[PATCH v2] huge_memory: return -EINVAL in folio split functions when THP is disabled

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

From: Pankaj Raghav <p.raghav@samsung.com>

split_huge_page_to_list_[to_order](), split_huge_page() and
try_folio_split() return 0 on success and error codes on failure.

When THP is disabled, these functions return 0 indicating success even
though an error code should be returned as it is not possible to split a
folio when THP is disabled.

Make all these functions return -EINVAL to indicate failure instead of
0. As large folios depend on CONFIG_THP, issue warning as this function
should not be called without a large folio.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
This issue was discovered while experimenting enabling large folios
without THP and found that returning 0 in these functions is resulting in
undefined behavior in truncate operations. This change fixes the issue.

 include/linux/huge_mm.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 29ef70022da1..23f124493c47 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -588,22 +588,30 @@ static inline int
 split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		unsigned int new_order)
 {
-	return 0;
+	struct folio *folio = page_folio(page);
+
+	VM_WARN_ON_ONCE_FOLIO(1, folio);
+	return -EINVAL;
 }
 static inline int split_huge_page(struct page *page)
 {
-	return 0;
+	struct folio *folio = page_folio(page);
+
+	VM_WARN_ON_ONCE_FOLIO(1, folio);
+	return -EINVAL;
 }
 
 static inline int split_folio_to_list(struct folio *folio, struct list_head *list)
 {
-	return 0;
+	VM_WARN_ON_ONCE_FOLIO(1, folio);
+	return -EINVAL;
 }
 
 static inline int try_folio_split(struct folio *folio, struct page *page,
 		struct list_head *list)
 {
-	return 0;
+	VM_WARN_ON_ONCE_FOLIO(1, folio);
+	return -EINVAL;
 }
 
 static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}

base-commit: 291634ccfd2820c09f6e8c4982c2dee8155d09ae
-- 
2.50.1