[PATCH] mm: simplify size2index conversion of __kmalloc_index

[Dawei Li <set_pte_at@outlook.com>]

Current size2index is implemented by one to one hardcode mapping,
which can be improved by order_base_2().
Must be careful to not violate compile-time optimization rule.

Generated code for caller of kmalloc:
48 8b 3d 9f 0b 6b 01 mov    0x16b0b9f(%rip),%rdi
                            # ffffffff826d1568 <kmalloc_caches+0x48>
ba 08 01 00 00       mov    $0x108,%edx
be c0 0d 00 00       mov    $0xdc0,%esi
e8 98 d7 2e 00       callq  ffffffff8130e170 <kmem_cache_alloc_trace>

Signed-off-by: Dawei Li <set_pte_at@outlook.com>
---
 include/linux/slab.h | 34 +++++++++-------------------------
 1 file changed, 9 insertions(+), 25 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 0fefdf528e0d..66452a4357c6 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -17,7 +17,7 @@
 #include <linux/types.h>
 #include <linux/workqueue.h>
 #include <linux/percpu-refcount.h>
-
+#include <linux/log2.h>
 
 /*
  * Flags to pass to kmem_cache_create().
@@ -394,31 +394,16 @@ static __always_inline unsigned int __kmalloc_index(size_t size,
 
 	if (KMALLOC_MIN_SIZE <= 32 && size > 64 && size <= 96)
 		return 1;
+
 	if (KMALLOC_MIN_SIZE <= 64 && size > 128 && size <= 192)
 		return 2;
-	if (size <=          8) return 3;
-	if (size <=         16) return 4;
-	if (size <=         32) return 5;
-	if (size <=         64) return 6;
-	if (size <=        128) return 7;
-	if (size <=        256) return 8;
-	if (size <=        512) return 9;
-	if (size <=       1024) return 10;
-	if (size <=   2 * 1024) return 11;
-	if (size <=   4 * 1024) return 12;
-	if (size <=   8 * 1024) return 13;
-	if (size <=  16 * 1024) return 14;
-	if (size <=  32 * 1024) return 15;
-	if (size <=  64 * 1024) return 16;
-	if (size <= 128 * 1024) return 17;
-	if (size <= 256 * 1024) return 18;
-	if (size <= 512 * 1024) return 19;
-	if (size <= 1024 * 1024) return 20;
-	if (size <=  2 * 1024 * 1024) return 21;
-	if (size <=  4 * 1024 * 1024) return 22;
-	if (size <=  8 * 1024 * 1024) return 23;
-	if (size <=  16 * 1024 * 1024) return 24;
-	if (size <=  32 * 1024 * 1024) return 25;
+
+	if (size <= 8)
+		return 3;
+
+	/* Following compile-time optimization rule is mandatory. */
+	if (size <= 32 * 1024 * 1024)
+		return order_base_2(size);
 
 	if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant)
 		BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
@@ -700,7 +685,6 @@ static inline __alloc_size(1, 2) void *kcalloc_node(size_t n, size_t size, gfp_t
 	return kmalloc_array_node(n, size, flags | __GFP_ZERO, node);
 }
 
-
 #ifdef CONFIG_NUMA
 extern void *__kmalloc_node_track_caller(size_t size, gfp_t flags, int node,
 					 unsigned long caller) __alloc_size(1);
-- 
2.25.1