mm/slub: perform free consistency checks before call_rcu

For SLAB_TYPESAFE_BY_RCU caches we use call_rcu to perform empty slab
freeing. The rcu callback rcu_free_slab() calls __free_slab() that
currently includes checking the slab consistency for caches with
SLAB_CONSISTENCY_CHECKS flags. This check needs the slab->objects field
to be intact.

Because in the next patch we want to allow rcu_head in struct slab to
become larger in debug configurations and thus potentially overwrite
more fields through a union than slab_list, we want to limit the fields
used in rcu_free_slab().  Thus move the consistency checks to
free_slab() before call_rcu(). This can be done safely even for
SLAB_TYPESAFE_BY_RCU caches where accesses to the objects can still
occur after freeing them.

As a result, only the slab->slab_cache field has to be physically
separate from rcu_head for the freeing callback to work. We also save
some cycles in the rcu callback for caches with consistency checks
enabled.
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

mm/slub.c

@@ -1999,14 +1999,6 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
 	int order = folio_order(folio);
 	int pages = 1 << order;
 
-	if (kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
-		void *p;
-
-		slab_pad_check(s, slab);
-		for_each_object(p, s, slab_address(slab), slab->objects)
-			check_object(s, slab, p, SLUB_RED_INACTIVE);
-	}
-
 	__slab_clear_pfmemalloc(slab);
 	__folio_clear_slab(folio);
 	folio->mapping = NULL;
@@ -2025,9 +2017,17 @@ static void rcu_free_slab(struct rcu_head *h)
 
 static void free_slab(struct kmem_cache *s, struct slab *slab)
 {
-	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU)) {
+	if (kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
+		void *p;
+
+		slab_pad_check(s, slab);
+		for_each_object(p, s, slab_address(slab), slab->objects)
+			check_object(s, slab, p, SLUB_RED_INACTIVE);
+	}
+
+	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
 		call_rcu(&slab->rcu_head, rcu_free_slab);
-	} else
+	else
 		__free_slab(s, slab);
 }