[PATCH] mm: make minimum slab alignment a runtime property
Vlastimil Babka
vbabka at suse.cz
Fri Apr 22 09:19:18 PDT 2022
On 4/22/22 14:39, Hyeonggon Yoo wrote:
> On Thu, Apr 21, 2022 at 10:16:25AM -0700, Peter Collingbourne wrote:
>> On Thu, Apr 21, 2022 at 5:30 AM Hyeonggon Yoo <42.hyeyoo at gmail.com> wrote:
>> >
>> > 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 at 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
>> > > #endifh
>> > >
>> >
>> > Why isn't it just ARCH_SLAB_MINALIGN?
>>
>> Because this is the minimum possible value of the minimum alignment
>> decided at runtime. I chose to give it a different name to
>> arch_slab_minalign() because the two have different meanings.
>>
>> Granted this isn't a great name because of the stuttering but
>> hopefully it will prompt folks to investigate the meaning of this
>> constant if necessary.
>
> To be honest I don't care much about the name but just thought it's just better
> to be consistent with Catalin's series: ARCH_KMALLOC_MINALIGN for static
> alignment and arch_kmalloc_minalign() for (possibly bigger) alignment decided
> at runtime.
Agree it should be consistent, one way or another. I would (not overly
strongly) prefer Catalin's approach as it's less churn. The name
ARCH_SLAB_MINALIGN is not wrong as the actual alignment can be only bigger
than that (or equal).
Realistically it seems only slab internals are going to use
arch_kmalloc_minalign(), so there shouldn't be too much need of "prompt
folks to investigate".
>> > > 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?
>>
>> The decision of whether to enable KASAN is made early, before the slab
>> allocator is initialized (start_kernel -> smp_prepare_boot_cpu ->
>> kasan_init_hw_tags vs start_kernel -> mm_init -> kmem_cache_init). If
>> you think about it, this needs to be the case for KASAN to operate
>> correctly because it influences the behavior of the slab allocator via
>> the kasan_*poison* hooks. So I don't think we can end up calling this
>> function before then.
>
> Sounds not bad. I wanted to make sure the value of arch_slab_minaligned()
> is not changed during its execution.
>
> Just some part of me thought something like this would be more
> intuitive/robust.
>
> if (systems_supports_mte() && kasan_arg != KASAN_ARG_OFF)
> return MTE_GRANULE_SIZE;
> else
> return __alignof__(unsigned long long);
Let's see if kasan or arm folks have an opinion here.
>
>> > > #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());
>> > > +}
I think this might not be necessary at all. There doesn't seem to be actual
connection to the slab+kasan constraints here. My brief digging into git
blame suggest they just used the ARCH_SLAB_MINALIGN constant because it
existed, e.g. commit 2952095c6b2ee includes in changelog "Arguably, this is
kind of hokey that the FLAT is semi-abusing defines it shouldn't."
So, there shouldn't be a reason to increase this due to KASAN/MTE granule
size, it was done unnecessarily as a side-effect before (AFAIU it shouldn't
have caused existing userspace binaries to break, but maybe in some corner
case it could?), and if this patch leaves out the binfmt_flat changes, the
alignment will be (IMHO correctly) decreased again.
>> > >
>> > > #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.
>>
>> What's the difference?
>>
>
> 231 /*
> 232 * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
> 233 * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MIN_MINALIGN
> 234 * aligned pointers.
> 235 */
> 236 #define __assume_kmalloc_alignment __assume_aligned(ARCH_KMALLOC_MINALIGN)
> 237 #define __assume_slab_alignment __assume_aligned(ARCH_SLAB_MIN_MINALIGN)
> 238 #define __assume_page_alignment __assume_aligned(PAGE_SIZE)
>
> I mean actual slab object size should be both ARCH_SLAB_MIN_MINALIGN-aligned and
> arch_slab_minalign()-aligned. Otherwise we are lying to the compiler.
>
> It's okay If we use just power-of-two alignment.
> But adding a comment wouldn't harm :)
Agreed, technically it's not ">=ARCH_SLAB_MIN_MINALIGN", but "a least common
multiple of ARCH_SLAB_MIN_MINALIGN and whatever the other alignment
requirements arch_slab_minalign() wants to guarantee". But AFAIK in practice
these constraints are always power-of-two.
> Thank you for the work. I think the patch makes sense as usually people
> don't build and install their kernel for arm64 machines.
>
>> Peter
>
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