, expected_refs);
+					   folio_cache_ref_count(new_folio) + 1);
 
 			if (do_lru)
 				lru_add_split_folio(folio, new_folio, lruvec, list);
@@ -3879,8 +3896,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
 		 * Otherwise, a parallel folio_try_get() can grab @folio
 		 * and its caller can see stale page cache entries.
 		 */
-		expected_refs = folio_expected_ref_count(folio) + 1;
+		folio_ref_unfreeze(folio, folio_cache_ref_count(folio) + 1);
-		folio_ref_unfreeze(folio, expected_refs);
 
 		if (do_lru)
 			unlock_page_lruvec(lruvec);
@@ -3929,40 +3945,27 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	struct folio *new_folio, *next;
 	int nr_shmem_dropped = 0;
 	int remap_flags = 0;
-	int extra_pins, ret;
+	int ret;
 	pgoff_t end = 0;
-	bool is_hzp;
 
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
 
-	if (folio != page_folio(split_at) || folio != page_folio(lock_at))
+	if (folio != page_folio(split_at) || folio != page_folio(lock_at)) {
-		return -EINVAL;
+		ret = -EINVAL;
-
+		goto out;
-	/*
-	 * Folios that just got truncated cannot get split. Signal to the
-	 * caller that there was a race.
-	 *
-	 * TODO: this will also currently refuse shmem folios that are in the
-	 * swapcache.
-	 */
-	if (!is_anon && !folio->mapping)
-		return -EBUSY;
-
-	if (new_order >= old_order)
-		return -EINVAL;
-
-	if (!folio_split_supported(folio, new_order, split_type, /* warn = */ true))
-		return -EINVAL;
-
-	is_hzp = is_huge_zero_folio(folio);
-	if (is_hzp) {
-		pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
-		return -EBUSY;
 	}
 
-	if (folio_test_writeback(folio))
+	if (new_order >= old_order) {
-		return -EBUSY;
+		ret = -EINVAL;
+		goto out;
+	}
+
+	ret = folio_check_splittable(folio, new_order, split_type);
+	if (ret) {
+		VM_WARN_ONCE(ret == -EINVAL, "Tried to split an unsplittable folio");
+		goto out;
+	}
 
 	if (is_anon) {
 		/*
@@ -4027,7 +4030,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	 * Racy check if we can split the page, before unmap_folio() will
 	 * split PMDs
 	 */
-	if (!can_split_folio(folio, 1, &extra_pins)) {
+	if (folio_expected_ref_count(folio) != folio_ref_count(folio) - 1) {
 		ret = -EAGAIN;
 		goto out_unlock;
 	}
@@ -4050,8 +4053,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	}
 
 	ret = __folio_freeze_and_split_unmapped(folio, new_order, split_at, &xas, mapping,
-						true, list, split_type, end, &nr_shmem_dropped,
+						true, list, split_type, end, &nr_shmem_dropped);
-						extra_pins);
 fail:
 	if (mapping)
 		xas_unlock(&xas);
@@ -4125,20 +4127,20 @@ out:
  */
 int folio_split_unmapped(struct folio *folio, unsigned int new_order)
 {
-	int extra_pins, ret = 0;
+	int ret = 0;
 
 	VM_WARN_ON_ONCE_FOLIO(folio_mapped(folio), folio);
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_anon(folio), folio);
 
-	if (!can_split_folio(folio, 1, &extra_pins))
+	if (folio_expected_ref_count(folio) != folio_ref_count(folio) - 1)
 		return -EAGAIN;
 
 	local_irq_disable();
 	ret = __folio_freeze_and_split_unmapped(folio, new_order, &folio->page, NULL,
 						NULL, false, NULL, SPLIT_TYPE_UNIFORM,
-						0, NULL, extra_pins);
+						0, NULL);
 	local_irq_enable();
 	return ret;
 }
@@ -4230,16 +4232,29 @@ int folio_split(struct folio *folio, unsigned int new_order,
 			     SPLIT_TYPE_NON_UNIFORM);
 }
 
-int min_order_for_split(struct folio *folio)
+/**
+ * min_order_for_split() - get the minimum order @folio can be split to
+ * @folio: folio to split
+ *
+ * min_order_for_split() tells the minimum order @folio can be split to.
+ * If a file-backed folio is truncated, 0 will be returned. Any subsequent
+ * split attempt should get -EBUSY from split checking code.
+ *
+ * Return: @folio's minimum order for split
+ */
+unsigned int min_order_for_split(struct folio *folio)
 {
 	if (folio_test_anon(folio))
 		return 0;
 
-	if (!folio->mapping) {
+	/*
-		if (folio_test_pmd_mappable(folio))
+	 * If the folio got truncated, we don't know the previous mapping and
-			count_vm_event(THP_SPLIT_PAGE_FAILED);
+	 * consequently the old min order. But it doesn't matter, as any split
-		return -EBUSY;
+	 * attempt will immediately fail with -EBUSY as the folio cannot get
-	}
+	 * split until freed.
+	 */
+	if (!folio->mapping)
+		return 0;
 
 	return mapping_min_folio_order(folio->mapping);
 }
@@ -4631,7 +4646,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 		 * can be split or not. So skip the check here.
 		 */
 		if (!folio_test_private(folio) &&
-		    !can_split_folio(folio, 0, NULL))
+		    folio_expected_ref_count(folio) != folio_ref_count(folio))
 			goto next;
 
 		if (!folio_trylock(folio))

mm/hugetlb.c

@@ -6579,6 +6579,7 @@ long hugetlb_reserve_pages(struct inode *inode,
 	struct resv_map *resv_map;
 	struct hugetlb_cgroup *h_cg = NULL;
 	long gbl_reserve, regions_needed = 0;
+	int err;
 
 	/* This should never happen */
 	if (from > to) {
@@ -6612,8 +6613,10 @@ long hugetlb_reserve_pages(struct inode *inode,
 	} else {
 		/* Private mapping. */
 		resv_map = resv_map_alloc();
-		if (!resv_map)
+		if (!resv_map) {
+			err = -ENOMEM;
 			goto out_err;
+		}
 
 		chg = to - from;
 
@@ -6621,11 +6624,15 @@ long hugetlb_reserve_pages(struct inode *inode,
 		set_vma_desc_resv_flags(desc, HPAGE_RESV_OWNER);
 	}
 
-	if (chg < 0)
+	if (chg < 0) {
+		/* region_chg() above can return -ENOMEM */
+		err = (chg == -ENOMEM) ? -ENOMEM : -EINVAL;
 		goto out_err;
+	}
 
-	if (hugetlb_cgroup_charge_cgroup_rsvd(hstate_index(h),
+	err = hugetlb_cgroup_charge_cgroup_rsvd(hstate_index(h),
-				chg * pages_per_huge_page(h), &h_cg) < 0)
+				chg * pages_per_huge_page(h), &h_cg);
+	if (err < 0)
 		goto out_err;
 
 	if (desc && !(desc->vm_flags & VM_MAYSHARE) && h_cg) {
@@ -6641,14 +6648,17 @@ long hugetlb_reserve_pages(struct inode *inode,
 	 * reservations already in place (gbl_reserve).
 	 */
 	gbl_reserve = hugepage_subpool_get_pages(spool, chg);
-	if (gbl_reserve < 0)
+	if (gbl_reserve < 0) {
+		err = gbl_reserve;
 		goto out_uncharge_cgroup;
+	}
 
 	/*
 	 * Check enough hugepages are available for the reservation.
 	 * Hand the pages back to the subpool if there are not
 	 */
-	if (hugetlb_acct_memory(h, gbl_reserve) < 0)
+	err = hugetlb_acct_memory(h, gbl_reserve);
+	if (err < 0)
 		goto out_put_pages;
 
 	/*
@@ -6667,6 +6677,7 @@ long hugetlb_reserve_pages(struct inode *inode,
 
 		if (unlikely(add < 0)) {
 			hugetlb_acct_memory(h, -gbl_reserve);
+			err = add;
 			goto out_put_pages;
 		} else if (unlikely(chg > add)) {
 			/*
@@ -6726,7 +6737,7 @@ out_err:
 		kref_put(&resv_map->refs, resv_map_release);
 		set_vma_desc_resv_map(desc, NULL);
 	}
-	return chg < 0 ? chg : add < 0 ? add : -EINVAL;
+	return err;
 }
 
 long hugetlb_unreserve_pages(struct inode *inode, long start, long end,

mm/vmscan.c

@@ -1284,7 +1284,8 @@ retry:
 					goto keep_locked;
 				if (folio_test_large(folio)) {
 					/* cannot split folio, skip it */
-					if (!can_split_folio(folio, 1, NULL))
+					if (folio_expected_ref_count(folio) !=
+					    folio_ref_count(folio) - 1)
 						goto activate_locked;
 					/*
 					 * Split partially mapped folios right away.
@@ -4540,7 +4541,8 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	int scanned = 0;
 	int isolated = 0;
 	int skipped = 0;
-	int remaining = min(nr_to_scan, MAX_LRU_BATCH);
+	int scan_batch = min(nr_to_scan, MAX_LRU_BATCH);
+	int remaining = scan_batch;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 
@@ -4600,7 +4602,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	count_memcg_events(memcg, item, isolated);
 	count_memcg_events(memcg, PGREFILL, sorted);
 	__count_vm_events(PGSCAN_ANON + type, isolated);
-	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, MAX_LRU_BATCH,
+	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, scan_batch,
 				scanned, skipped, isolated,
 				type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
 	if (type == LRU_GEN_FILE)