[PATCH v2 3/4] mm/huge_memory: make min_order_for_split() always return an order

[Zi Yan <ziy@nvidia.com>]

min_order_for_split() returns -EBUSY when the folio is truncated and cannot
be split. In commit 77008e1b2ef7 ("mm/huge_memory: do not change
split_huge_page*() target order silently"), memory_failure() does not
handle it and pass -EBUSY to try_to_split_thp_page() directly.
try_to_split_thp_page() returns -EINVAL since -EBUSY becomes 0xfffffff0 as
new_order is unsigned int in __folio_split() and this large new_order is
rejected as an invalid input. The code does not cause a bug.
soft_offline_in_use_page() also uses min_order_for_split() but it always
passes 0 as new_order for split.

Fix it by making min_order_for_split() always return an order. When the
given folio is truncated, namely folio->mapping == NULL, return 0 and let
a subsequent split function handle the situation and return -EBUSY.

Add kernel-doc to min_order_for_split() to clarify its use.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 include/linux/huge_mm.h |  6 +++---
 mm/huge_memory.c        | 25 +++++++++++++++++++------
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 1ecaeccf39c9..9b3a4e2b0668 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -372,7 +372,7 @@ enum split_type {
 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);
 int folio_check_splittable(struct folio *folio, unsigned int new_order,
 			   enum split_type split_type, bool warns);
@@ -634,10 +634,10 @@ 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)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6c821c1c0ac3..ebc3ba0907fd 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4230,16 +4230,29 @@ 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);
 }
-- 
2.51.0