Re: [PATCH v2] mm: kmalloc_index: make compiler break when size is not supported

[Vlastimil Babka]

On 5/10/21 5:15 PM, Christoph Lameter wrote:
> I guess this needs to be reviewed and tested by the users of architectures
> that can use large MAXORDER pages such as powerpc and Itanium.

AFAICS large MAX_ORDER should matter as KMALLOC_SHIFT_HIGH will be always capped
to 25. But sure, let the bots complain if something is wrong. I'm more
interested if we shake out some compiler that can't do the compile-time
evaluation properly and we didn't know until now.

> On Tue, 11 May 2021, Hyeonggon Yoo wrote:
> 
>> updated patch. let me know if something is wrong!
>>
> 
> 
> 0001-mm-kmalloc_index-make-compiler-break-when-size-is-no.patch
> 
>>From 8fe7ecdfb0f5bd5b08771512303d72f1c6447362 Mon Sep 17 00:00:00 2001
> From: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> Date: Mon, 10 May 2021 23:57:34 +0900
> Subject: [PATCH] mm: kmalloc_index: make compiler break when size is not
>  supported
> 
> currently when size is not supported by kmalloc_index, compiler will not
> break. so changed BUG to BUILD_BUG_ON_MSG to make compiler break if size is
> wrong. this is done in compile time.
> 
> also removed code that allocates more than 32MB because current
> implementation supports only up to 32MB.
> 
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> ---
>  include/linux/slab.h | 7 +++++--
>  mm/slab_common.c     | 7 +++----
>  2 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 0c97d788762c..fd0c7229d105 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -346,6 +346,9 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
>   * 1 =  65 .. 96 bytes
>   * 2 = 129 .. 192 bytes
>   * n = 2^(n-1)+1 .. 2^n
> + *
> + * Note: you don't need to optimize kmalloc_index because it's evaluated
> + * in compile-time.
>   */
>  static __always_inline unsigned int kmalloc_index(size_t size)
>  {
> @@ -382,8 +385,8 @@ static __always_inline unsigned int kmalloc_index(size_t size)
>  	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();
> +
> +	BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
>  
>  	/* Will never be reached. Needed because the compiler may complain */
>  	return -1;
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index f8833d3e5d47..39d4eca8cf9b 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -745,8 +745,8 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
>  
>  /*
>   * kmalloc_info[] is to make slub_debug=,kmalloc-xx option work at boot time.
> - * kmalloc_index() supports up to 2^26=64MB, so the final entry of the table is
> - * kmalloc-67108864.
> + * kmalloc_index() supports up to 2^25=32MB, so the final entry of the table is
> + * kmalloc-33554432.
>   */
>  const struct kmalloc_info_struct kmalloc_info[] __initconst = {
>  	INIT_KMALLOC_INFO(0, 0),
> @@ -774,8 +774,7 @@ const struct kmalloc_info_struct kmalloc_info[] __initconst = {
>  	INIT_KMALLOC_INFO(4194304, 4M),
>  	INIT_KMALLOC_INFO(8388608, 8M),
>  	INIT_KMALLOC_INFO(16777216, 16M),
> -	INIT_KMALLOC_INFO(33554432, 32M),
> -	INIT_KMALLOC_INFO(67108864, 64M)
> +	INIT_KMALLOC_INFO(33554432, 32M)
>  };
>  
>  /*
>