[RFC PATCH 1/3] mm/huge_memory: prevent NULL pointer dereference in try_folio_split_to_order()

[Zi Yan]

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. Add the check to prevent NULL pointer dereference.

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.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 include/linux/huge_mm.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 1d439de1ca2c..0d55354e3a34 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -407,6 +407,13 @@ 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)
 {
+	/*
+	 * Folios that just got truncated cannot get split. Signal to the
+	 * caller that there was a race.
+	 */
+	if (!folio_test_anon(folio) && !folio->mapping)
+		return -EBUSY;
+
 	if (!folio_split_supported(folio, new_order, SPLIT_TYPE_NON_UNIFORM, /* warns= */ false))
 		return split_huge_page_to_order(&folio->page, new_order);
 	return folio_split(folio, new_order, page, NULL);
-- 
2.51.0