mm, slub: introduce static key for slub_debug()

One advantage of CONFIG_SLUB_DEBUG is that a generic distro kernel can be
built with the option enabled, but it's inactive until simply enabled on
boot, without rebuilding the kernel.  With a static key, we can further
eliminate the overhead of checking whether a cache has a particular debug
flag enabled if we know that there are no such caches (slub_debug was not
enabled during boot).  We use the same mechanism also for e.g.
page_owner, debug_pagealloc or kmemcg functionality.

This patch introduces the static key and makes the general check for
per-cache debug flags kmem_cache_debug() use it.  This benefits several
call sites, including (slow path but still rather frequent) __slab_free().
The next patches will add more uses.

Signed-off-by: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Vijayanand Jitta <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Pekka Enberg <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/slub.c

@@ -114,13 +114,21 @@
  * 			the fast path and disables lockless freelists.
  */
 
+#ifdef CONFIG_SLUB_DEBUG
+#ifdef CONFIG_SLUB_DEBUG_ON
+DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
+#else
+DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
+#endif
+#endif
+
 static inline int kmem_cache_debug(struct kmem_cache *s)
 {
 #ifdef CONFIG_SLUB_DEBUG
-	return unlikely(s->flags & SLAB_DEBUG_FLAGS);
-#else
-	return 0;
+	if (static_branch_unlikely(&slub_debug_enabled))
+		return s->flags & SLAB_DEBUG_FLAGS;
 #endif
+	return 0;
 }
 
 void *fixup_red_left(struct kmem_cache *s, void *p)
@@ -1389,6 +1397,8 @@ static int __init setup_slub_debug(char *str)
 		slub_debug_string = saved_str;
 	}
 out:
+	if (slub_debug != 0 || slub_debug_string)
+		static_branch_enable(&slub_debug_enabled);
 	if ((static_branch_unlikely(&init_on_alloc) ||
 	     static_branch_unlikely(&init_on_free)) &&
 	    (slub_debug & SLAB_POISON))