Re: [RESEND v4 4/7] mm, slab: Return ZERO_SIZE_ALLOC for zero sized kmalloc requests

[David Rientjes]

On Mon, 16 Sep 2019, Pengfei Li wrote:

> 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 

Acked-by: David Rientjes 

[RESEND v4 4/7] mm, slab: Return ZERO_SIZE_ALLOC for zero sized kmalloc requests

[Pengfei Li]

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 
---
 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)0x1000U)
 
 /*
- * 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