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Date:   Wed, 2 Jan 2019 12:14:36 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
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Subject: Re: [PATCH v2 1/3] kasan, arm64: use ARCH_SLAB_MINALIGN instead of
 manual aligning
Message-Id: <20190102121436.5c2b72d1b0ec49affadc9692@linux-foundation.org>
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On Wed,  2 Jan 2019 18:36:06 +0100 Andrey Konovalov <andreyknvl@google.com> wrote:

> Instead of changing cache->align to be aligned to KASAN_SHADOW_SCALE_SIZE
> in kasan_cache_create() we can reuse the ARCH_SLAB_MINALIGN macro.
> 
> ...
>
> --- a/arch/arm64/include/asm/kasan.h
> +++ b/arch/arm64/include/asm/kasan.h
> @@ -36,6 +36,10 @@
>  #define KASAN_SHADOW_OFFSET     (KASAN_SHADOW_END - (1ULL << \
>  					(64 - KASAN_SHADOW_SCALE_SHIFT)))
>  
> +#ifdef CONFIG_KASAN_SW_TAGS
> +#define ARCH_SLAB_MINALIGN	(1ULL << KASAN_SHADOW_SCALE_SHIFT)
> +#endif
> +
>  void kasan_init(void);
>  void kasan_copy_shadow(pgd_t *pgdir);
>  asmlinkage void kasan_early_init(void);
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 11b45f7ae405..d87f913ab4e8 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -16,6 +16,7 @@
>  #include <linux/overflow.h>
>  #include <linux/types.h>
>  #include <linux/workqueue.h>
> +#include <linux/kasan.h>
>  

This still seems unadvisable.  Like other architectures, arm defines
ARCH_SLAB_MINALIGN in arch/arm/include/asm/cache.h. 
arch/arm/include/asm64/cache.h doesn't define ARCH_SLAB_MINALIGN
afaict.

If arch/arm/include/asm64/cache.h later gets a definition of
ARCH_SLAB_MINALIGN then we again face the risk that different .c files
will see different values of ARCH_SLAB_MINALIGN depending on which
headers they include.

So what to say about this?  The architecture's ARCH_SLAB_MINALIGN
should be defined in the architecture's cache.h, end of story.  Not in
slab.h, not in kasan.h.