patches/old/memcontrol-add-a-new-function-to-traverse-online-only-memcg-hierarchy.patch

From: Nhat Pham <nphamcs@gmail.com>
Subject: memcontrol: add a new function to traverse online-only memcg hierarchy
Date: Mon, 27 Nov 2023 15:45:56 -0800

The new zswap writeback scheme requires an online-only memcg hierarchy
traversal. Add this functionality via the new mem_cgroup_iter_online()
function - the old mem_cgroup_iter() is a special case of this new
function.

Link: https://lkml.kernel.org/r/20231127234600.2971029-3-nphamcs@gmail.com
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/memcontrol.h |   13 +++++++++++++
 mm/memcontrol.c            |   29 +++++++++++++++++++++++++----
 2 files changed, 38 insertions(+), 4 deletions(-)

--- a/include/linux/memcontrol.h~memcontrol-add-a-new-function-to-traverse-online-only-memcg-hierarchy
+++ a/include/linux/memcontrol.h
@@ -833,6 +833,10 @@ static inline void mem_cgroup_put(struct
 struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
 				   struct mem_cgroup *,
 				   struct mem_cgroup_reclaim_cookie *);
+struct mem_cgroup *mem_cgroup_iter_online(struct mem_cgroup *root,
+				   struct mem_cgroup *prev,
+				   struct mem_cgroup_reclaim_cookie *reclaim,
+				   bool online);
 void mem_cgroup_iter_break(struct mem_cgroup *, struct mem_cgroup *);
 void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 			   int (*)(struct task_struct *, void *), void *arg);
@@ -1385,6 +1389,15 @@ mem_cgroup_iter(struct mem_cgroup *root,
 {
 	return NULL;
 }
+
+static inline struct mem_cgroup *
+mem_cgroup_iter_online(struct mem_cgroup *root,
+		struct mem_cgroup *prev,
+		struct mem_cgroup_reclaim_cookie *reclaim,
+		bool online)
+{
+	return NULL;
+}
 
 static inline void mem_cgroup_iter_break(struct mem_cgroup *root,
 					 struct mem_cgroup *prev)
--- a/mm/memcontrol.c~memcontrol-add-a-new-function-to-traverse-online-only-memcg-hierarchy
+++ a/mm/memcontrol.c
@@ -1111,10 +1111,11 @@ again:
 }
 
 /**
- * mem_cgroup_iter - iterate over memory cgroup hierarchy
+ * mem_cgroup_iter_online - iterate over memory cgroup hierarchy
  * @root: hierarchy root
  * @prev: previously returned memcg, NULL on first invocation
  * @reclaim: cookie for shared reclaim walks, NULL for full walks
+ * @online: whether to skip offline memcgs
  *
  * Returns references to children of the hierarchy below @root, or
  * @root itself, or %NULL after a full round-trip.
@@ -1123,13 +1124,16 @@ again:
  * invocations for reference counting, or use mem_cgroup_iter_break()
  * to cancel a hierarchy walk before the round-trip is complete.
  *
+ * Caller can skip offline memcgs by passing true for @online.
+ *
  * Reclaimers can specify a node in @reclaim to divide up the memcgs
  * in the hierarchy among all concurrent reclaimers operating on the
  * same node.
  */
-struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
+struct mem_cgroup *mem_cgroup_iter_online(struct mem_cgroup *root,
 				   struct mem_cgroup *prev,
-				   struct mem_cgroup_reclaim_cookie *reclaim)
+				   struct mem_cgroup_reclaim_cookie *reclaim,
+				   bool online)
 {
 	struct mem_cgroup_reclaim_iter *iter;
 	struct cgroup_subsys_state *css = NULL;
@@ -1199,7 +1203,8 @@ struct mem_cgroup *mem_cgroup_iter(struc
 		 * is provided by the caller, so we know it's alive
 		 * and kicking, and don't take an extra reference.
 		 */
-		if (css == &root->css || css_tryget(css)) {
+		if (css == &root->css || (!online && css_tryget(css)) ||
+				css_tryget_online(css)) {
 			memcg = mem_cgroup_from_css(css);
 			break;
 		}
@@ -1229,6 +1234,22 @@ out_unlock:
 }
 
 /**
+ * mem_cgroup_iter - iterate over memory cgroup hierarchy
+ * @root: hierarchy root
+ * @prev: previously returned memcg, NULL on first invocation
+ * @reclaim: cookie for shared reclaim walks, NULL for full walks
+ *
+ * Perform an iteration on the memory cgroup hierarchy without skipping
+ * offline memcgs.
+ */
+struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
+				   struct mem_cgroup *prev,
+				   struct mem_cgroup_reclaim_cookie *reclaim)
+{
+	return mem_cgroup_iter_online(root, prev, reclaim, false);
+}
+
+/**
  * mem_cgroup_iter_break - abort a hierarchy walk prematurely
  * @root: hierarchy root
  * @prev: last visited hierarchy member as returned by mem_cgroup_iter()
_