Calling is_huge_zero_folio(NULL) should not be legal - it makes no sense, and a different (theoretical) implementation may dereference the pointer. But currently, lacking any explicit documentation, this call is possible. But if somebody really passes NULL, the function should not return true - this isn't the huge zero folio after all! However, if the `huge_zero_folio` hasn't been allocated yet, it's NULL, and is_huge_zero_folio(NULL) just happens to return true, which is a lie. This weird side effect prevented me from reproducing a kernel crash that occurred when the elements of a folio_batch were NULL - since folios_put_refs() skips huge zero folios, this sometimes causes a crash, but sometimes does not. For debugging, it is better to reveal such bugs reliably and not hide them behind random preconditions like "has the huge zero folio already been created?" To improve detection of such bugs, David Hildenbrand suggested adding a VM_WARN_ON_ONCE(). Signed-off-by: Max Kellermann --- v1->v2: using VM_WARN_ON_ONCE() instead of checking huge_zero_folio v2->v3: use "!" to check NULL; removed the #include --- include/linux/huge_mm.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index 7748489fde1b..96ac47603d97 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -479,6 +479,8 @@ extern unsigned long huge_zero_pfn; static inline bool is_huge_zero_folio(const struct folio *folio) { + VM_WARN_ON_ONCE(!folio); + return READ_ONCE(huge_zero_folio) == folio; } -- 2.47.2