[PATCH v3] mm: make minimum slab alignment a runtime property

[Peter Collingbourne]

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>
Reviewed-by: Andrey Konovalov <andreyknvl at gmail.com>
---
v3:
- go back to ARCH_SLAB_MINALIGN
- revert changes to fs/binfmt_flat.c
- update arch_slab_minalign() comment to say that it must be a power of two

v2:
- use max instead of max_t in flat_stack_align()

 arch/arm64/include/asm/cache.h | 17 ++++++++++++-----
 include/linux/slab.h           | 12 ++++++++++++
 mm/slab.c                      |  7 +++----
 mm/slab_common.c               |  3 +--
 mm/slob.c                      |  6 +++---
 5 files changed, 31 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
index a074459f8f2f..22b22dc1b1b5 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
@@ -49,16 +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 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
 
-#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 373b3ef99f4e..2c7190db4cc0 100644
--- 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 size_t 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 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