linux-kernel - Re: [PATCH v3] mm: change run-time assertion in kmalloc

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Date:   Tue, 11 May 2021 11:59:46 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Matthew Wilcox <willy@...radead.org>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: change run-time assertion in kmalloc_index() to
 compile-time

>From c2b8fc4144f2c44b0ac5957a06207c385cc94d15 Mon Sep 17 00:00:00 2001
From: Hyeonggon Yoo <42.hyeyoo@...il.com>
Date: Tue, 11 May 2021 11:53:23 +0900
Subject: [PATCH] mm, slub: change run-time assertion in kmalloc_index() to
 compile-time

currently when size is not supported by kmalloc_index, compiler will
generate a run-time BUG() while compile-time error is also possible,
and better. so changed BUG to BUILD_BUG_ON_MSG to make compile-time
check possible.

also removed code that allocates more than 32MB because current
implementation supports only up to 32MB.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
---
 include/linux/slab.h | 7 +++++--
 mm/slab_common.c     | 7 +++----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 6d454886bcaf..df1937309df2 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -345,6 +345,9 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
  * 1 =  65 .. 96 bytes
  * 2 = 129 .. 192 bytes
  * n = 2^(n-1)+1 .. 2^n
+ *
+ * Note: there's no need to optimize kmalloc_index because it's evaluated
+ * in compile-time.
  */
 static __always_inline unsigned int kmalloc_index(size_t size)
 {
@@ -381,8 +384,8 @@ static __always_inline unsigned int kmalloc_index(size_t size)
 	if (size <=  8 * 1024 * 1024) return 23;
 	if (size <=  16 * 1024 * 1024) return 24;
 	if (size <=  32 * 1024 * 1024) return 25;
-	if (size <=  64 * 1024 * 1024) return 26;
-	BUG();
+
+	BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
 
 	/* Will never be reached. Needed because the compiler may complain */
 	return -1;
diff --git a/mm/slab_common.c b/mm/slab_common.c
index fe8b68482670..97664bbe8147 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1192,8 +1192,8 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
 
 /*
  * kmalloc_info[] is to make slub_debug=,kmalloc-xx option work at boot time.
- * kmalloc_index() supports up to 2^26=64MB, so the final entry of the table is
- * kmalloc-67108864.
+ * kmalloc_index() supports up to 2^25=32MB, so the final entry of the table is
+ * kmalloc-32M.
  */
 const struct kmalloc_info_struct kmalloc_info[] __initconst = {
 	INIT_KMALLOC_INFO(0, 0),
@@ -1221,8 +1221,7 @@ const struct kmalloc_info_struct kmalloc_info[] __initconst = {
 	INIT_KMALLOC_INFO(4194304, 4M),
 	INIT_KMALLOC_INFO(8388608, 8M),
 	INIT_KMALLOC_INFO(16777216, 16M),
-	INIT_KMALLOC_INFO(33554432, 32M),
-	INIT_KMALLOC_INFO(67108864, 64M)
+	INIT_KMALLOC_INFO(33554432, 32M)
 };
 
 /*
-- 
2.25.1