[PATCH] pagemap: Add alert to mapping_set_release_always() for mapping with no release_folio

[Deepakkumar Karn]

AS_RELEASE_ALWAYS tells the memory management code to always call
->release_folio() when releasing a folio, even if it has no private
data. Setting this flag without providing release_folio callback
leads to try_to_free_buffers() being called on folios without
buffer_heads, causing a NULL pointer dereference.

Add a VM_WARN_ONCE() alert to mapping_set_release_always() to catch
this programming error early and help prevent similar bugs in other
filesystems.

Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Deepakkumar Karn <dkarn@xxxxxxxxxx>
---
 include/linux/pagemap.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 31a848485ad9..cc352e87ac2d 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -296,6 +296,9 @@ static inline bool mapping_release_always(const struct address_space *mapping)
 
 static inline void mapping_set_release_always(struct address_space *mapping)
 {
+	/* Alert while setting the flag with no release_folio callback */
+	VM_WARN_ONCE(!mapping->a_ops->release_folio,
+		     "Setting AS_RELEASE_ALWAYS with no release_folio");
 	set_bit(AS_RELEASE_ALWAYS, &mapping->flags);
 }
 
-- 
2.52.0