[PATCH v2 1/2] mm: slab: Introduce __GFP_PACKED for smaller kmalloc() alignments

[Catalin Marinas]

By default kmalloc() returns objects aligned to ARCH_KMALLOC_MINALIGN.
This can be somewhat large on architectures defining ARCH_DMA_MINALIGN
(e.g. 128 on arm64) and significant memory is wasted through small
kmalloc() allocations.

Reduce the minimum alignment for kmalloc() to the default
KMALLOC_MIN_SIZE (8 for slub, 32 for slab) but align the
requested size to the bigger ARCH_KMALLOC_MINALIGN unless a newly added
__GFP_PACKED flag is passed. With this gfp flag, the alignment is
reduced to KMALLOC_PACKED_ALIGN, at least sizeof(unsigned long long).

There's no slob support.

Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
---
 include/linux/gfp_types.h | 10 ++++++++--
 include/linux/slab.h      | 22 ++++++++++++++++++----
 mm/slab_common.c          |  3 ++-
 3 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index d88c46ca82e1..305cb8cb6f8b 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h
@@ -55,8 +55,9 @@ typedef unsigned int __bitwise gfp_t;
 #define ___GFP_SKIP_KASAN_UNPOISON	0
 #define ___GFP_SKIP_KASAN_POISON	0
 #endif
+#define ___GFP_PACKED		0x8000000u
 #ifdef CONFIG_LOCKDEP
-#define ___GFP_NOLOCKDEP	0x8000000u
+#define ___GFP_NOLOCKDEP	0x10000000u
 #else
 #define ___GFP_NOLOCKDEP	0
 #endif
@@ -243,6 +244,10 @@ typedef unsigned int __bitwise gfp_t;
  *
  * %__GFP_SKIP_KASAN_POISON makes KASAN skip poisoning on page deallocation.
  * Typically, used for userspace pages. Only effective in HW_TAGS mode.
+ *
+ * %__GFP_PACKED returns a pointer aligned to the possibly smaller
+ * KMALLOC_PACKED_ALIGN rather than ARCH_KMALLOC_MINALIGN. Useful for small
+ * object allocation on architectures that define large ARCH_DMA_MINALIGN.
  */
 #define __GFP_NOWARN	((__force gfp_t)___GFP_NOWARN)
 #define __GFP_COMP	((__force gfp_t)___GFP_COMP)
@@ -251,12 +256,13 @@ typedef unsigned int __bitwise gfp_t;
 #define __GFP_SKIP_ZERO ((__force gfp_t)___GFP_SKIP_ZERO)
 #define __GFP_SKIP_KASAN_UNPOISON ((__force gfp_t)___GFP_SKIP_KASAN_UNPOISON)
 #define __GFP_SKIP_KASAN_POISON   ((__force gfp_t)___GFP_SKIP_KASAN_POISON)
+#define __GFP_PACKED	((__force gfp_t)___GFP_PACKED)
 
 /* Disable lockdep for GFP context tracking */
 #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
 
 /* Room for N __GFP_FOO bits */
-#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))
+#define __GFP_BITS_SHIFT (28 + IS_ENABLED(CONFIG_LOCKDEP))
 #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
 
 /**
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 90877fcde70b..0f59585b5fbf 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -223,8 +223,6 @@ void kmem_dump_obj(void *object);
  */
 #if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
 #define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN
-#define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN
-#define KMALLOC_SHIFT_LOW ilog2(ARCH_DMA_MINALIGN)
 #else
 #define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
 #endif
@@ -310,6 +308,11 @@ static inline unsigned int arch_slab_minalign(void)
 #define KMALLOC_MIN_SIZE (1 << KMALLOC_SHIFT_LOW)
 #endif
 
+/*
+ * This alignment should be at least sizeof(unsigned long long).
+ */
+#define KMALLOC_PACKED_ALIGN	(KMALLOC_MIN_SIZE)
+
 /*
  * This restriction comes from byte sized index implementation.
  * Page size is normally 2^12 bytes and, in this case, if we want to use
@@ -382,6 +385,17 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
 		return KMALLOC_CGROUP;
 }
 
+/*
+ * Align the size to ARCH_KMALLOC_MINALIGN unless __GFP_PACKED is passed.
+ */
+static __always_inline size_t kmalloc_size_align(size_t size, gfp_t flags)
+{
+	if (ARCH_KMALLOC_MINALIGN > KMALLOC_PACKED_ALIGN &&
+	    !(flags & __GFP_PACKED))
+		size = ALIGN(size, ARCH_KMALLOC_MINALIGN);
+	return size;
+}
+
 /*
  * Figure out which kmalloc slab an allocation of a certain size
  * belongs to.
@@ -568,7 +582,7 @@ static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
 		if (size > KMALLOC_MAX_CACHE_SIZE)
 			return kmalloc_large(size, flags);
 #ifndef CONFIG_SLOB
-		index = kmalloc_index(size);
+		index = kmalloc_index(kmalloc_size_align(size, flags));
 
 		if (!index)
 			return ZERO_SIZE_PTR;
@@ -590,7 +604,7 @@ static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t fla
 		if (size > KMALLOC_MAX_CACHE_SIZE)
 			return kmalloc_large_node(size, flags, node);
 
-		index = kmalloc_index(size);
+		index = kmalloc_index(kmalloc_size_align(size, flags));
 
 		if (!index)
 			return ZERO_SIZE_PTR;
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 33b1886b06eb..0e4ea396cd4f 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -627,7 +627,7 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
 		unsigned int useroffset, unsigned int usersize)
 {
 	int err;
-	unsigned int align = ARCH_KMALLOC_MINALIGN;
+	unsigned int align = KMALLOC_PACKED_ALIGN;
 
 	s->name = name;
 	s->size = s->object_size = size;
@@ -720,6 +720,7 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
 {
 	unsigned int index;
 
+	size = kmalloc_size_align(size, flags);
 	if (size <= 192) {
 		if (!size)
 			return ZERO_SIZE_PTR;