[PATCH v2 1/4] mm/huge_memory: change folio_split_supported() to folio_check_splittable()

[Zi Yan <ziy@nvidia.com>]

folio_split_supported() used in try_folio_split_to_order() requires
folio->mapping to be non NULL, but current try_folio_split_to_order() does
not check it. There is no issue in the current code, since
try_folio_split_to_order() is only used in truncate_inode_partial_folio(),
where folio->mapping is not NULL.

To prevent future misuse, move folio->mapping NULL check (i.e., folio is
truncated) into folio_split_supported(). Since folio->mapping NULL check
returns -EBUSY and folio_split_supported() == false means -EINVAL, change
folio_split_supported() return type from bool to int and return error
numbers accordingly. Rename folio_split_supported() to
folio_check_splittable() to match the return type change.

While at it, move is_huge_zero_folio() check and folio_test_writeback()
check into folio_check_splittable() and add kernel-doc.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/huge_mm.h | 10 ++++--
 mm/huge_memory.c        | 74 +++++++++++++++++++++++++----------------
 2 files changed, 53 insertions(+), 31 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 1d439de1ca2c..97686fb46e30 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -375,8 +375,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
 int folio_split_unmapped(struct folio *folio, unsigned int new_order);
 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,
-		enum split_type split_type, bool warns);
+int folio_check_splittable(struct folio *folio, unsigned int new_order,
+			   enum split_type split_type, bool warns);
 int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
 		struct list_head *list);
 
@@ -407,7 +407,11 @@ 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))
+	int ret;
+
+	ret = folio_check_splittable(folio, new_order, SPLIT_TYPE_NON_UNIFORM,
+				     /* warns= */ false);
+	if (ret)
 		return split_huge_page_to_order(&folio->page, new_order);
 	return folio_split(folio, new_order, page, NULL);
 }
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 041b554c7115..c1f1055165dd 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3688,15 +3688,43 @@ 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
+ * @warns: whether gives warnings or not for the checks in the function
+ *
+ * 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, bool warns)
 {
+	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 shmem folios that are in the
+	 * swapcache.
+	 */
+	if (!folio_test_anon(folio) && !folio->mapping)
+		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)) {
@@ -3719,7 +3747,7 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
 			 */
 			VM_WARN_ONCE(warns,
 				"Cannot split file folio to non-0 order");
-			return false;
+			return -EINVAL;
 		}
 	}
 
@@ -3734,10 +3762,18 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
 	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)) {
+		pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
+		return -EINVAL;
+	}
+
+	if (folio_test_writeback(folio))
+		return -EBUSY;
+
+	return 0;
 }
 
 static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
@@ -3922,7 +3958,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	int remap_flags = 0;
 	int extra_pins, 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);
@@ -3930,30 +3965,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	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;
-
-	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))
-		return -EBUSY;
+	ret = folio_check_splittable(folio, new_order, split_type,
+				     /* warn = */ true);
+	if (ret)
+		return ret;
 
 	if (is_anon) {
 		/*
-- 
2.51.0