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>

3 files changed, 15 insertions, 11 deletions

diff --git a/mm/slab.c b/mm/slab.c
index 0edb474edef1..8fb259511305 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()=%u\n", objp,
+		       arch_slab_minalign());
 	}
 	return objp;
 }
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 2b3206a2c3b5..987e6917d850 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(align, arch_slab_minalign());
 
 	return ALIGN(align, sizeof(void *));
 }
diff --git a/mm/slob.c b/mm/slob.c
index 40ea6e2d4ccd..f47811f09aca 100644
--- 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;
 }