From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>

8 byte minimal SLAB alignment interferes with KASAN's granularity of 16
bytes. It causes a lot of out-of-bounds errors for unaligned 8 byte
allocations.

Compared to a kernel with KASAN disabled, the memory footprint increases
because all kmalloc-8 allocations now are realized as kmalloc-16, which
has twice the object size. But more meaningfully, when compared to a
kernel with generic KASAN enabled, there is no difference. Because of
redzones in generic KASAN, kmalloc-8' and kmalloc-16' object size is the
same (48 bytes). So changing the minimal SLAB alignment of the tag-based
mode doesn't have any negative impact when compared to the other
software KASAN mode.

Adjust x86 minimal SLAB alignment to match KASAN granularity size.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
---
Changelog v6:
- Add Andrey's Reviewed-by tag.

Changelog v4:
- Extend the patch message with some more context and impact
  information.

Changelog v3:
- Fix typo in patch message 4 -> 16.
- Change define location to arch/x86/include/asm/cache.c.

 arch/x86/include/asm/cache.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/cache.h b/arch/x86/include/asm/cache.h
index 69404eae9983..3232583b5487 100644
--- a/arch/x86/include/asm/cache.h
+++ b/arch/x86/include/asm/cache.h
@@ -21,4 +21,8 @@
 #endif
 #endif
 
+#ifdef CONFIG_KASAN_SW_TAGS
+#define ARCH_SLAB_MINALIGN (1ULL << KASAN_SHADOW_SCALE_SHIFT)
+#endif
+
 #endif /* _ASM_X86_CACHE_H */
-- 
2.51.0

> diff --git a/arch/x86/include/asm/cache.h b/arch/x86/include/asm/cache.h
> index 69404eae9983..3232583b5487 100644
> --- a/arch/x86/include/asm/cache.h
> +++ b/arch/x86/include/asm/cache.h
> @@ -21,4 +21,8 @@
>  #endif
>  #endif
>
> +#ifdef CONFIG_KASAN_SW_TAGS
> +#define ARCH_SLAB_MINALIGN (1ULL << KASAN_SHADOW_SCALE_SHIFT)

I don't think linux/linkage.h (the only header included here) defines
KASAN_SHADOW_SCALE_SHIFT, does it?

On 2025-11-10 at 18:48:35 +0100, Alexander Potapenko wrote:
>> diff --git a/arch/x86/include/asm/cache.h b/arch/x86/include/asm/cache.h
>> index 69404eae9983..3232583b5487 100644
>> --- a/arch/x86/include/asm/cache.h
>> +++ b/arch/x86/include/asm/cache.h
>> @@ -21,4 +21,8 @@
>>  #endif
>>  #endif
>>
>> +#ifdef CONFIG_KASAN_SW_TAGS
>> +#define ARCH_SLAB_MINALIGN (1ULL << KASAN_SHADOW_SCALE_SHIFT)
>
>I don't think linux/linkage.h (the only header included here) defines
>KASAN_SHADOW_SCALE_SHIFT, does it?

I revised all the x86 and non-arch places where ARCH_SLAB_MINALIGN is used and
all these places also include linux/slab.h which does include
KASAN_SHADOW_SCALE_SHIFT. So there are no cases where it's undefined.

The minalign makes sense defined here but including kasan headers causes
compilation errors all over the place. And I don't think moving
KASAN_SHADOW_SCALE_SHIFT here makes much sense?