linux-kernel - [PATCH 2/2] mm: slab: fix comment for __assume_kmalloc

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Message-Id: <84d8142747230f2015eaf9705ee7c2e1a9f56596.1651161548.git.andreyknvl@google.com>
Date:   Thu, 28 Apr 2022 17:59:40 +0200
From:   andrey.konovalov@...ux.dev
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andrey Konovalov <andreyknvl@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Peter Collingbourne <pcc@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: [PATCH 2/2] mm: slab: fix comment for __assume_kmalloc_alignment

From: Andrey Konovalov <andreyknvl@...gle.com>

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 <andreyknvl@...gle.com>
---
 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