linux-kernel - [PATCH v4 4/7] mm, slab: return ZERO_SIZE_ALLOC for zero sized kmalloc requests

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Date:   Mon, 16 Sep 2019 00:51:14 +0800
From:   Pengfei Li <lpf.vector@...il.com>
To:     akpm@...ux-foundation.org
Cc:     vbabka@...e.cz, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, guro@...com,
        Pengfei Li <lpf.vector@...il.com>
Subject: [PATCH v4 4/7] mm, slab: return ZERO_SIZE_ALLOC for zero sized kmalloc requests

This is a preparation patch, just replace 0 with ZERO_SIZE_ALLOC
as the return value of zero sized requests.

Signed-off-by: Pengfei Li <lpf.vector@...il.com>
---
 include/linux/slab.h | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index e773e5764b7b..1f05f68f2c3e 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -121,14 +121,20 @@
 #define SLAB_DEACTIVATED	((slab_flags_t __force)0x10000000U)
 
 /*
- * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
+ * ZERO_SIZE_ALLOC will be returned by kmalloc_index() if it was zero sized
+ * requests.
  *
+ * After that, ZERO_SIZE_PTR will be returned by the function that called
+ * kmalloc_index().
+
  * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault.
  *
  * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can.
  * Both make kfree a no-op.
  */
-#define ZERO_SIZE_PTR ((void *)16)
+#define ZERO_SIZE_ALLOC		(UINT_MAX)
+
+#define ZERO_SIZE_PTR		((void *)16)
 
 #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
 				(unsigned long)ZERO_SIZE_PTR)
@@ -350,7 +356,7 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
 static __always_inline unsigned int kmalloc_index(size_t size)
 {
 	if (!size)
-		return 0;
+		return ZERO_SIZE_ALLOC;
 
 	if (size <= KMALLOC_MIN_SIZE)
 		return KMALLOC_SHIFT_LOW;
@@ -546,7 +552,7 @@ static __always_inline void *kmalloc(size_t size, gfp_t flags)
 #ifndef CONFIG_SLOB
 		index = kmalloc_index(size);
 
-		if (!index)
+		if (index == ZERO_SIZE_ALLOC)
 			return ZERO_SIZE_PTR;
 
 		return kmem_cache_alloc_trace(
@@ -564,7 +570,7 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 		size <= KMALLOC_MAX_CACHE_SIZE) {
 		unsigned int i = kmalloc_index(size);
 
-		if (!i)
+		if (i == ZERO_SIZE_ALLOC)
 			return ZERO_SIZE_PTR;
 
 		return kmem_cache_alloc_node_trace(
-- 
2.21.0