Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time

[Hyeonggon Yoo]

On Thu, May 13, 2021 at 12:31:38PM +0200, Marco Elver wrote:

> ------ >8 ------
> 
> From: Marco Elver <elver@xxxxxxxxxx>
> Subject: [PATCH] kfence: test: fix for "mm, slub: change run-time assertion in
>  kmalloc_index() to compile-time"
> 
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
>  include/linux/slab.h    | 9 +++++++--
>  mm/kfence/kfence_test.c | 5 +++--
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 27d142564557..7a10bdc4b7a9 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -350,7 +350,8 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
>   * 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;
> @@ -386,11 +387,15 @@ 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();

what about checking size it on top of kmalloc_index? because by definition of 
KMALLOC_SHIFT_HIGH, it's not always 25. it can be less than 25. for some
situations.

below is what I suggested beofre. for just reference:

--- 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;