SLUB: Use unique end pointer for each slab page.

We use a NULL pointer on freelists to signal that there are no more objects.
However the NULL pointers of all slabs match in contrast to the pointers to
the real objects which are in different ranges for different slab pages.

Change the end pointer to be a pointer to the first object and set bit 0.
Every slab will then have a different end pointer. This is necessary to ensure
that end markers can be matched to the source slab during cmpxchg_local.

Bring back the use of the mapping field by SLUB since we would otherwise have
to call a relatively expensive function page_address() in __slab_alloc().  Use
of the mapping field allows avoiding a call to page_address() in various other
functions as well.

There is no need to change the page_mapping() function since bit 0 is set on
the mapping as also for anonymous pages.  page_mapping(slab_page) will
therefore still return NULL although the mapping field is overloaded.
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm_types.h

@@ -64,7 +64,10 @@ struct page {
 #if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
 	    spinlock_t ptl;
 #endif
+	    struct {
 		   struct kmem_cache *slab;	/* SLUB: Pointer to slab */
+		   void *end;			/* SLUB: end marker */
+	    };
 	    struct page *first_page;	/* Compound tail pages */
 	};
 	union {

mm/slub.c

@@ -280,15 +280,32 @@ static inline struct kmem_cache_cpu *get_cpu_slab(struct kmem_cache *s, int cpu)
 #endif
 }
 
+/*
+ * The end pointer in a slab is special. It points to the first object in the
+ * slab but has bit 0 set to mark it.
+ *
+ * Note that SLUB relies on page_mapping returning NULL for pages with bit 0
+ * in the mapping set.
+ */
+static inline int is_end(void *addr)
+{
+	return (unsigned long)addr & PAGE_MAPPING_ANON;
+}
+
+void *slab_address(struct page *page)
+{
+	return page->end - PAGE_MAPPING_ANON;
+}
+
 static inline int check_valid_pointer(struct kmem_cache *s,
 				struct page *page, const void *object)
 {
 	void *base;
 
-	if (!object)
+	if (object == page->end)
 		return 1;
 
-	base = page_address(page);
+	base = slab_address(page);
 	if (object < base || object >= base + s->objects * s->size ||
 		(object - base) % s->size) {
 		return 0;
@@ -321,7 +338,8 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
 
 /* Scan freelist */
 #define for_each_free_object(__p, __s, __free) \
-	for (__p = (__free); __p; __p = get_freepointer((__s), __p))
+	for (__p = (__free); (__p) != page->end; __p = get_freepointer((__s),\
+		__p))
 
 /* Determine object index from a given position */
 static inline int slab_index(void *p, struct kmem_cache *s, void *addr)
@@ -473,7 +491,7 @@ static void slab_fix(struct kmem_cache *s, char *fmt, ...)
 static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
 {
 	unsigned int off;	/* Offset of last byte */
-	u8 *addr = page_address(page);
+	u8 *addr = slab_address(page);
 
 	print_tracking(s, p);
 
@@ -651,7 +669,7 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
 	if (!(s->flags & SLAB_POISON))
 		return 1;
 
-	start = page_address(page);
+	start = slab_address(page);
 	end = start + (PAGE_SIZE << s->order);
 	length = s->objects * s->size;
 	remainder = end - (start + length);
@@ -718,7 +736,7 @@ static int check_object(struct kmem_cache *s, struct page *page,
 		 * of the free objects in this slab. May cause
 		 * another error because the object count is now wrong.
 		 */
-		set_freepointer(s, p, NULL);
+		set_freepointer(s, p, page->end);
 		return 0;
 	}
 	return 1;
@@ -752,18 +770,18 @@ static int on_freelist(struct kmem_cache *s, struct page *page, void *search)
 	void *fp = page->freelist;
 	void *object = NULL;
 
-	while (fp && nr <= s->objects) {
+	while (fp != page->end && nr <= s->objects) {
 		if (fp == search)
 			return 1;
 		if (!check_valid_pointer(s, page, fp)) {
 			if (object) {
 				object_err(s, page, object,
 					"Freechain corrupt");
-				set_freepointer(s, object, NULL);
+				set_freepointer(s, object, page->end);
 				break;
 			} else {
 				slab_err(s, page, "Freepointer corrupt");
-				page->freelist = NULL;
+				page->freelist = page->end;
 				page->inuse = s->objects;
 				slab_fix(s, "Freelist cleared");
 				return 0;
@@ -869,7 +887,7 @@ static int alloc_debug_processing(struct kmem_cache *s, struct page *page,
 		 */
 		slab_fix(s, "Marking all objects used");
 		page->inuse = s->objects;
-		page->freelist = NULL;
+		page->freelist = page->end;
 	}
 	return 0;
 }
@@ -910,7 +928,7 @@ static int free_debug_processing(struct kmem_cache *s, struct page *page,
 	}
 
 	/* Special debug activities for freeing objects */
-	if (!SlabFrozen(page) && !page->freelist)
+	if (!SlabFrozen(page) && page->freelist == page->end)
 		remove_full(s, page);
 	if (s->flags & SLAB_STORE_USER)
 		set_track(s, object, TRACK_FREE, addr);
@@ -1102,6 +1120,7 @@ static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
 		SetSlabDebug(page);
 
 	start = page_address(page);
+	page->end = start + 1;
 
 	if (unlikely(s->flags & SLAB_POISON))
 		memset(start, POISON_INUSE, PAGE_SIZE << s->order);
@@ -1113,7 +1132,7 @@ static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
 		last = p;
 	}
 	setup_object(s, page, last);
-	set_freepointer(s, last, NULL);
+	set_freepointer(s, last, page->end);
 
 	page->freelist = start;
 	page->inuse = 0;
@@ -1129,7 +1148,7 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
 		void *p;
 
 		slab_pad_check(s, page);
-		for_each_object(p, s, page_address(page))
+		for_each_object(p, s, slab_address(page))
 			check_object(s, page, p, 0);
 		ClearSlabDebug(page);
 	}
@@ -1139,6 +1158,7 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
 		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE,
 		-pages);
 
+	page->mapping = NULL;
 	__free_pages(page, s->order);
 }
 
@@ -1341,7 +1361,7 @@ static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)
 	ClearSlabFrozen(page);
 	if (page->inuse) {
 
-		if (page->freelist)
+		if (page->freelist != page->end)
 			add_partial(n, page, tail);
 		else if (SlabDebug(page) && (s->flags & SLAB_STORE_USER))
 			add_full(n, page);
@@ -1377,8 +1397,12 @@ static void deactivate_slab(struct kmem_cache *s, struct kmem_cache_cpu *c)
 	 * Merge cpu freelist into freelist. Typically we get here
 	 * because both freelists are empty. So this is unlikely
 	 * to occur.
+	 *
+	 * We need to use _is_end here because deactivate slab may
+	 * be called for a debug slab. Then c->freelist may contain
+	 * a dummy pointer.
 	 */
-	while (unlikely(c->freelist)) {
+	while (unlikely(!is_end(c->freelist))) {
 		void **object;
 
 		tail = 0;	/* Hot objects. Put the slab first */
@@ -1478,7 +1502,7 @@ static void *__slab_alloc(struct kmem_cache *s,
 		goto another_slab;
 load_freelist:
 	object = c->page->freelist;
-	if (unlikely(!object))
+	if (unlikely(object == c->page->end))
 		goto another_slab;
 	if (unlikely(SlabDebug(c->page)))
 		goto debug;
@@ -1486,7 +1510,7 @@ static void *__slab_alloc(struct kmem_cache *s,
 	object = c->page->freelist;
 	c->freelist = object[c->offset];
 	c->page->inuse = s->objects;
-	c->page->freelist = NULL;
+	c->page->freelist = c->page->end;
 	c->node = page_to_nid(c->page);
 	slab_unlock(c->page);
 	return object;
@@ -1550,7 +1574,7 @@ static __always_inline void *slab_alloc(struct kmem_cache *s,
 
 	local_irq_save(flags);
 	c = get_cpu_slab(s, smp_processor_id());
-	if (unlikely(!c->freelist || !node_match(c, node)))
+	if (unlikely(is_end(c->freelist) || !node_match(c, node)))
 
 		object = __slab_alloc(s, gfpflags, node, addr, c);
 
@@ -1614,7 +1638,7 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 	 * was not on the partial list before
 	 * then add it.
 	 */
-	if (unlikely(!prior))
+	if (unlikely(prior == page->end))
 		add_partial(get_node(s, page_to_nid(page)), page, 1);
 
 out_unlock:
@@ -1622,7 +1646,7 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 	return;
 
 slab_empty:
-	if (prior)
+	if (prior != page->end)
 		/*
 		 * Slab still on the partial list.
 		 */
@@ -1842,7 +1866,7 @@ static void init_kmem_cache_cpu(struct kmem_cache *s,
 			struct kmem_cache_cpu *c)
 {
 	c->page = NULL;
-	c->freelist = NULL;
+	c->freelist = (void *)PAGE_MAPPING_ANON;
 	c->node = 0;
 	c->offset = s->offset / sizeof(void *);
 	c->objsize = s->objsize;
@@ -3105,7 +3129,7 @@ static int validate_slab(struct kmem_cache *s, struct page *page,
 						unsigned long *map)
 {
 	void *p;
-	void *addr = page_address(page);
+	void *addr = slab_address(page);
 
 	if (!check_slab(s, page) ||
 			!on_freelist(s, page, NULL))
@@ -3385,7 +3409,7 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
 static void process_slab(struct loc_track *t, struct kmem_cache *s,
 		struct page *page, enum track_item alloc)
 {
-	void *addr = page_address(page);
+	void *addr = slab_address(page);
 	DECLARE_BITMAP(map, s->objects);
 	void *p;