quoted

 include/linux/slab.h    |   12 ++++++++----
 mm/kfence/kfence_test.c |    4 ++--
 2 files changed, 10 insertions(+), 6 deletions(-)

--- a/include/linux/slab.h~b
+++ a/include/linux/slab.h

@@ -374,7 +374,8 @@ static __always_inline enum kmalloc_cach
  * Note: there's no need to optimize kmalloc_index because it's evaluated
  * in compile-time.
  */
-static __always_inline unsigned int kmalloc_index(size_t size)
+static __always_inline unsigned int kmalloc_index(size_t size,
+						  bool size_is_constant)
 {
 	if (!size)
 		return 0;

@@ -410,7 +411,10 @@ static __always_inline unsigned int kmal
 	if (size <=  16 * 1024 * 1024) return 24;
 	if (size <=  32 * 1024 * 1024) return 25;
 
-	BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
+	if (size_is_constant)
+		BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
+	else
+		BUG();

kfence is randomly generating size. because kfence is using non-constant
size, we should do run-time assertion or compile-time assertion depending
on situation.

I think we can use __builtin_constant_p here. we don't need to modify
kmalloc_index's prototype.

so what about this?
if you think it makes sense, I'll send patch v4.

I used KMALLOC_MAX_CACHE_SIZE to assure it's safe size.
it's safer than putting BUILD_BUG_ON_MSG(1, ...) to below if statements
because KMALLOC_MAX_CACHE_SIZE can be less than 32MB.

--- include/linux/slab.h.orig	2021-05-12 17:56:54.504738768 +0900
+++ include/linux/slab.h	2021-05-13 15:06:25.724565850 +0900

@@ -346,9 +346,18 @@ static __always_inline enum kmalloc_cach
  * 1 =  65 .. 96 bytes
  * 2 = 129 .. 192 bytes
  * n = 2^(n-1)+1 .. 2^n
+ *
+ * Note: there's no need to optimize kmalloc_index because it's evaluated
+ * in compile-time.
  */
 static __always_inline unsigned int kmalloc_index(size_t size)
 {
+	if (__builtin_constant_p(size)) {
+		BUILD_BUG_ON_MSG(size > KMALLOC_MAX_CACHE_SIZE , "unexpected size in kmalloc_index()");
+	} else if (size > KMALLOC_MAX_CACHE_SIZE) {
+		BUG();
+	}
+
 	if (!size)
 		return 0;

@@ -382,8 +391,6 @@ static __always_inline unsigned int kmal
 	if (size <=  8 * 1024 * 1024) return 23;
 	if (size <=  16 * 1024 * 1024) return 24;
 	if (size <=  32 * 1024 * 1024) return 25;
-	if (size <=  64 * 1024 * 1024) return 26;
-	BUG();

 	/* Will never be reached. Needed because the compiler may complain */
 	return -1;