[PATCH] kfence: test: fix for "mm, slub: change run-time assertion in kmalloc_index() to compile-time"

[Marco Elver <elver@google.com>]

Enable using kmalloc_index() in allocator test modules again where the
size may be non-constant, while ensuring normal usage always passes a
constant size.

Split the definition into __kmalloc_index(size, size_is_constant), and a
definition of kmalloc_index(s), matching the old kmalloc_index()
interface, but that still requires size_is_constant==true. This ensures
that normal usage of kmalloc_index() always passes a constant size.

While the __-prefix should make it clearer that the function is to be
used with care, also rewrite the "Note" to highlight the restriction
(and add a hint to kmalloc_slab()).

The alternative considered here is to export kmalloc_slab(), but given
it is internal to mm/ and not in <linux/slab.h>, we should probably
avoid exporting it. Allocator test modules will work just fine by using
__kmalloc_index(s, false).

Link: https://lkml.kernel.org/r/20210512195227.245000695c9014242e9a00e5@linux-foundation.org
Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/slab.h    | 15 +++++++++++----
 mm/kfence/kfence_test.c |  5 +++--
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 27d142564557..465c5d9d9630 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -347,10 +347,13 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
  * 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.
+ * Note: __kmalloc_index() is compile-time optimized, and not runtime optimized;
+ * typical usage is via kmalloc_index() and therefore evaluated at compile-time.
+ * Callers where !size_is_constant should only be test modules, where runtime
+ * overheads of __kmalloc_index() can be tolerated.  Also see kmalloc_slab().
  */
-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;
@@ -386,11 +389,15 @@ static __always_inline unsigned int kmalloc_index(size_t size)
 	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();
 
 	/* Will never be reached. Needed because the compiler may complain */
 	return -1;
 }
+#define kmalloc_index(s) __kmalloc_index(s, true)
 #endif /* !CONFIG_SLOB */
 
 void *__kmalloc(size_t size, gfp_t flags) __assume_kmalloc_alignment __malloc;
diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
index 4acf4251ee04..7f24b9bcb2ec 100644
--- a/mm/kfence/kfence_test.c
+++ b/mm/kfence/kfence_test.c
@@ -197,7 +197,7 @@ static void test_cache_destroy(void)
 
 static inline size_t kmalloc_cache_alignment(size_t size)
 {
-	return kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size)]->align;
+	return kmalloc_caches[kmalloc_type(GFP_KERNEL)][__kmalloc_index(size, false)]->align;
 }
 
 /* Must always inline to match stack trace against caller. */
@@ -267,7 +267,8 @@ static void *test_alloc(struct kunit *test, size_t size, gfp_t gfp, enum allocat
 
 		if (is_kfence_address(alloc)) {
 			struct page *page = virt_to_head_page(alloc);
-			struct kmem_cache *s = test_cache ?: kmalloc_caches[kmalloc_type(GFP_KERNEL)][kmalloc_index(size)];
+			struct kmem_cache *s = test_cache ?:
+					kmalloc_caches[kmalloc_type(GFP_KERNEL)][__kmalloc_index(size, false)];
 
 			/*
 			 * Verify that various helpers return the right values
-- 
2.31.1.607.g51e8a6a459-goog