[PATCH -mm 4/6] memcg: add pointer to owner cache to memcg_cache_params

[Vladimir Davydov <vdavydov@parallels.com>]

We don't keep a pointer to the owner kmem cache in the
memcg_cache_params struct, because we can always get the cache by
reading the slot corresponding to the owner memcg in the root cache's
memcg_caches array (see memcg_params_to_cache).

However, this means that offline css's, which can be zombieing around
for quite a long time, will occupy slots in memcg_caches arrays, making
them grow larger and larger, which doesn't sound good. Therefore I'm
going to make memcg release the slots on offline, which will render
memcg_params_to_cache invalid. So I'm removing it and adding a back
pointer to memcg_cache_params instead.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
---
 include/linux/slab.h |    2 ++
 mm/memcontrol.c      |   20 ++++----------------
 2 files changed, 6 insertions(+), 16 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 14888328e96b..e6e6ddb769c7 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -523,6 +523,7 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
  *
  * Child caches will hold extra metadata needed for its operation. Fields are:
  *
+ * @cachep: cache which this struct is for
  * @memcg: pointer to the memcg this cache belongs to
  * @list: list_head for the list of all caches in this memcg
  * @root_cache: pointer to the global, root cache, this cache was derived from
@@ -536,6 +537,7 @@ struct memcg_cache_params {
 			struct kmem_cache *memcg_caches[0];
 		};
 		struct {
+			struct kmem_cache *cachep;
 			struct mem_cgroup *memcg;
 			struct list_head list;
 			struct kmem_cache *root_cache;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cc1064a504cc..aa3111ac3b7e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2790,19 +2790,6 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg)
 	return memcg ? memcg->kmemcg_id : -1;
 }
 
-/*
- * This is a bit cumbersome, but it is rarely used and avoids a backpointer
- * in the memcg_cache_params struct.
- */
-static struct kmem_cache *memcg_params_to_cache(struct memcg_cache_params *p)
-{
-	struct kmem_cache *cachep;
-
-	VM_BUG_ON(p->is_root_cache);
-	cachep = p->root_cache;
-	return cache_from_memcg_idx(cachep, memcg_cache_id(p->memcg));
-}
-
 #ifdef CONFIG_SLABINFO
 static int mem_cgroup_slabinfo_read(struct seq_file *m, void *v)
 {
@@ -2816,7 +2803,7 @@ static int mem_cgroup_slabinfo_read(struct seq_file *m, void *v)
 
 	mutex_lock(&memcg_slab_mutex);
 	list_for_each_entry(params, &memcg->memcg_slab_caches, list)
-		cache_show(memcg_params_to_cache(params), m);
+		cache_show(params->cachep, m);
 	mutex_unlock(&memcg_slab_mutex);
 
 	return 0;
@@ -2979,6 +2966,7 @@ int memcg_alloc_cache_params(struct mem_cgroup *memcg, struct kmem_cache *s,
 		return -ENOMEM;
 
 	if (memcg) {
+		s->memcg_params->cachep = s;
 		s->memcg_params->memcg = memcg;
 		s->memcg_params->root_cache = root_cache;
 		css_get(&memcg->css);
@@ -3124,7 +3112,6 @@ int __memcg_cleanup_cache_params(struct kmem_cache *s)
 
 static void memcg_unregister_all_caches(struct mem_cgroup *memcg)
 {
-	struct kmem_cache *cachep;
 	struct memcg_cache_params *params, *tmp;
 
 	if (!memcg_kmem_is_active(memcg))
@@ -3132,7 +3119,8 @@ static void memcg_unregister_all_caches(struct mem_cgroup *memcg)
 
 	mutex_lock(&memcg_slab_mutex);
 	list_for_each_entry_safe(params, tmp, &memcg->memcg_slab_caches, list) {
-		cachep = memcg_params_to_cache(params);
+		struct kmem_cache *cachep = params->cachep;
+
 		kmem_cache_shrink(cachep);
 		if (atomic_read(&cachep->memcg_params->nr_pages) == 0)
 			memcg_unregister_cache(cachep);
-- 
1.7.10.4

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