On Wed, 27 Apr 2022 12:58:20 -0700 Peter Collingbourne <pcc@xxxxxxxxxx> 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):
>
> ...
>
> --- 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());
printf/printk know about size_t. Use %zu, no cast needed. But...
> }
> 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());
max_t/min_t are nature's way of telling us "you screwed up the types".
So what type _is_ slab alignment? size_t seems sensible, but the code
prefers unsigned int. So how about we stick with that?
This compiles. Still some max_t's in slob.c because I was too lazy to
go fix the type of ARCH_KMALLOC_MINALIGN.
Shrug, I don't know if we can be bothered. You decide :)
arch/arm64/include/asm/cache.h | 2 +-
include/linux/slab.h | 2 +-
mm/slab.c | 4 ++--
mm/slab_common.c | 2 +-
mm/slob.c | 16 +++++++++++-----
5 files changed, 16 insertions(+), 10 deletions(-)
--- a/arch/arm64/include/asm/cache.h~mm-make-minimum-slab-alignment-a-runtime-property-fix
+++ a/arch/arm64/include/asm/cache.h
@@ -58,7 +58,7 @@
#ifdef CONFIG_KASAN_SW_TAGS
#define ARCH_SLAB_MINALIGN (1ULL << KASAN_SHADOW_SCALE_SHIFT)
#elif defined(CONFIG_KASAN_HW_TAGS)
-static inline size_t arch_slab_minalign(void)
+static inline unsigned int arch_slab_minalign(void)
{
return kasan_hw_tags_enabled() ? MTE_GRANULE_SIZE :
__alignof__(unsigned long long);
--- a/include/linux/slab.h~mm-make-minimum-slab-alignment-a-runtime-property-fix
+++ a/include/linux/slab.h
@@ -215,7 +215,7 @@ void kmem_dump_obj(void *object);
* of two and >= ARCH_SLAB_MINALIGN.
*/
#ifndef arch_slab_minalign
-static inline size_t arch_slab_minalign(void)
+static inline unsigned int arch_slab_minalign(void)
{
return ARCH_SLAB_MINALIGN;
}
--- a/mm/slab.c~mm-make-minimum-slab-alignment-a-runtime-property-fix
+++ a/mm/slab.c
@@ -3010,8 +3010,8 @@ static void *cache_alloc_debugcheck_afte
if (cachep->ctor && cachep->flags & SLAB_POISON)
cachep->ctor(objp);
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());
+ pr_err("0x%px: not aligned to arch_slab_minalign()=%u\n", objp,
+ arch_slab_minalign());
}
return objp;
}
--- a/mm/slab_common.c~mm-make-minimum-slab-alignment-a-runtime-property-fix
+++ a/mm/slab_common.c
@@ -154,7 +154,7 @@ static unsigned int calculate_alignment(
align = max(align, ralign);
}
- align = max_t(size_t, align, arch_slab_minalign());
+ align = max(align, arch_slab_minalign());
return ALIGN(align, sizeof(void *));
}
--- a/mm/slob.c~mm-make-minimum-slab-alignment-a-runtime-property-fix
+++ a/mm/slob.c
@@ -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
* 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;
}
_
