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: Peter Collingbourne <pcc@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Vlastimil 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: Andrew Morton <akpm@linux-foundation.org>

diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
index a074459f8f2fb7761c046518d7ce0ac947ff0cda..7c2181c721168416e5eb9bb8c6ca817c8bce1b13 100644 (file)
--- 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
@@ -49,16 +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;

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 373b3ef99f4e5204b347ef280407885d4650bb7f..3d2f2a3ca17e59b8a64e63b6d3c32eca5b21770c 100644 (file)
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -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

diff --git a/mm/slab.c b/mm/slab.c
index 0edb474edef1839662d4e1a4edeacfceea14bb41..8fb259511305c3d6d9e6f12e99d248e306b41c1c 100644 (file)
--- 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()=%u\n", objp,
+                      arch_slab_minalign());
        }
        return objp;
 }

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 2b3206a2c3b51064f8813ab03412152d4f6fc043..987e6917d850b07ec233fd66146ef229274118cf 100644 (file)
--- 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(align, arch_slab_minalign());
 
        return ALIGN(align, sizeof(void *));
 }

diff --git a/mm/slob.c b/mm/slob.c
index 40ea6e2d4ccd3f67c1a558cbdb35475c81bcaaf3..f47811f09aca0ffc1ddb6d31732dce0f24a56e30 100644 (file)
--- a/mm/slob.c
+++ b/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, 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;
 }