[PATCH -mm 3/6] slub: introduce __kfree_rcu

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Introduce __kfree_rcu() for kfree_rcu()

It like __kfree_rcu() in slab.c.
We can calculate the object poiter from a poiter inside this
object in slub.c, so we can use it for rcu callback and free
the object.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/mm/slub.c b/mm/slub.c
index 0280eee..b438ec5 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2756,6 +2756,30 @@ void kfree(const void *x)
 }
 EXPORT_SYMBOL(kfree);
 
+static void kfree_rcu_callback(struct rcu_head *rcu)
+{
+	void *portion = rcu;
+	struct page *page = virt_to_head_page(portion);
+
+	if (unlikely(!PageSlab(page))) {
+		BUG_ON(!PageCompound(page));
+		put_page(page);
+	} else {
+		struct kmem_cache *s = page->slab;
+		void *start = page_address(page);
+		unsigned int index = (portion - start) / s->size;
+		void *object = start + s->size * index;
+
+		slab_free(s, page, object, _RET_IP_);
+	}
+}
+
+void __kfree_rcu(const void *x, struct rcu_head *rcu)
+{
+	call_rcu(rcu, kfree_rcu_callback);
+}
+EXPORT_SYMBOL(__kfree_rcu);
+
 /*
  * kmem_cache_shrink removes empty slabs from the partial lists and sorts
  * the remaining slabs by the number of items in use. The slabs with the