Re: [PATCH v3] mm: make minimum slab alignment a runtime property

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Fri, Apr 22, 2022 at 01:18:30PM -0700, Peter Collingbourne wrote:
> When CONFIG_KASAN_HW_TAGS is enabled we currently increase the minimum
> slab alignment to 16. This happens even if MTE is not supported in
> hardware or disabled via kasan=off, which creates an unnecessary
> memory overhead in those cases. Eliminate this overhead by making
> the minimum slab alignment a runtime property and only aligning to
> 16 if KASAN is enabled at runtime.
> 
> On a DragonBoard 845c (non-MTE hardware) with a kernel built with
> CONFIG_KASAN_HW_TAGS, waiting for quiescence after a full Android
> boot I see the following Slab measurements in /proc/meminfo (median
> of 3 reboots):
> 
> Before: 169020 kB
> After:  167304 kB
> 
> Link: https://linux-review.googlesource.com/id/I752e725179b43b144153f4b6f584ceb646473ead
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
> ---
> v3:
> - go back to ARCH_SLAB_MINALIGN
> - revert changes to fs/binfmt_flat.c
> - update arch_slab_minalign() comment to say that it must be a power of two
> 
> v2:
> - use max instead of max_t in flat_stack_align()
> 
>  arch/arm64/include/asm/cache.h | 17 ++++++++++++-----
>  include/linux/slab.h           | 12 ++++++++++++
>  mm/slab.c                      |  7 +++----
>  mm/slab_common.c               |  3 +--
>  mm/slob.c                      |  6 +++---
>  5 files changed, 31 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
> index a074459f8f2f..22b22dc1b1b5 100644
> --- a/arch/arm64/include/asm/cache.h
> +++ b/arch/arm64/include/asm/cache.h
> @@ -6,6 +6,7 @@
>  #define __ASM_CACHE_H
>  
>  #include <asm/cputype.h>
> +#include <asm/mte-def.h>
>  
>  #define CTR_L1IP_SHIFT		14
>  #define CTR_L1IP_MASK		3
> @@ -49,16 +50,22 @@
>   */
>  #define ARCH_DMA_MINALIGN	(128)
>  
> +#ifndef __ASSEMBLY__
> +
> +#include <linux/bitops.h>
> +#include <linux/kasan-enabled.h>
> +
>  #ifdef CONFIG_KASAN_SW_TAGS
>  #define ARCH_SLAB_MINALIGN	(1ULL << KASAN_SHADOW_SCALE_SHIFT)
>  #elif defined(CONFIG_KASAN_HW_TAGS)
> -#define ARCH_SLAB_MINALIGN	MTE_GRANULE_SIZE
> +static inline size_t arch_slab_minalign(void)
> +{
> +	return kasan_hw_tags_enabled() ? MTE_GRANULE_SIZE :
> +					 __alignof__(unsigned long long);
> +}
> +#define arch_slab_minalign() arch_slab_minalign()
>  #endif
>  
> -#ifndef __ASSEMBLY__
> -
> -#include <linux/bitops.h>
> -
>  #define ICACHEF_ALIASING	0
>  #define ICACHEF_VPIPT		1
>  extern unsigned long __icache_flags;
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 373b3ef99f4e..2c7190db4cc0 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -209,6 +209,18 @@ void kmem_dump_obj(void *object);
>  #define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
>  #endif
>  
> +/*
> + * Arches can define this function if they want to decide the minimum slab
> + * alignment at runtime. The value returned by the function must be a power
> + * of two and >= ARCH_SLAB_MINALIGN.
> + */
> +#ifndef arch_slab_minalign
> +static inline size_t arch_slab_minalign(void)
> +{
> +	return ARCH_SLAB_MINALIGN;
> +}
> +#endif
> +
>  /*
>   * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
>   * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
> diff --git a/mm/slab.c b/mm/slab.c
> index 0edb474edef1..97b756976c8b 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3009,10 +3009,9 @@ static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
>  	objp += obj_offset(cachep);
>  	if (cachep->ctor && cachep->flags & SLAB_POISON)
>  		cachep->ctor(objp);
> -	if (ARCH_SLAB_MINALIGN &&
> -	    ((unsigned long)objp & (ARCH_SLAB_MINALIGN-1))) {
> -		pr_err("0x%px: not aligned to ARCH_SLAB_MINALIGN=%d\n",
> -		       objp, (int)ARCH_SLAB_MINALIGN);
> +	if ((unsigned long)objp & (arch_slab_minalign() - 1)) {
> +		pr_err("0x%px: not aligned to arch_slab_minalign()=%d\n", objp,
> +		       (int)arch_slab_minalign());
>  	}
>  	return objp;
>  }
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 2b3206a2c3b5..33cc49810a54 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -154,8 +154,7 @@ static unsigned int calculate_alignment(slab_flags_t flags,
>  		align = max(align, ralign);
>  	}
>  
> -	if (align < ARCH_SLAB_MINALIGN)
> -		align = ARCH_SLAB_MINALIGN;
> +	align = max_t(size_t, align, arch_slab_minalign());
>  
>  	return ALIGN(align, sizeof(void *));
>  }
> diff --git a/mm/slob.c b/mm/slob.c
> index 40ea6e2d4ccd..3bd2669bd690 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -478,7 +478,7 @@ static __always_inline void *
>  __do_kmalloc_node(size_t size, gfp_t gfp, int node, unsigned long caller)
>  {
>  	unsigned int *m;
> -	int minalign = max_t(size_t, ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> +	int minalign = max_t(size_t, ARCH_KMALLOC_MINALIGN, arch_slab_minalign());
>  	void *ret;
>  
>  	gfp &= gfp_allowed_mask;
> @@ -555,7 +555,7 @@ void kfree(const void *block)
>  
>  	sp = virt_to_folio(block);
>  	if (folio_test_slab(sp)) {
> -		int align = max_t(size_t, ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> +		int align = max_t(size_t, ARCH_KMALLOC_MINALIGN, arch_slab_minalign());
>  		unsigned int *m = (unsigned int *)(block - align);
>  		slob_free(m, *m + align);
>  	} else {
> @@ -584,7 +584,7 @@ size_t __ksize(const void *block)
>  	if (unlikely(!folio_test_slab(folio)))
>  		return folio_size(folio);
>  
> -	align = max_t(size_t, ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
> +	align = max_t(size_t, ARCH_KMALLOC_MINALIGN, arch_slab_minalign());
>  	m = (unsigned int *)(block - align);
>  	return SLOB_UNITS(*m) * SLOB_UNIT;
>  }
> -- 
> 2.36.0.rc2.479.g8af0fa9b8e-goog
> 

Looks good to me.
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

And work properly on my arm64 machine (no MTE support)
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

Thanks!

-- 
Thanks,
Hyeonggon