Commit d949a815 authored by Peter Collingbourne's avatar Peter Collingbourne Committed by Andrew Morton
Browse files

mm: make minimum slab alignment a runtime property 

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

[akpm@linux-foundation.org: make slab alignment type `unsigned int' to avoid casting]
Link: https://linux-review.googlesource.com/id/I752e725179b43b144153f4b6f584ceb646473ead
Link: https://lkml.kernel.org/r/20220427195820.1716975-2-pcc@google.com


Signed-off-by: default avatarPeter Collingbourne <pcc@google.com>
Reviewed-by: default avatarAndrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: default avatarHyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: default avatarHyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: default avatarDavid Rientjes <rientjes@google.com>
Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 534aa1dc

arch/arm64/include/asm/cache.h

+12 −5
Original line number Diff line number Diff line
@@ -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 unsigned int 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;

include/linux/slab.h

+12 −0
Original line number Diff line number Diff line
@@ -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 unsigned int 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

mm/slab.c

+3 −4
Original line number Diff line number Diff line
@@ -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()=%u\n", objp,

		       arch_slab_minalign());

	}

	return objp;

}

mm/slab_common.c

+1 −2
Original line number Diff line number Diff line
@@ -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(align, arch_slab_minalign());



	return ALIGN(align, sizeof(void *));

}

mm/slob.c

+11 −5
Original line number Diff line number Diff line
@@ -478,9 +478,11 @@ 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);

	unsigned int minalign;

	void *ret;



	minalign = max_t(unsigned int, ARCH_KMALLOC_MINALIGN,

			 arch_slab_minalign());

	gfp &= gfp_allowed_mask;



	might_alloc(gfp);
@@ -493,7 +495,7 @@ __do_kmalloc_node(size_t size, gfp_t gfp, int node, unsigned long caller) 
		 * kmalloc()'d objects.

		 */

		if (is_power_of_2(size))

			align = max(minalign, (int) size);

			align = max_t(unsigned int, minalign, size);



		if (!size)

			return ZERO_SIZE_PTR;
@@ -555,8 +557,11 @@ 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);

		unsigned int align = max_t(unsigned int,

					   ARCH_KMALLOC_MINALIGN,

					   arch_slab_minalign());

		unsigned int *m = (unsigned int *)(block - align);



		slob_free(m, *m + align);

	} else {

		unsigned int order = folio_order(sp);
@@ -573,7 +578,7 @@ EXPORT_SYMBOL(kfree); 
size_t __ksize(const void *block)

{

	struct folio *folio;

	int align;

	unsigned int align;

	unsigned int *m;



	BUG_ON(!block);
@@ -584,7 +589,8 @@ 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(unsigned int, ARCH_KMALLOC_MINALIGN,

		      arch_slab_minalign());

	m = (unsigned int *)(block - align);

	return SLOB_UNITS(*m) * SLOB_UNIT;

}