[PATCH v8 2/6] memcontrol: implement mem_cgroup_tryget_online()

[Nhat Pham <nphamcs@gmail.com>]

This patch implements a helper function that try to get a reference to
an memcg's css, as well as checking if it is online. This new function
is almost exactly the same as the existing mem_cgroup_tryget(), except
for the onlineness check. In the !CONFIG_MEMCG case, it always returns
true, analogous to mem_cgroup_tryget(). This is useful for e.g to the
new zswap writeback scheme, where we need to select the next online
memcg as a candidate for the global limit reclaim.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 include/linux/memcontrol.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 7bdcf3020d7a..2bd7d14ace78 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -821,6 +821,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
 	return !memcg || css_tryget(&memcg->css);
 }
 
+static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
+{
+	return !memcg || css_tryget_online(&memcg->css);
+}
+
 static inline void mem_cgroup_put(struct mem_cgroup *memcg)
 {
 	if (memcg)
@@ -1349,6 +1354,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
 	return true;
 }
 
+static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
+{
+	return true;
+}
+
 static inline void mem_cgroup_put(struct mem_cgroup *memcg)
 {
 }
-- 
2.34.1