mm: memcontrol: remove kmemcg_id reparenting

Since slab objects and kmem pages are charged to object cgroup instead
of memory cgroup, memcg_reparent_objcgs() will reparent this cgroup and
all its descendants to its parent cgroup.  This already makes further
list_lru_add()'s add elements to the parent's list.  So it is
unnecessary to change kmemcg_id of an offline cgroup to its parent's id.
It just wastes CPU cycles.  Just remove the redundant code.

Link: https://lkml.kernel.org/r/20211025125102.56533-1-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e4fcb6e..1e2d3f3 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3650,8 +3650,7 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 
 static void memcg_offline_kmem(struct mem_cgroup *memcg)
 {
-       struct cgroup_subsys_state *css;
-       struct mem_cgroup *parent, *child;
+       struct mem_cgroup *parent;
        int kmemcg_id;
 
        if (memcg->kmem_state != KMEM_ONLINE)
@@ -3669,21 +3668,11 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
        BUG_ON(kmemcg_id < 0);
 
        /*
-        * Change kmemcg_id of this cgroup and all its descendants to the
-        * parent's id, and then move all entries from this cgroup's list_lrus
-        * to ones of the parent. After we have finished, all list_lrus
-        * corresponding to this cgroup are guaranteed to remain empty. The
-        * ordering is imposed by list_lru_node->lock taken by
+        * After we have finished memcg_reparent_objcgs(), all list_lrus
+        * corresponding to this cgroup are guaranteed to remain empty.
+        * The ordering is imposed by list_lru_node->lock taken by
         * memcg_drain_all_list_lrus().
         */
-       rcu_read_lock(); /* can be called from css_free w/o cgroup_mutex */
-       css_for_each_descendant_pre(css, &memcg->css) {
-               child = mem_cgroup_from_css(css);
-               BUG_ON(child->kmemcg_id != kmemcg_id);
-               child->kmemcg_id = parent->kmemcg_id;
-       }
-       rcu_read_unlock();
-
        memcg_drain_all_list_lrus(kmemcg_id, parent);
 
        memcg_free_cache_id(kmemcg_id);