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

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

On Wed, Apr 20, 2022 at 08:17:38PM -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>
> ---
>  arch/arc/include/asm/cache.h        |  4 ++--
>  arch/arm/include/asm/cache.h        |  2 +-
>  arch/arm64/include/asm/cache.h      | 19 +++++++++++++------
>  arch/microblaze/include/asm/page.h  |  2 +-
>  arch/riscv/include/asm/cache.h      |  2 +-
>  arch/sparc/include/asm/cache.h      |  2 +-
>  arch/xtensa/include/asm/processor.h |  2 +-
>  fs/binfmt_flat.c                    |  9 ++++++---
>  include/crypto/hash.h               |  2 +-
>  include/linux/slab.h                | 22 +++++++++++++++++-----
>  mm/slab.c                           |  7 +++----
>  mm/slab_common.c                    |  3 +--
>  mm/slob.c                           |  6 +++---
>  13 files changed, 51 insertions(+), 31 deletions(-)

[+Cc slab people, Catalin and affected subsystems' folks]

just FYI, There is similar discussion about kmalloc caches' alignment.
https://lore.kernel.org/linux-mm/20220405135758.774016-1-catalin.marinas@arm.com/

It seems this is another demand for runtime resolution of slab
alignment, But slightly different from kmalloc as there is no requirement
for DMA alignment.

> 
> diff --git a/arch/arc/include/asm/cache.h b/arch/arc/include/asm/cache.h
> index f0f1fc5d62b6..b6a7763fd5d6 100644
> --- a/arch/arc/include/asm/cache.h
> +++ b/arch/arc/include/asm/cache.h
> @@ -55,11 +55,11 @@
>   * Make sure slab-allocated buffers are 64-bit aligned when atomic64_t uses
>   * ARCv2 64-bit atomics (LLOCKD/SCONDD). This guarantess runtime 64-bit
>   * alignment for any atomic64_t embedded in buffer.
> - * Default ARCH_SLAB_MINALIGN is __alignof__(long long) which has a relaxed
> + * Default ARCH_SLAB_MIN_MINALIGN is __alignof__(long long) which has a relaxed
>   * value of 4 (and not 8) in ARC ABI.
>   */
>  #if defined(CONFIG_ARC_HAS_LL64) && defined(CONFIG_ARC_HAS_LLSC)
> -#define ARCH_SLAB_MINALIGN	8
> +#define ARCH_SLAB_MIN_MINALIGN	8
>  #endif
> 

Why isn't it just ARCH_SLAB_MINALIGN?

>  extern int ioc_enable;
> diff --git a/arch/arm/include/asm/cache.h b/arch/arm/include/asm/cache.h
> index e3ea34558ada..3e1018bb9805 100644
> --- a/arch/arm/include/asm/cache.h
> +++ b/arch/arm/include/asm/cache.h
> @@ -21,7 +21,7 @@
>   * With EABI on ARMv5 and above we must have 64-bit aligned slab pointers.
>   */
>  #if defined(CONFIG_AEABI) && (__LINUX_ARM_ARCH__ >= 5)
> -#define ARCH_SLAB_MINALIGN 8
> +#define ARCH_SLAB_MIN_MINALIGN 8
>  #endif
>  
>  #define __read_mostly __section(".data..read_mostly")
> diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
> index a074459f8f2f..38f171591c3f 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
> @@ -49,15 +50,21 @@
>   */
>  #define ARCH_DMA_MINALIGN	(128)
>  
> -#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
> -#endif
> -
>  #ifndef __ASSEMBLY__
>  
>  #include <linux/bitops.h>
> +#include <linux/kasan-enabled.h>
> +
> +#ifdef CONFIG_KASAN_SW_TAGS
> +#define ARCH_SLAB_MIN_MINALIGN	(1ULL << KASAN_SHADOW_SCALE_SHIFT)
> +#elif defined(CONFIG_KASAN_HW_TAGS)
> +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
>

kasan_hw_tags_enabled() is also false when kasan is just not initialized yet.
What about writing a new helper something like kasan_is_disabled()
instead?

>  #define ICACHEF_ALIASING	0
>  #define ICACHEF_VPIPT		1
> diff --git a/arch/microblaze/include/asm/page.h b/arch/microblaze/include/asm/page.h
> index 4b8b2fa78fc5..ccdbc1da3c3e 100644
> --- a/arch/microblaze/include/asm/page.h
> +++ b/arch/microblaze/include/asm/page.h
> @@ -33,7 +33,7 @@
>  /* MS be sure that SLAB allocates aligned objects */
>  #define ARCH_DMA_MINALIGN	L1_CACHE_BYTES
>  
> -#define ARCH_SLAB_MINALIGN	L1_CACHE_BYTES
> +#define ARCH_SLAB_MIN_MINALIGN	L1_CACHE_BYTES
>  
>  /*
>   * PAGE_OFFSET -- the first address of the first page of memory. With MMU
> diff --git a/arch/riscv/include/asm/cache.h b/arch/riscv/include/asm/cache.h
> index 9b58b104559e..7beb3b5d27c7 100644
> --- a/arch/riscv/include/asm/cache.h
> +++ b/arch/riscv/include/asm/cache.h
> @@ -16,7 +16,7 @@
>   * the flat loader aligns it accordingly.
>   */
>  #ifndef CONFIG_MMU
> -#define ARCH_SLAB_MINALIGN	16
> +#define ARCH_SLAB_MIN_MINALIGN	16
>  #endif
>  
>  #endif /* _ASM_RISCV_CACHE_H */
> diff --git a/arch/sparc/include/asm/cache.h b/arch/sparc/include/asm/cache.h
> index e62fd0e72606..9d8cb4687b7e 100644
> --- a/arch/sparc/include/asm/cache.h
> +++ b/arch/sparc/include/asm/cache.h
> @@ -8,7 +8,7 @@
>  #ifndef _SPARC_CACHE_H
>  #define _SPARC_CACHE_H
>  
> -#define ARCH_SLAB_MINALIGN	__alignof__(unsigned long long)
> +#define ARCH_SLAB_MIN_MINALIGN	__alignof__(unsigned long long)
>  
>  #define L1_CACHE_SHIFT 5
>  #define L1_CACHE_BYTES 32
> diff --git a/arch/xtensa/include/asm/processor.h b/arch/xtensa/include/asm/processor.h
> index 4489a27d527a..e3ea278e3fcf 100644
> --- a/arch/xtensa/include/asm/processor.h
> +++ b/arch/xtensa/include/asm/processor.h
> @@ -18,7 +18,7 @@
>  #include <asm/types.h>
>  #include <asm/regs.h>
>  
> -#define ARCH_SLAB_MINALIGN XTENSA_STACK_ALIGNMENT
> +#define ARCH_SLAB_MIN_MINALIGN XTENSA_STACK_ALIGNMENT
>  
>  /*
>   * User space process size: 1 GB.
> diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
> index 626898150011..8ff1bf7d1e87 100644
> --- a/fs/binfmt_flat.c
> +++ b/fs/binfmt_flat.c
> @@ -64,7 +64,10 @@
>   * Here we can be a bit looser than the data sections since this
>   * needs to only meet arch ABI requirements.
>   */
> -#define FLAT_STACK_ALIGN	max_t(unsigned long, sizeof(void *), ARCH_SLAB_MINALIGN)
> +static size_t flat_stack_align(void)
> +{
> +	return max_t(unsigned long, sizeof(void *), arch_slab_minalign());
> +}
>  
>  #define RELOC_FAILED 0xff00ff01		/* Relocation incorrect somewhere */
>  #define UNLOADED_LIB 0x7ff000ff		/* Placeholder for unused library */
> @@ -148,7 +151,7 @@ static int create_flat_tables(struct linux_binprm *bprm, unsigned long arg_start
>  		sp -= 2; /* argvp + envp */
>  	sp -= 1;  /* &argc */
>  
> -	current->mm->start_stack = (unsigned long)sp & -FLAT_STACK_ALIGN;
> +	current->mm->start_stack = (unsigned long)sp & -flat_stack_align();
>  	sp = (unsigned long __user *)current->mm->start_stack;
>  
>  	if (put_user(bprm->argc, sp++))
> @@ -966,7 +969,7 @@ static int load_flat_binary(struct linux_binprm *bprm)
>  #endif
>  	stack_len += (bprm->argc + 1) * sizeof(char *);   /* the argv array */
>  	stack_len += (bprm->envc + 1) * sizeof(char *);   /* the envp array */
> -	stack_len = ALIGN(stack_len, FLAT_STACK_ALIGN);
> +	stack_len = ALIGN(stack_len, flat_stack_align());
>  
>  	res = load_flat_file(bprm, &libinfo, 0, &stack_len);
>  	if (res < 0)
> diff --git a/include/crypto/hash.h b/include/crypto/hash.h
> index f140e4643949..442c290f458c 100644
> --- a/include/crypto/hash.h
> +++ b/include/crypto/hash.h
> @@ -149,7 +149,7 @@ struct ahash_alg {
>  
>  struct shash_desc {
>  	struct crypto_shash *tfm;
> -	void *__ctx[] __aligned(ARCH_SLAB_MINALIGN);
> +	void *__ctx[] __aligned(ARCH_SLAB_MIN_MINALIGN);
>  };
>  
>  #define HASH_MAX_DIGESTSIZE	 64
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 373b3ef99f4e..80e517593372 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -201,21 +201,33 @@ void kmem_dump_obj(void *object);
>  #endif
>  
>  /*
> - * Setting ARCH_SLAB_MINALIGN in arch headers allows a different alignment.
> + * Setting ARCH_SLAB_MIN_MINALIGN in arch headers allows a different alignment.
>   * Intended for arches that get misalignment faults even for 64 bit integer
>   * aligned buffers.
>   */
> -#ifndef ARCH_SLAB_MINALIGN
> -#define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
> +#ifndef ARCH_SLAB_MIN_MINALIGN
> +#define ARCH_SLAB_MIN_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
> + * >= ARCH_SLAB_MIN_MINALIGN.
> + */

Not only the value should be bigger than or equal to ARCH_SLAB_MIN_MINALIGN,
it should be compatible with ARCH_SLAB_MIN_MINALIGN.

> +#ifndef arch_slab_minalign
> +static inline size_t arch_slab_minalign(void)
> +{
> +	return ARCH_SLAB_MIN_MINALIGN;
> +}
>  #endif
>  
>  /*
>   * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
> - * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
> + * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MIN_MINALIGN
>   * aligned pointers.
>   */
>  #define __assume_kmalloc_alignment __assume_aligned(ARCH_KMALLOC_MINALIGN)
> -#define __assume_slab_alignment __assume_aligned(ARCH_SLAB_MINALIGN)
> +#define __assume_slab_alignment __assume_aligned(ARCH_SLAB_MIN_MINALIGN)
>  #define __assume_page_alignment __assume_aligned(PAGE_SIZE)
>  
>  /*
> 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.rc0.470.gd361397f0d-goog
> 
> 

-- 
Thanks,
Hyeonggon

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