Re: [PATCH v2 08/11] zsmalloc: encode 1-based obj_idx so valid obj is never 0

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] zsmalloc: ZS_MIN_ALLOC_SIZE aligns to ZS_SIZE_CLASS_DELTA
  instead of ZS_ALIGN
--

commit 0c61b7dd8c43af4f863baff5b649ef0e8a7b35b0
Author: Sergey Senozhatsky <senozhatsky@chromium.org>

zsmalloc: encode 1-based obj_idx so valid obj is never 0

This commit encodes obj_idx + 1 in the object handle so that a valid encoded
object is never 0, and updates ZS_MIN_ALLOC_SIZE so the maximum object index
fits in ZS_OBJ_IDX_MASK.

> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 3a6a7253fd01c..aa436df0cc4da 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c

[ ... ]

> @@ -169,9 +170,6 @@
>  static_assert(ZS_OBJ_IDX_BITS >= ZS_PAGES_PER_ZSPAGE_BITS,
>  	      "zsmalloc: ZS_MIN_ALLOC_SIZE would exceed ZS_MAX_ALLOC_SIZE");
>  
> -/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
> -#define ZS_MIN_ALLOC_SIZE \
> -	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> ZS_OBJ_IDX_BITS))
>  /* each chunk includes extra space to keep handle */
>  #define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
>  
> @@ -189,6 +187,11 @@ static_assert(ZS_OBJ_IDX_BITS >= ZS_PAGES_PER_ZSPAGE_BITS,
>   *  (reason above)
>   */
>  #define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> CLASS_BITS)
> +#define _ZS_MIN_ALLOC_RAW \
> +	DIV_ROUND_UP(ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT, ZS_OBJ_IDX_MASK)
> +#define ZS_MIN_ALLOC_SIZE \
> +	MIN(ZS_MAX_ALLOC_SIZE, \
> +	    MAX(32UL, ALIGN(_ZS_MIN_ALLOC_RAW, ZS_SIZE_CLASS_DELTA)))
>  #define ZS_SIZE_CLASSES	(DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
>  				      ZS_SIZE_CLASS_DELTA) + 1)

[Severity: Medium]
Should ZS_MIN_ALLOC_SIZE align _ZS_MIN_ALLOC_RAW to ZS_ALIGN instead of
ZS_SIZE_CLASS_DELTA?

Because _ZS_MIN_ALLOC_RAW uses DIV_ROUND_UP() with a non-zero numerator, it
is always at least 1.  On 16KB and 64KB page kernels, ZS_SIZE_CLASS_DELTA is
64 and 256 respectively, so ALIGN(_ZS_MIN_ALLOC_RAW, ZS_SIZE_CLASS_DELTA)
rounds _ZS_MIN_ALLOC_RAW up to 64 or 256 and overrides the 32-byte floor in
MAX(32UL, ...), eliminating small size classes even when ZS_OBJ_IDX_MASK can
index 32-byte objects.  As noted in the comment above ZS_SIZE_CLASS_DELTA,
ZS_MIN_ALLOC_SIZE only needs to be a multiple of ZS_ALIGN.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261110054836.1515030-1-senozhatsky@chromium.org?part=8