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Date: Fri, 22 Apr 2022 18:04:47 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Peter Collingbourne <pcc@...gle.com>
Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Catalin Marinas <catalin.marinas@....com>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux Memory Management List <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Vlastimil Babka <vbabka@...e.cz>,
Pekka Enberg <penberg@...nel.org>, roman.gushchin@...ux.dev,
Joonsoo Kim <iamjoonsoo.kim@....com>,
David Rientjes <rientjes@...gle.com>,
Herbert Xu <herbert@...dor.apana.org.au>,
Andrey Ryabinin <ryabinin.a.a@...il.com>,
Alexander Potapenko <glider@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
kasan-dev <kasan-dev@...glegroups.com>,
Eric Biederman <ebiederm@...ssion.com>,
Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] mm: make minimum slab alignment a runtime property
On Thu, Apr 21, 2022 at 11:16 PM Peter Collingbourne <pcc@...gle.com> 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
Thanks for the improvement, Peter!
Overall, the patch looks good to me:
Reviewed-by: Andrey Konovalov <andreyknvl@...il.com>
While looking at the code, I noticed a couple of issues in the already
existing comments. Not sure if they are worth fixing, but I'll point
them out just in case.
> Link: https://linux-review.googlesource.com/id/I752e725179b43b144153f4b6f584ceb646473ead
> Signed-off-by: Peter Collingbourne <pcc@...gle.com>
> ---
> v2:
> - use max instead of max_t in flat_stack_align()
>
> 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(-)
>
> 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
>
> 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
>
> #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..23ce3439eafa 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(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.
> + */
> +#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.
This comment is not precise: kmalloc relies on kmem_cache_alloc, so
kmalloc technically returns pointers aligned to
max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MIN_MINALIGN). See
create_kmalloc_cache()->create_boot_cache()->calculate_alignment() for
SLAB and SLUB and __do_kmalloc_node() for SLOB. This alignment is
stronger than the one is specified for __assume_kmalloc_alignment
below, so the code should be fine. However, the comment is confusing.
Also, the comment next to the ARCH_KMALLOC_MINALIGN definition says
"Setting ARCH_KMALLOC_MINALIGN in arch headers" while it should say
"Setting ARCH_DMA_MINALIGN in arch headers".
> */
> #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.rc2.479.g8af0fa9b8e-goog
>
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