Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777 warning

[Christoph Lameter <clameter@sgi.com>]

On Fri, 1 Jun 2007, Linus Torvalds wrote:

> I'm not seeing the path on the freeing side that silently accepts BADPTR.
> 
> It is not ok to derefence BADPTR, but it's obviously ok to _free_ it.

Ok.

> Also, if I read the patch correctly, you _also_ return BADPTR for slabs 
> that are too large. No?  That would be wrong - those need to return NULL 
> for "out of memory".

A too large alloc is >32MB or MAX_ORDER << PAGE_SIZE. A BUG_ON in 
kmalloc_slab() will trigger.

Here is the updated patch. It works fine here:


SLUB: Return BADPTR instead of warning for kmalloc(0)

Remove the WARN_ON_ONCE and simply return BADPTR.

BADPTR can be used legitimately as long as it is not dereferenced.
Can even be freed.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/slub_def.h |   18 ++++++++----------
 mm/slub.c                |   10 +++++-----
 2 files changed, 13 insertions(+), 15 deletions(-)

Index: slub/include/linux/slub_def.h
===================================================================
--- slub.orig/include/linux/slub_def.h	2007-06-01 16:19:36.000000000 -0700
+++ slub/include/linux/slub_def.h	2007-06-01 16:43:57.000000000 -0700
@@ -12,6 +12,8 @@
 #include <linux/kobject.h>
 #include <linux/log2.h>
 
+#define BADPTR ((void *)16)
+
 struct kmem_cache_node {
 	spinlock_t list_lock;	/* Protect partial list and nr_partial */
 	unsigned long nr_partial;
@@ -74,13 +76,9 @@ extern struct kmem_cache kmalloc_caches[
  */
 static inline int kmalloc_index(size_t size)
 {
-	/*
-	 * We should return 0 if size == 0 (which would result in the
-	 * kmalloc caller to get NULL) but we use the smallest object
-	 * here for legacy reasons. Just issue a warning so that
-	 * we can discover locations where we do 0 sized allocations.
-	 */
-	WARN_ON_ONCE(size == 0);
+
+	if (!size)
+		return 0;
 
 	if (size > KMALLOC_MAX_SIZE)
 		return -1;
@@ -133,7 +131,7 @@ static inline void *kmalloc(size_t size,
 		struct kmem_cache *s = kmalloc_slab(size);
 
 		if (!s)
-			return NULL;
+			return BADPTR;
 
 		return kmem_cache_alloc(s, flags);
 	} else
@@ -146,7 +144,7 @@ static inline void *kzalloc(size_t size,
 		struct kmem_cache *s = kmalloc_slab(size);
 
 		if (!s)
-			return NULL;
+			return BADPTR;
 
 		return kmem_cache_zalloc(s, flags);
 	} else
@@ -162,7 +160,7 @@ static inline void *kmalloc_node(size_t 
 		struct kmem_cache *s = kmalloc_slab(size);
 
 		if (!s)
-			return NULL;
+			return BADPTR;
 
 		return kmem_cache_alloc_node(s, flags, node);
 	} else
Index: slub/mm/slub.c
===================================================================
--- slub.orig/mm/slub.c	2007-06-01 16:21:00.000000000 -0700
+++ slub/mm/slub.c	2007-06-01 16:43:27.000000000 -0700
@@ -2286,7 +2286,7 @@ void *__kmalloc(size_t size, gfp_t flags
 
 	if (s)
 		return slab_alloc(s, flags, -1, __builtin_return_address(0));
-	return NULL;
+	return BADPTR;
 }
 EXPORT_SYMBOL(__kmalloc);
 
@@ -2297,7 +2297,7 @@ void *__kmalloc_node(size_t size, gfp_t 
 
 	if (s)
 		return slab_alloc(s, flags, node, __builtin_return_address(0));
-	return NULL;
+	return BADPTR;
 }
 EXPORT_SYMBOL(__kmalloc_node);
 #endif
@@ -2338,7 +2338,7 @@ void kfree(const void *x)
 	struct kmem_cache *s;
 	struct page *page;
 
-	if (!x)
+	if (!x || x == BADPTR)
 		return;
 
 	page = virt_to_head_page(x);
@@ -2707,7 +2707,7 @@ void *__kmalloc_track_caller(size_t size
 	struct kmem_cache *s = get_slab(size, gfpflags);
 
 	if (!s)
-		return NULL;
+		return BADPTR;
 
 	return slab_alloc(s, gfpflags, -1, caller);
 }
@@ -2718,7 +2718,7 @@ void *__kmalloc_node_track_caller(size_t
 	struct kmem_cache *s = get_slab(size, gfpflags);
 
 	if (!s)
-		return NULL;
+		return BADPTR;
 
 	return slab_alloc(s, gfpflags, node, caller);
 }
-
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