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 <[e-mail]>
Cc: Pekka Enberg <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mm_types.h

···
 #if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
 	    spinlock_t ptl;
 #endif
-	    struct kmem_cache *slab;	/* SLUB: Pointer to slab */
+	    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

···
 #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;
···
 
 /* 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)
···
 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);
 
···
 	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);
···
 		 * 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;
···
 	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;
···
 		 */
 		slab_fix(s, "Marking all objects used");
 		page->inuse = s->objects;
-		page->freelist = NULL;
+		page->freelist = page->end;
 	}
 	return 0;
 }
···
 	}
 
 	/* 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);
···
 		SetSlabDebug(page);
 
 	start = page_address(page);
+	page->end = start + 1;
 
 	if (unlikely(s->flags & SLAB_POISON))
 		memset(start, POISON_INUSE, PAGE_SIZE << s->order);
···
 		last = p;
 	}
 	setup_object(s, page, last);
-	set_freepointer(s, last, NULL);
+	set_freepointer(s, last, page->end);
 
 	page->freelist = start;
 	page->inuse = 0;
···
 		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);
 	}
···
 		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE,
 		-pages);
 
+	page->mapping = NULL;
 	__free_pages(page, s->order);
 }
 
···
 	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);
···
 	 * 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 */
···
 		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;
···
 	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;
···
 
 	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);
 
···
 	 * 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:
···
 	return;
 
 slab_empty:
-	if (prior)
+	if (prior != page->end)
 		/*
 		 * Slab still on the partial list.
 		 */
···
 			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;
···
 						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))
···
 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;