[PATCH v2 04/21] mm: memcontrol: do it in mem_cgroup_css_online to make the kmem online

[Muchun Song]

If we do it in the mem_cgroup_css_online() to make the kmem online,
we do not need to set ->kmemcg_id when the kmem is offline. And we
also can remove memcg_free_kmem(). So just do that to simplify the
code.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 42 +++++++++++++++---------------------------
 1 file changed, 15 insertions(+), 27 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 23a9fc8dc143..377ec9847179 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3629,7 +3629,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 	if (cgroup_memory_nokmem)
 		return 0;
 
-	BUG_ON(memcg->kmemcg_id >= 0);
+	if (unlikely(mem_cgroup_is_root(memcg)))
+		return 0;
 
 	memcg_id = memcg_alloc_cache_id();
 	if (memcg_id < 0)
@@ -3658,6 +3659,9 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 	if (cgroup_memory_nokmem)
 		return;
 
+	if (unlikely(mem_cgroup_is_root(memcg)))
+		return;
+
 	parent = parent_mem_cgroup(memcg);
 	if (!parent)
 		parent = root_mem_cgroup;
@@ -3665,20 +3669,11 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 	memcg_reparent_objcgs(memcg, parent);
 
 	kmemcg_id = memcg->kmemcg_id;
-	BUG_ON(kmemcg_id < 0);
 
 	/* memcg_reparent_objcgs() must be called before this. */
 	memcg_drain_all_list_lrus(kmemcg_id, parent);
 
 	memcg_free_cache_id(kmemcg_id);
-	memcg->kmemcg_id = -1;
-}
-
-static void memcg_free_kmem(struct mem_cgroup *memcg)
-{
-	/* css_alloc() failed, offlining didn't happen */
-	if (unlikely(memcg->kmemcg_id != -1))
-		memcg_offline_kmem(memcg);
 }
 #else
 static int memcg_online_kmem(struct mem_cgroup *memcg)
@@ -3688,9 +3683,6 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 static void memcg_offline_kmem(struct mem_cgroup *memcg)
 {
 }
-static void memcg_free_kmem(struct mem_cgroup *memcg)
-{
-}
 #endif /* CONFIG_MEMCG_KMEM */
 
 static int memcg_update_kmem_max(struct mem_cgroup *memcg,
@@ -5219,7 +5211,6 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 {
 	struct mem_cgroup *parent = mem_cgroup_from_css(parent_css);
 	struct mem_cgroup *memcg, *old_memcg;
-	long error = -ENOMEM;
 
 	old_memcg = set_active_memcg(parent);
 	memcg = mem_cgroup_alloc();
@@ -5249,38 +5240,36 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 	}
 
 	/* The following stuff does not apply to the root */
-	error = memcg_online_kmem(memcg);
-	if (error)
-		goto fail;
-
 	if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket)
 		static_branch_inc(&memcg_sockets_enabled_key);
 
 	return &memcg->css;
-fail:
-	mem_cgroup_id_remove(memcg);
-	mem_cgroup_free(memcg);
-	return ERR_PTR(error);
 }
 
 static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 
+	if (memcg_online_kmem(memcg))
+		goto remove_id;
+
 	/*
 	 * A memcg must be visible for expand_shrinker_info()
 	 * by the time the maps are allocated. So, we allocate maps
 	 * here, when for_each_mem_cgroup() can't skip it.
 	 */
-	if (alloc_shrinker_info(memcg)) {
-		mem_cgroup_id_remove(memcg);
-		return -ENOMEM;
-	}
+	if (alloc_shrinker_info(memcg))
+		goto offline_kmem;
 
 	/* Online state pins memcg ID, memcg ID pins CSS */
 	refcount_set(&memcg->id.ref, 1);
 	css_get(css);
 	return 0;
+offline_kmem:
+	memcg_offline_kmem(memcg);
+remove_id:
+	mem_cgroup_id_remove(memcg);
+	return -ENOMEM;
 }
 
 static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
@@ -5338,7 +5327,6 @@ static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
 	cancel_work_sync(&memcg->high_work);
 	mem_cgroup_remove_from_trees(memcg);
 	free_shrinker_info(memcg);
-	memcg_free_kmem(memcg);
 	mem_cgroup_free(memcg);
 }
 
-- 
2.11.0