[PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure

[Kirill Tkhai <ktkhai@virtuozzo.com>]

In case of memcg_online_kmem() fail, memcg_cgroup::id remains hashed
in mem_cgroup_idr even after memcg memory is freed. This leads to leak
of ID in mem_cgroup_idr.

This patch adds removing into mem_cgroup_css_alloc(), which fixes
the problem. For better readability, it adds generic helper, which
will be used in mem_cgroup_alloc() and mem_cgroup_id_put_many() too.

Fixes 73f576c04b94 "mm: memcontrol: fix cgroup creation failure after many small jobs"
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 mm/memcontrol.c |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3e7942c301a8..448db08d97a0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4263,6 +4263,14 @@ static struct cftype mem_cgroup_legacy_files[] = {
 
 static DEFINE_IDR(mem_cgroup_idr);
 
+static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
+{
+	if (memcg->id.id > 0) {
+		idr_remove(&mem_cgroup_idr, memcg->id.id);
+		memcg->id.id = 0;
+	}
+}
+
 static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
 {
 	VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0);
@@ -4273,8 +4281,7 @@ static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
 {
 	VM_BUG_ON(atomic_read(&memcg->id.ref) < n);
 	if (atomic_sub_and_test(n, &memcg->id.ref)) {
-		idr_remove(&mem_cgroup_idr, memcg->id.id);
-		memcg->id.id = 0;
+		mem_cgroup_id_remove(memcg);
 
 		/* Memcg ID pins CSS */
 		css_put(&memcg->css);
@@ -4411,8 +4418,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
 	return memcg;
 fail:
-	if (memcg->id.id > 0)
-		idr_remove(&mem_cgroup_idr, memcg->id.id);
+	mem_cgroup_id_remove(memcg);
 	__mem_cgroup_free(memcg);
 	return NULL;
 }
@@ -4471,6 +4477,7 @@ 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);
 }