1 files changed, 90 insertions, 75 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f7c565f11a98..40cf59301c21 100644
--- a/mm/huge_memory.c
+++ b/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,
-				"Cannot split file folio to non-0 order");
-			return false;
+			return -EINVAL;
 		}
 	}
 
@@ -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,
-			"Cannot split swapcache folio to non-0 order");
-		return false;
+		return -EINVAL;
 	}
 
-	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, expected_refs);
+			folio_ref_unfreeze(new_folio,
+					   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, expected_refs);
+		folio_ref_unfreeze(folio, folio_cache_ref_count(folio) + 1);
 
 		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))
-		return -EINVAL;
-
-	/*
-	 * 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;
+	if (folio != page_folio(split_at) || folio != page_folio(lock_at)) {
+		ret = -EINVAL;
+		goto out;
+	}
 
-	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 (new_order >= old_order) {
+		ret = -EINVAL;
+		goto out;
 	}
 
-	if (folio_test_writeback(folio))
-		return -EBUSY;
+	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,
-						extra_pins);
+						true, list, split_type, end, &nr_shmem_dropped);
 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))
-			count_vm_event(THP_SPLIT_PAGE_FAILED);
-		return -EBUSY;
-	}
+	/*
+	 * If the folio got truncated, we don't know the previous mapping and
+	 * consequently the old min order. But it doesn't matter, as any split
+	 * 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))