2022-05-02 15:39:58

by andrey.konovalov

Subject: [PATCH 2/2] mm: slab: fix comment for __assume_kmalloc_alignment

From: Andrey Konovalov <[email protected]>

The comment next to the __assume_kmalloc_alignment definition is not
precise: kmalloc relies on kmem_cache_alloc, so kmalloc technically returns
pointers aligned to both ARCH_KMALLOC_MINALIGN and ARCH_SLAB_MINALIGN,
not only to ARCH_KMALLOC_MINALIGN.

(See create_kmalloc_cache()->create_boot_cache()->calculate_alignment()
for SLAB and SLUB and __do_kmalloc_node() for SLOB.)

Clarify the comment.

The assumption specified by __assume_kmalloc_alignment is still correct,
although it can be made stronger. I'll leave this to a separate patch.

Signed-off-by: Andrey Konovalov <[email protected]>
---
include/linux/slab.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 8cc1d54e56ad..06323a4beff0 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -210,9 +210,9 @@ void kmem_dump_obj(void *object);
#endif

/*
- * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
- * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
- * aligned pointers.
+ * kmem_cache_alloc and friends return pointers aligned to ARCH_SLAB_MINALIGN.
+ * kmalloc and friends return pointers aligned to both ARCH_KMALLOC_MINALIGN
+ * and ARCH_SLAB_MINALIGN, but here we only assume the former alignment.
*/
#define __assume_kmalloc_alignment __assume_aligned(ARCH_KMALLOC_MINALIGN)
#define __assume_slab_alignment __assume_aligned(ARCH_SLAB_MINALIGN)
--
2.25.1

2022-05-03 00:46:57

by David Rientjes

Subject: Re: [PATCH 2/2] mm: slab: fix comment for __assume_kmalloc_alignment

On Thu, 28 Apr 2022, [email protected] wrote:

> From: Andrey Konovalov <[email protected]>
>
> The comment next to the __assume_kmalloc_alignment definition is not
> precise: kmalloc relies on kmem_cache_alloc, so kmalloc technically returns
> pointers aligned to both ARCH_KMALLOC_MINALIGN and ARCH_SLAB_MINALIGN,
> not only to ARCH_KMALLOC_MINALIGN.
>
> (See create_kmalloc_cache()->create_boot_cache()->calculate_alignment()
> for SLAB and SLUB and __do_kmalloc_node() for SLOB.)
>
> Clarify the comment.
>
> The assumption specified by __assume_kmalloc_alignment is still correct,
> although it can be made stronger. I'll leave this to a separate patch.
>
> Signed-off-by: Andrey Konovalov <[email protected]>

Acked-by: David Rientjes <[email protected]>