mm, slub: move reset of c->page and freelist out of deactivate_slab()

deactivate_slab() removes the cpu slab by merging the cpu freelist with slab's
freelist and putting the slab on the proper node's list. It also sets the
respective kmem_cache_cpu pointers to NULL.

By extracting the kmem_cache_cpu operations from the function, we can make it
not dependent on disabled irqs.

Also if we return a single free pointer from ___slab_alloc, we no longer have
to assign kmem_cache_cpu.page before deactivation or care if somebody preempted
us and assigned a different page to our kmem_cache_cpu in the process.
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slub.c

@@ -2209,10 +2209,13 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
 }
 
 /*
- * Remove the cpu slab
+ * Finishes removing the cpu slab. Merges cpu's freelist with page's freelist,
+ * unfreezes the slabs and puts it on the proper list.
+ * Assumes the slab has been already safely taken away from kmem_cache_cpu
+ * by the caller.
  */
 static void deactivate_slab(struct kmem_cache *s, struct page *page,
-				void *freelist, struct kmem_cache_cpu *c)
+			    void *freelist)
 {
 	enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
 	struct kmem_cache_node *n = get_node(s, page_to_nid(page));
@@ -2341,9 +2344,6 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
 		discard_slab(s, page);
 		stat(s, FREE_SLAB);
 	}
-
-	c->page = NULL;
-	c->freelist = NULL;
 }
 
 /*
@@ -2468,10 +2468,16 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 
 static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c)
 {
-	stat(s, CPUSLAB_FLUSH);
-	deactivate_slab(s, c->page, c->freelist, c);
+	void *freelist = c->freelist;
+	struct page *page = c->page;
 
+	c->page = NULL;
+	c->freelist = NULL;
 	c->tid = next_tid(c->tid);
+
+	deactivate_slab(s, page, freelist);
+
+	stat(s, CPUSLAB_FLUSH);
 }
 
 /*
@@ -2769,7 +2775,10 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 		local_irq_restore(flags);
 		goto reread_page;
 	}
-	deactivate_slab(s, page, c->freelist, c);
+	freelist = c->freelist;
+	c->page = NULL;
+	c->freelist = NULL;
+	deactivate_slab(s, page, freelist);
 	local_irq_restore(flags);
 
 new_slab:
@@ -2848,11 +2857,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 return_single:
 
 	local_irq_save(flags);
-	if (unlikely(c->page))
-		flush_slab(s, c);
-	c->page = page;
-
-	deactivate_slab(s, page, get_freepointer(s, freelist), c);
+	deactivate_slab(s, page, get_freepointer(s, freelist));
 	local_irq_restore(flags);
 	return freelist;
 }