mm: memcg/slab: use a single set of kmem_caches for all accounted allocations

This is fairly big but mostly red patch, which makes all accounted slab
allocations use a single set of kmem_caches instead of creating a separate
set for each memory cgroup.

Because the number of non-root kmem_caches is now capped by the number of
root kmem_caches, there is no need to shrink or destroy them prematurely.
They can be perfectly destroyed together with their root counterparts.
This allows to dramatically simplify the management of non-root
kmem_caches and delete a ton of code.

This patch performs the following changes:
1) introduces memcg_params.memcg_cache pointer to represent the
   kmem_cache which will be used for all non-root allocations
2) reuses the existing memcg kmem_cache creation mechanism
   to create memcg kmem_cache on the first allocation attempt
3) memcg kmem_caches are named <kmemcache_name>-memcg,
   e.g. dentry-memcg
4) simplifies memcg_kmem_get_cache() to just return memcg kmem_cache
   or schedule it's creation and return the root cache
5) removes almost all non-root kmem_cache management code
   (separate refcounter, reparenting, shrinking, etc)
6) makes slab debugfs to display root_mem_cgroup css id and never
   show :dead and :deact flags in the memcg_slabinfo attribute.

Following patches in the series will simplify the kmem_cache creation.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/20200623174037.3951353-13-guro@fb.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -4204,36 +4204,6 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 	return ret;
 }
 
-#ifdef CONFIG_MEMCG
-void __kmemcg_cache_deactivate_after_rcu(struct kmem_cache *s)
-{
-	/*
-	 * Called with all the locks held after a sched RCU grace period.
-	 * Even if @s becomes empty after shrinking, we can't know that @s
-	 * doesn't have allocations already in-flight and thus can't
-	 * destroy @s until the associated memcg is released.
-	 *
-	 * However, let's remove the sysfs files for empty caches here.
-	 * Each cache has a lot of interface files which aren't
-	 * particularly useful for empty draining caches; otherwise, we can
-	 * easily end up with millions of unnecessary sysfs files on
-	 * systems which have a lot of memory and transient cgroups.
-	 */
-	if (!__kmem_cache_shrink(s))
-		sysfs_slab_remove(s);
-}
-
-void __kmemcg_cache_deactivate(struct kmem_cache *s)
-{
-	/*
-	 * Disable empty slabs caching. Used to avoid pinning offline
-	 * memory cgroups by kmem pages that can be freed.
-	 */
-	slub_set_cpu_partial(s, 0);
-	s->min_partial = 0;
-}
-#endif	/* CONFIG_MEMCG */
-
 static int slab_mem_going_offline_callback(void *arg)
 {
 	struct kmem_cache *s;
@@ -4390,7 +4360,7 @@ static struct kmem_cache * __init bootstrap(struct kmem_cache *static_cache)
 	}
 	slab_init_memcg_params(s);
 	list_add(&s->list, &slab_caches);
-	memcg_link_cache(s, NULL);
+	memcg_link_cache(s);
 	return s;
 }
 
@@ -4458,7 +4428,8 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
 		s->object_size = max(s->object_size, size);
 		s->inuse = max(s->inuse, ALIGN(size, sizeof(void *)));
 
-		for_each_memcg_cache(c, s) {
+		c = memcg_cache(s);
+		if (c) {
 			c->object_size = s->object_size;
 			c->inuse = max(c->inuse, ALIGN(size, sizeof(void *)));
 		}
@@ -5591,7 +5562,8 @@ static ssize_t slab_attr_store(struct kobject *kobj,
 		 * directly either failed or succeeded, in which case we loop
 		 * through the descendants with best-effort propagation.
 		 */
-		for_each_memcg_cache(c, s)
+		c = memcg_cache(s);
+		if (c)
 			attribute->store(c, buf, len);
 		mutex_unlock(&slab_mutex);
 	}