cgroup: add css_parent()

Currently, controllers have to explicitly follow the cgroup hierarchy
to find the parent of a given css.  cgroup is moving towards using
cgroup_subsys_state as the main controller interface construct, so
let's provide a way to climb the hierarchy using just csses.

This patch implements css_parent() which, given a css, returns its
parent.  The function is guarnateed to valid non-NULL parent css as
long as the target css is not at the top of the hierarchy.

freezer, cpuset, cpu, cpuacct, hugetlb, memory, net_cls and devices
are converted to use css_parent() instead of accessing cgroup->parent
directly.

* __parent_ca() is dropped from cpuacct and its usage is replaced with
  parent_ca().  The only difference between the two was NULL test on
  cgroup->parent which is now embedded in css_parent() making the
  distinction moot.  Note that eventually a css->parent field will be
  added to css and the NULL check in css_parent() will go away.

This patch shouldn't cause any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

2 files changed, 12 insertions, 33 deletions

diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index 95585a0b9c8d..57ecb5d2513f 100644
--- a/mm/hugetlb_cgroup.c
+++ b/mm/hugetlb_cgroup.c
@@ -59,11 +59,7 @@ static inline bool hugetlb_cgroup_is_root(struct hugetlb_cgroup *h_cg)
 static inline struct hugetlb_cgroup *
 parent_hugetlb_cgroup(struct hugetlb_cgroup *h_cg)
 {
-	struct cgroup *parent = h_cg->css.cgroup->parent;
-
-	if (!parent)
-		return NULL;
-	return hugetlb_cgroup_from_cgroup(parent);
+	return hugetlb_cgroup_from_css(css_parent(&h_cg->css));
 }
 
 static inline bool hugetlb_cgroup_have_usage(struct hugetlb_cgroup *h_cg)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 11d659e3b08e..69b3e520f921 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1524,10 +1524,8 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
 
 int mem_cgroup_swappiness(struct mem_cgroup *memcg)
 {
-	struct cgroup *cgrp = memcg->css.cgroup;
-
 	/* root ? */
-	if (cgrp->parent == NULL)
+	if (!css_parent(&memcg->css))
 		return vm_swappiness;
 
 	return memcg->swappiness;
@@ -5026,11 +5024,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
 {
 	int retval = 0;
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
-	struct cgroup *parent = cont->parent;
-	struct mem_cgroup *parent_memcg = NULL;
-
-	if (parent)
-		parent_memcg = mem_cgroup_from_cont(parent);
+	struct mem_cgroup *parent_memcg = mem_cgroup_from_css(css_parent(&memcg->css));
 
 	mutex_lock(&memcg_create_mutex);
 
@@ -5282,18 +5276,15 @@ static int mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
 static void memcg_get_hierarchical_limit(struct mem_cgroup *memcg,
 		unsigned long long *mem_limit, unsigned long long *memsw_limit)
 {
-	struct cgroup *cgroup;
 	unsigned long long min_limit, min_memsw_limit, tmp;
 
 	min_limit = res_counter_read_u64(&memcg->res, RES_LIMIT);
 	min_memsw_limit = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
-	cgroup = memcg->css.cgroup;
 	if (!memcg->use_hierarchy)
 		goto out;
 
-	while (cgroup->parent) {
-		cgroup = cgroup->parent;
-		memcg = mem_cgroup_from_cont(cgroup);
+	while (css_parent(&memcg->css)) {
+		memcg = mem_cgroup_from_css(css_parent(&memcg->css));
 		if (!memcg->use_hierarchy)
 			break;
 		tmp = res_counter_read_u64(&memcg->res, RES_LIMIT);
@@ -5523,16 +5514,11 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
 				       u64 val)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
-	struct mem_cgroup *parent;
-
-	if (val > 100)
-		return -EINVAL;
+	struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(&memcg->css));
 
-	if (cgrp->parent == NULL)
+	if (val > 100 || !parent)
 		return -EINVAL;
 
-	parent = mem_cgroup_from_cont(cgrp->parent);
-
 	mutex_lock(&memcg_create_mutex);
 
 	/* If under hierarchy, only empty-root can set this value */
@@ -5861,14 +5847,12 @@ static int mem_cgroup_oom_control_write(struct cgroup *cgrp,
 	struct cftype *cft, u64 val)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
-	struct mem_cgroup *parent;
+	struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(&memcg->css));
 
 	/* cannot set to root cgroup and only 0 and 1 are allowed */
-	if (!cgrp->parent || !((val == 0) || (val == 1)))
+	if (!parent || !((val == 0) || (val == 1)))
 		return -EINVAL;
 
-	parent = mem_cgroup_from_cont(cgrp->parent);
-
 	mutex_lock(&memcg_create_mutex);
 	/* oom-kill-disable is a flag for subhierarchy. */
 	if ((parent->use_hierarchy) || memcg_has_children(memcg)) {
@@ -6266,15 +6250,14 @@ free_out:
 static int
 mem_cgroup_css_online(struct cgroup *cont)
 {
-	struct mem_cgroup *memcg, *parent;
+	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
+	struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(&memcg->css));
 	int error = 0;
 
-	if (!cont->parent)
+	if (!parent)
 		return 0;
 
 	mutex_lock(&memcg_create_mutex);
-	memcg = mem_cgroup_from_cont(cont);
-	parent = mem_cgroup_from_cont(cont->parent);
 
 	memcg->use_hierarchy = parent->use_hierarchy;
 	memcg->oom_kill_disable = parent->oom_kill_disable;