[PATCH] mm, slab: extend kmalloc() alignment for non power-of-two sizes

Vlastimil Babka
2024-07-02T15:58:12Z

Slab allocators have been guaranteeing natural alignment for
power-of-two sizes since commit 59bb47985c1d ("mm, sl[aou]b: guarantee
natural alignment for kmalloc(power-of-two)"), while any other sizes are
aligned only to ARCH_KMALLOC_MINALIGN bytes.

Rust's allocator API specifies size and alignment per allocation, which
have to satisfy the following rules, per Alice Ryhl [1]:

1. The alignment is a power of two.
2. The size is non-zero.
3. When you round up the size to the next multiple of the alignment,
then it must not overflow the signed type isize / ssize_t.

In order to map this to kmalloc()'s guarantees, some requested
allocation sizes have to be enlarged to the next power-of-two size [2].
For example, an allocation of size 96 and alignment of 32 will be
enlarged to an allocation of size 128, because the existing kmalloc-96
bucket doesn't guarantee alignent above ARCH_KMALLOC_MINALIGN. Without
slab debugging active, the layout of the kmalloc-96 slabs however
naturally aligns the objects to 32 bytes, so extending the size to 128
bytes is wasteful.

To improve the situation we can extend the kmalloc() alignment
guarantees in a way that

1) doesn't change the current slab layout (and thus does not increase
internal fragmentation) when slab debugging is not active
2) reduces waste in the Rust allocator use case
3) is a superset of the current guarantee for power-of-two sizes.

The extended guarantee is that alignment is at least the largest
power-of-two divisor of the requested size. For power-of-two sizes the
largest divisor is the size itself, but let's keep this case documented
separately for clarity.

For current kmalloc size buckets, it means kmalloc-96 will guarantee
alignment of 32 bytes and kmalloc-196 will guarantee 64 bytes.

This covers the rules 1 and 2 above of Rust's API as long as the size is
a multiple of the alignment. The Rust layer should now only need to
round up the size to the next multiple if it isn't, while enforcing the
rule 3.

Implementation-wise, this changes the alignment calculation in
create_boot_cache(). While at it also do the calulation only for caches
with the SLAB_KMALLOC flag, because the function is also used to create
the initial kmem_cache and kmem_cache_node caches, where no alignment
guarantee is necessary.

Link: https://lore.kernel.org/all/CAH5fLggjrbdUuT-H-5vbQfMazjRDpp2%2Bk3%3DYhPyS17ezEqxwcw@mail.gmail.com/ [1]
Link: https://lore.kernel.org/all/CAH5fLghsZRemYUwVvhk77o6y1foqnCeDzW4WZv6ScEWna2+_jw@mail.gmail.com/ [2]
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
Documentation/core-api/memory-allocation.rst | 6 ++++--
include/linux/slab.h | 3 ++-
mm/slab_common.c | 9 +++++----
3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
index 1c58d883b273..8b84eb4bdae7 100644
--- a/Documentation/core-api/memory-allocation.rst
+++ b/Documentation/core-api/memory-allocation.rst
@@ -144,8 +144,10 @@ configuration, but it is a good practice to use `kmalloc` for objects
smaller than page size.

The address of a chunk allocated with `kmalloc` is aligned to at least
-ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
-alignment is also guaranteed to be at least the respective size.
+ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
+alignment is also guaranteed to be at least the respective size. For other
+sizes, the alignment is guaranteed to be at least the largest power-of-two
+divisor of the size.

Chunks allocated with kmalloc() can be resized with krealloc(). Similarly
to kmalloc_array(): a helper for resizing arrays is provided in the form of
diff --git a/include/linux/slab.h b/include/linux/slab.h
index ed6bee5ec2b6..640cea6e6323 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -604,7 +604,8 @@ void *__kmalloc_large_node_noprof(size_t size, gfp_t flags, int node)
*
* The allocated object address is aligned to at least ARCH_KMALLOC_MINALIGN
* bytes. For @size of power of two bytes, the alignment is also guaranteed
- * to be at least to the size.
+ * to be at least to the size. For other sizes, the alignment is guaranteed to
+ * be at least the largest power-of-two divisor of @size.
*
* The @flags argument may be one of the GFP flags defined at
* include/linux/gfp_types.h and described at
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 1560a1546bb1..7272ef7bc55f 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -617,11 +617,12 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
s->size = s->object_size = size;

/*
- * For power of two sizes, guarantee natural alignment for kmalloc
- * caches, regardless of SL*B debugging options.
+ * kmalloc caches guarantee alignment of at least the largest
+ * power-of-two divisor of the size. For power-of-two sizes,
+ * it is the size itself.
*/
- if (is_power_of_2(size))
- align = max(align, size);
+ if (flags & SLAB_KMALLOC)
+ align = max(align, 1U << (ffs(size) - 1));
s->align = calculate_alignment(flags, align, size);

#ifdef CONFIG_HARDENED_USERCOPY
--
2.45.2