[PATCH] mm: memcontrol: simplify memcg idr allocation and error unwinding

[Johannes Weiner]

The memcg ID is allocated early in the multi-step memcg creation
process, which needs 2-step ID allocation and IDR publishing, as well
as two separate IDR cleanup/unwind sites on error.

Defer the IDR allocation until the last second during onlining to
eliminate all this complexity. There is no requirement to have the ID
and IDR entry earlier than that. And the root reference to the ID is
put in the offline path, so this matches nicely.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/memcontrol.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 12205159b462..12339ae779ca 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4151,12 +4151,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	if (!memcg)
 		return NULL;
 
-	memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
-				 1, MEM_CGROUP_ID_MAX,
-				 GFP_KERNEL);
-	if (memcg->id.id < 0)
-		goto fail;
-
 	memcg->stat_cpu = alloc_percpu(struct mem_cgroup_stat_cpu);
 	if (!memcg->stat_cpu)
 		goto fail;
@@ -4183,10 +4177,8 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 #ifdef CONFIG_CGROUP_WRITEBACK
 	INIT_LIST_HEAD(&memcg->cgwb_list);
 #endif
-	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
 	return memcg;
 fail:
-	mem_cgroup_id_remove(memcg);
 	__mem_cgroup_free(memcg);
 	return NULL;
 }
@@ -4245,7 +4237,6 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 
 	return &memcg->css;
 fail:
-	mem_cgroup_id_remove(memcg);
 	mem_cgroup_free(memcg);
 	return ERR_PTR(-ENOMEM);
 }
@@ -4253,10 +4244,17 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+	int i;
+
+	i = idr_alloc(&mem_cgroup_idr, memcg, 1, MEM_CGROUP_ID_MAX, GFP_KERNEL);
+	if (i < 0)
+		return i;
 
 	/* Online state pins memcg ID, memcg ID pins CSS */
+	memcg->id.id = i;
 	atomic_set(&memcg->id.ref, 1);
 	css_get(css);
+
 	return 0;
 }
 
-- 
2.18.0