slab: embed memcg_cache_params to kmem_cache

Currently, kmem_cache stores a pointer to struct memcg_cache_params
instead of embedding it.  The rationale is to save memory when kmem
accounting is disabled.  However, the memcg_cache_params has shrivelled
drastically since it was first introduced:

* Initially:

struct memcg_cache_params {
	bool is_root_cache;
	union {
		struct kmem_cache *memcg_caches[0];
		struct {
			struct mem_cgroup *memcg;
			struct list_head list;
			struct kmem_cache *root_cache;
			bool dead;
			atomic_t nr_pages;
			struct work_struct destroy;
		};
	};
};

* Now:

struct memcg_cache_params {
	bool is_root_cache;
	union {
		struct {
			struct rcu_head rcu_head;
			struct kmem_cache *memcg_caches[0];
		};
		struct {
			struct mem_cgroup *memcg;
			struct kmem_cache *root_cache;
		};
	};
};

So the memory saving does not seem to be a clear win anymore.

OTOH, keeping a pointer to memcg_cache_params struct instead of embedding
it results in touching one more cache line on kmem alloc/free hot paths.
Besides, it makes linking kmem caches in a list chained by a field of
struct memcg_cache_params really painful due to a level of indirection,
while I want to make them linked in the following patch.  That said, let
us embed it.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

7 files changed, 111 insertions, 103 deletions

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 2e3b448cfa2d..1e03c11bbfbd 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -473,14 +473,14 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 #ifndef ARCH_SLAB_MINALIGN
 #define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
 #endif
+
+struct memcg_cache_array {
+        struct rcu_head rcu;
+        struct kmem_cache *entries[0];
+};
+
 /*
  * This is the main placeholder for memcg-related information in kmem caches.
- * struct kmem_cache will hold a pointer to it, so the memory cost while
- * disabled is 1 pointer. The runtime cost while enabled, gets bigger than it
- * would otherwise be if that would be bundled in kmem_cache: we'll need an
- * extra pointer chase. But the trade off clearly lays in favor of not
- * penalizing non-users.
- *
  * Both the root cache and the child caches will have it. For the root cache,
  * this will hold a dynamically allocated array large enough to hold
  * information about the currently limited memcgs in the system. To allow the
@@ -495,10 +495,7 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 struct memcg_cache_params {
         bool is_root_cache;
         union {
-                struct {
-                        struct rcu_head rcu_head;
-                        struct kmem_cache *memcg_caches[0];
-                };
+                struct memcg_cache_array __rcu *memcg_caches;
                 struct {
                         struct mem_cgroup *memcg;
                         struct kmem_cache *root_cache;
diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
index b869d1662ba3..33d049066c3d 100644
--- a/include/linux/slab_def.h
+++ b/include/linux/slab_def.h
@@ -70,7 +70,7 @@ struct kmem_cache {
         int obj_offset;
 #endif /* CONFIG_DEBUG_SLAB */
 #ifdef CONFIG_MEMCG_KMEM
-        struct memcg_cache_params *memcg_params;
+        struct memcg_cache_params memcg_params;
 #endif
 
         struct kmem_cache_node *node[MAX_NUMNODES];
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index d82abd40a3c0..9abf04ed0999 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -85,7 +85,7 @@ struct kmem_cache {
         struct kobject kobj;    /* For sysfs */
 #endif
 #ifdef CONFIG_MEMCG_KMEM
-        struct memcg_cache_params *memcg_params;
+        struct memcg_cache_params memcg_params;
         int max_attr_size; /* for propagation, maximum size of a stored attr */
 #ifdef CONFIG_SYSFS
         struct kset *memcg_kset;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index afa55bb38cbd..6f3c0fcd7a2d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -332,7 +332,7 @@ struct mem_cgroup {
         struct cg_proto tcp_mem;
 #endif
 #if defined(CONFIG_MEMCG_KMEM)
-        /* Index in the kmem_cache->memcg_params->memcg_caches array */
+        /* Index in the kmem_cache->memcg_params.memcg_caches array */
         int kmemcg_id;
 #endif
 
@@ -531,7 +531,7 @@ static void disarm_sock_keys(struct mem_cgroup *memcg)
 
 #ifdef CONFIG_MEMCG_KMEM
 /*
- * This will be the memcg's index in each cache's ->memcg_params->memcg_caches.
+ * This will be the memcg's index in each cache's ->memcg_params.memcg_caches.
  * The main reason for not using cgroup id for this:
  *  this works better in sparse environments, where we have a lot of memcgs,
  *  but only a few kmem-limited. Or also, if we have, for instance, 200
@@ -2667,8 +2667,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep)
         struct mem_cgroup *memcg;
         struct kmem_cache *memcg_cachep;
 
-        VM_BUG_ON(!cachep->memcg_params);
-        VM_BUG_ON(!cachep->memcg_params->is_root_cache);
+        VM_BUG_ON(!is_root_cache(cachep));
 
         if (current->memcg_kmem_skip_account)
                 return cachep;
@@ -2702,7 +2701,7 @@ out:
 void __memcg_kmem_put_cache(struct kmem_cache *cachep)
 {
         if (!is_root_cache(cachep))
-                css_put(&cachep->memcg_params->memcg->css);
+                css_put(&cachep->memcg_params.memcg->css);
 }
 
 /*
@@ -2778,7 +2777,7 @@ struct mem_cgroup *__mem_cgroup_from_kmem(void *ptr)
         if (PageSlab(page)) {
                 cachep = page->slab_cache;
                 if (!is_root_cache(cachep))
-                        memcg = cachep->memcg_params->memcg;
+                        memcg = cachep->memcg_params.memcg;
         } else
                 /* page allocated by alloc_kmem_pages */
                 memcg = page->mem_cgroup;
diff --git a/mm/slab.h b/mm/slab.h
index 90430d6f665e..53a623f85931 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -86,8 +86,6 @@ extern struct kmem_cache *create_kmalloc_cache(const char *name, size_t size,
 extern void create_boot_cache(struct kmem_cache *, const char *name,
                         size_t size, unsigned long flags);
 
-struct mem_cgroup;
-
 int slab_unmergeable(struct kmem_cache *s);
 struct kmem_cache *find_mergeable(size_t size, size_t align,
                 unsigned long flags, const char *name, void (*ctor)(void *));
@@ -167,14 +165,13 @@ ssize_t slabinfo_write(struct file *file, const char __user *buffer,
 #ifdef CONFIG_MEMCG_KMEM
 static inline bool is_root_cache(struct kmem_cache *s)
 {
-        return !s->memcg_params || s->memcg_params->is_root_cache;
+        return s->memcg_params.is_root_cache;
 }
 
 static inline bool slab_equal_or_root(struct kmem_cache *s,
-                                        struct kmem_cache *p)
+                                      struct kmem_cache *p)
 {
-        return (p == s) ||
-                (s->memcg_params && (p == s->memcg_params->root_cache));
+        return p == s || p == s->memcg_params.root_cache;
 }
 
 /*
@@ -185,37 +182,30 @@ static inline bool slab_equal_or_root(struct kmem_cache *s,
 static inline const char *cache_name(struct kmem_cache *s)
 {
         if (!is_root_cache(s))
-                return s->memcg_params->root_cache->name;
+                s = s->memcg_params.root_cache;
         return s->name;
 }
 
 /*
  * Note, we protect with RCU only the memcg_caches array, not per-memcg caches.
- * That said the caller must assure the memcg's cache won't go away. Since once
- * created a memcg's cache is destroyed only along with the root cache, it is
- * true if we are going to allocate from the cache or hold a reference to the
- * root cache by other means. Otherwise, we should hold either the slab_mutex
- * or the memcg's slab_caches_mutex while calling this function and accessing
- * the returned value.
+ * That said the caller must assure the memcg's cache won't go away by either
+ * taking a css reference to the owner cgroup, or holding the slab_mutex.
  */
 static inline struct kmem_cache *
 cache_from_memcg_idx(struct kmem_cache *s, int idx)
 {
         struct kmem_cache *cachep;
-        struct memcg_cache_params *params;
-
-        if (!s->memcg_params)
-                return NULL;
+        struct memcg_cache_array *arr;
 
         rcu_read_lock();
-        params = rcu_dereference(s->memcg_params);
+        arr = rcu_dereference(s->memcg_params.memcg_caches);
 
         /*
          * Make sure we will access the up-to-date value. The code updating
          * memcg_caches issues a write barrier to match this (see
-         * memcg_register_cache()).
+         * memcg_create_kmem_cache()).
          */
-        cachep = lockless_dereference(params->memcg_caches[idx]);
+        cachep = lockless_dereference(arr->entries[idx]);
         rcu_read_unlock();
 
         return cachep;
@@ -225,7 +215,7 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
 {
         if (is_root_cache(s))
                 return s;
-        return s->memcg_params->root_cache;
+        return s->memcg_params.root_cache;
 }
 
 static __always_inline int memcg_charge_slab(struct kmem_cache *s,
@@ -235,7 +225,7 @@ static __always_inline int memcg_charge_slab(struct kmem_cache *s,
                 return 0;
         if (is_root_cache(s))
                 return 0;
-        return memcg_charge_kmem(s->memcg_params->memcg, gfp, 1 << order);
+        return memcg_charge_kmem(s->memcg_params.memcg, gfp, 1 << order);
 }
 
 static __always_inline void memcg_uncharge_slab(struct kmem_cache *s, int order)
@@ -244,9 +234,13 @@ static __always_inline void memcg_uncharge_slab(struct kmem_cache *s, int order)
                 return;
         if (is_root_cache(s))
                 return;
-        memcg_uncharge_kmem(s->memcg_params->memcg, 1 << order);
+        memcg_uncharge_kmem(s->memcg_params.memcg, 1 << order);
 }
-#else
+
+extern void slab_init_memcg_params(struct kmem_cache *);
+
+#else /* !CONFIG_MEMCG_KMEM */
+
 static inline bool is_root_cache(struct kmem_cache *s)
 {
         return true;
@@ -282,7 +276,11 @@ static inline int memcg_charge_slab(struct kmem_cache *s, gfp_t gfp, int order)
 static inline void memcg_uncharge_slab(struct kmem_cache *s, int order)
 {
 }
-#endif
+
+static inline void slab_init_memcg_params(struct kmem_cache *s)
+{
+}
+#endif /* CONFIG_MEMCG_KMEM */
 
 static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
 {
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 23f5fcde6043..7cc32cf126ef 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -106,62 +106,66 @@ static inline int kmem_cache_sanity_check(const char *name, size_t size)
 #endif
 
 #ifdef CONFIG_MEMCG_KMEM
-static int memcg_alloc_cache_params(struct mem_cgroup *memcg,
-                struct kmem_cache *s, struct kmem_cache *root_cache)
+void slab_init_memcg_params(struct kmem_cache *s)
 {
-        size_t size;
+        s->memcg_params.is_root_cache = true;
+        RCU_INIT_POINTER(s->memcg_params.memcg_caches, NULL);
+}
+
+static int init_memcg_params(struct kmem_cache *s,
+                struct mem_cgroup *memcg, struct kmem_cache *root_cache)
+{
+        struct memcg_cache_array *arr;
 
-        if (!memcg_kmem_enabled())
+        if (memcg) {
+                s->memcg_params.is_root_cache = false;
+                s->memcg_params.memcg = memcg;
+                s->memcg_params.root_cache = root_cache;
                 return 0;
+        }
 
-        if (!memcg) {
-                size = offsetof(struct memcg_cache_params, memcg_caches);
-                size += memcg_nr_cache_ids * sizeof(void *);
-        } else
-                size = sizeof(struct memcg_cache_params);
+        slab_init_memcg_params(s);
 
-        s->memcg_params = kzalloc(size, GFP_KERNEL);
-        if (!s->memcg_params)
-                return -ENOMEM;
+        if (!memcg_nr_cache_ids)
+                return 0;
 
-        if (memcg) {
-                s->memcg_params->memcg = memcg;
-                s->memcg_params->root_cache = root_cache;
-        } else
-                s->memcg_params->is_root_cache = true;
+        arr = kzalloc(sizeof(struct memcg_cache_array) +
+                      memcg_nr_cache_ids * sizeof(void *),
+                      GFP_KERNEL);
+        if (!arr)
+                return -ENOMEM;
 
+        RCU_INIT_POINTER(s->memcg_params.memcg_caches, arr);
         return 0;
 }
 
-static void memcg_free_cache_params(struct kmem_cache *s)
+static void destroy_memcg_params(struct kmem_cache *s)
 {
-        kfree(s->memcg_params);
+        if (is_root_cache(s))
+                kfree(rcu_access_pointer(s->memcg_params.memcg_caches));
 }
 
-static int memcg_update_cache_params(struct kmem_cache *s, int num_memcgs)
+static int update_memcg_params(struct kmem_cache *s, int new_array_size)
 {
-        int size;
-        struct memcg_cache_params *new_params, *cur_params;
+        struct memcg_cache_array *old, *new;
 
-        BUG_ON(!is_root_cache(s));
-
-        size = offsetof(struct memcg_cache_params, memcg_caches);
-        size += num_memcgs * sizeof(void *);
+        if (!is_root_cache(s))
+                return 0;
 
-        new_params = kzalloc(size, GFP_KERNEL);
-        if (!new_params)
+        new = kzalloc(sizeof(struct memcg_cache_array) +
+                      new_array_size * sizeof(void *), GFP_KERNEL);
+        if (!new)
                 return -ENOMEM;
 
-        cur_params = s->memcg_params;
-        memcpy(new_params->memcg_caches, cur_params->memcg_caches,
-               memcg_nr_cache_ids * sizeof(void *));
-
-        new_params->is_root_cache = true;
-
-        rcu_assign_pointer(s->memcg_params, new_params);
-        if (cur_params)
-                kfree_rcu(cur_params, rcu_head);
+        old = rcu_dereference_protected(s->memcg_params.memcg_caches,
+                                        lockdep_is_held(&slab_mutex));
+        if (old)
+                memcpy(new->entries, old->entries,
+                       memcg_nr_cache_ids * sizeof(void *));
 
+        rcu_assign_pointer(s->memcg_params.memcg_caches, new);
+        if (old)
+                kfree_rcu(old, rcu);
         return 0;
 }
 
@@ -172,10 +176,7 @@ int memcg_update_all_caches(int num_memcgs)
 
         mutex_lock(&slab_mutex);
         list_for_each_entry(s, &slab_caches, list) {
-                if (!is_root_cache(s))
-                        continue;
-
-                ret = memcg_update_cache_params(s, num_memcgs);
+                ret = update_memcg_params(s, num_memcgs);
                 /*
                  * Instead of freeing the memory, we'll just leave the caches
                  * up to this point in an updated state.
@@ -187,13 +188,13 @@ int memcg_update_all_caches(int num_memcgs)
         return ret;
 }
 #else
-static inline int memcg_alloc_cache_params(struct mem_cgroup *memcg,
-                struct kmem_cache *s, struct kmem_cache *root_cache)
+static inline int init_memcg_params(struct kmem_cache *s,
+                struct mem_cgroup *memcg, struct kmem_cache *root_cache)
 {
         return 0;
 }
 
-static inline void memcg_free_cache_params(struct kmem_cache *s)
+static inline void destroy_memcg_params(struct kmem_cache *s)
 {
 }
 #endif /* CONFIG_MEMCG_KMEM */
@@ -311,7 +312,7 @@ do_kmem_cache_create(char *name, size_t object_size, size_t size, size_t align,
         s->align = align;
         s->ctor = ctor;
 
-        err = memcg_alloc_cache_params(memcg, s, root_cache);
+        err = init_memcg_params(s, memcg, root_cache);
         if (err)
                 goto out_free_cache;
 
@@ -327,7 +328,7 @@ out:
         return s;
 
 out_free_cache:
-        memcg_free_cache_params(s);
+        destroy_memcg_params(s);
         kmem_cache_free(kmem_cache, s);
         goto out;
 }
@@ -439,11 +440,15 @@ static int do_kmem_cache_shutdown(struct kmem_cache *s,
 
 #ifdef CONFIG_MEMCG_KMEM
         if (!is_root_cache(s)) {
-                struct kmem_cache *root_cache = s->memcg_params->root_cache;
-                int memcg_id = memcg_cache_id(s->memcg_params->memcg);
-
-                BUG_ON(root_cache->memcg_params->memcg_caches[memcg_id] != s);
-                root_cache->memcg_params->memcg_caches[memcg_id] = NULL;
+                int idx;
+                struct memcg_cache_array *arr;
+
+                idx = memcg_cache_id(s->memcg_params.memcg);
+                arr = rcu_dereference_protected(s->memcg_params.root_cache->
+                                                memcg_params.memcg_caches,
+                                                lockdep_is_held(&slab_mutex));
+                BUG_ON(arr->entries[idx] != s);
+                arr->entries[idx] = NULL;
         }
 #endif
         list_move(&s->list, release);
@@ -481,27 +486,32 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
                              struct kmem_cache *root_cache)
 {
         static char memcg_name_buf[NAME_MAX + 1]; /* protected by slab_mutex */
-        int memcg_id = memcg_cache_id(memcg);
+        struct memcg_cache_array *arr;
         struct kmem_cache *s = NULL;
         char *cache_name;
+        int idx;
 
         get_online_cpus();
         get_online_mems();
 
         mutex_lock(&slab_mutex);
 
+        idx = memcg_cache_id(memcg);
+        arr = rcu_dereference_protected(root_cache->memcg_params.memcg_caches,
+                                        lockdep_is_held(&slab_mutex));
+
         /*
          * Since per-memcg caches are created asynchronously on first
          * allocation (see memcg_kmem_get_cache()), several threads can try to
          * create the same cache, but only one of them may succeed.
          */
-        if (cache_from_memcg_idx(root_cache, memcg_id))
+        if (arr->entries[idx])
                 goto out_unlock;
 
         cgroup_name(mem_cgroup_css(memcg)->cgroup,
                     memcg_name_buf, sizeof(memcg_name_buf));
         cache_name = kasprintf(GFP_KERNEL, "%s(%d:%s)", root_cache->name,
-                               memcg_cache_id(memcg), memcg_name_buf);
+                               idx, memcg_name_buf);
         if (!cache_name)
                 goto out_unlock;
 
@@ -525,7 +535,7 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
          * initialized.
          */
         smp_wmb();
-        root_cache->memcg_params->memcg_caches[memcg_id] = s;
+        arr->entries[idx] = s;
 
 out_unlock:
         mutex_unlock(&slab_mutex);
@@ -545,7 +555,7 @@ void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
 
         mutex_lock(&slab_mutex);
         list_for_each_entry_safe(s, s2, &slab_caches, list) {
-                if (is_root_cache(s) || s->memcg_params->memcg != memcg)
+                if (is_root_cache(s) || s->memcg_params.memcg != memcg)
                         continue;
                 /*
                  * The cgroup is about to be freed and therefore has no charges
@@ -564,7 +574,7 @@ void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
 
 void slab_kmem_cache_release(struct kmem_cache *s)
 {
-        memcg_free_cache_params(s);
+        destroy_memcg_params(s);
         kfree(s->name);
         kmem_cache_free(kmem_cache, s);
 }
@@ -640,6 +650,9 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name, size_t siz
         s->name = name;
         s->size = s->object_size = size;
         s->align = calculate_alignment(flags, ARCH_KMALLOC_MINALIGN, size);
+
+        slab_init_memcg_params(s);
+
         err = __kmem_cache_create(s, flags);
 
         if (err)
@@ -980,7 +993,7 @@ int memcg_slab_show(struct seq_file *m, void *p)
 
         if (p == slab_caches.next)
                 print_slabinfo_header(m);
-        if (!is_root_cache(s) && s->memcg_params->memcg == memcg)
+        if (!is_root_cache(s) && s->memcg_params.memcg == memcg)
                 cache_show(s, m);
         return 0;
 }
diff --git a/mm/slub.c b/mm/slub.c
index 8b8508adf9c2..75d55fdfe3a1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3577,6 +3577,7 @@ static struct kmem_cache * __init bootstrap(struct kmem_cache *static_cache)
                         p->slab_cache = s;
 #endif
         }
+        slab_init_memcg_params(s);
         list_add(&s->list, &slab_caches);
         return s;
 }
@@ -4964,7 +4965,7 @@ static void memcg_propagate_slab_attrs(struct kmem_cache *s)
         if (is_root_cache(s))
                 return;
 
-        root_cache = s->memcg_params->root_cache;
+        root_cache = s->memcg_params.root_cache;
 
         /*
          * This mean this cache had no attribute written. Therefore, no point
@@ -5044,7 +5045,7 @@ static inline struct kset *cache_kset(struct kmem_cache *s)
 {
 #ifdef CONFIG_MEMCG_KMEM
         if (!is_root_cache(s))
-                return s->memcg_params->root_cache->memcg_kset;
+                return s->memcg_params.root_cache->memcg_kset;
 #endif
         return slab_kset;
 }