[RFC PATCH v2 14/18] mm: memcontrol: introduce memcg_reparent_ops

[Muchun Song <songmuchun@bytedance.com>]

In the previous patch, we know how to make the lruvec lock safe when the
LRU pages reparented. We should do something like following.

    memcg_reparent_objcgs(memcg)
        1) lock
        // lruvec belongs to memcg and lruvec_parent belongs to parent memcg.
        spin_lock(&lruvec->lru_lock);
        spin_lock(&lruvec_parent->lru_lock);

        2) do reparent
        // Move all the pages from the lruvec list to the parent lruvec list.

        3) unlock
        spin_unlock(&lruvec_parent->lru_lock);
        spin_unlock(&lruvec->lru_lock);

Apart from the page lruvec lock, the deferred split queue lock (THP only)
also needs to do something similar. So we extracted the necessary 3 steps
in the memcg_reparent_objcgs().

    memcg_reparent_objcgs(memcg)
        1) lock
        memcg_reparent_ops->lock(memcg, parent);

        2) reparent
        memcg_reparent_ops->reparent(memcg, reparent);

        3) unlock
        memcg_reparent_ops->unlock(memcg, reparent);

Now there are two different locks (e.g. lruvec lock and deferred split
queue lock) need to use this infrastructure. In the next patch, we will
use those APIs to make those locks safe when the LRU pages reparented.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/memcontrol.h | 11 +++++++++++
 mm/memcontrol.c            | 49 ++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 7e15be2bd47a..fdc385ecff55 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -355,6 +355,17 @@ struct mem_cgroup {
 	/* WARNING: nodeinfo must be the last member here */
 };
 
+struct memcg_reparent_ops {
+	struct list_head list;
+
+	/* Irq is disabled before calling those functions. */
+	void (*lock)(struct mem_cgroup *memcg, struct mem_cgroup *parent);
+	void (*unlock)(struct mem_cgroup *memcg, struct mem_cgroup *parent);
+	void (*reparent)(struct mem_cgroup *memcg, struct mem_cgroup *parent);
+};
+
+void __init register_memcg_repatent(struct memcg_reparent_ops *ops);
+
 /*
  * size of first charge trial. "32" comes from vmscan.c's magic value.
  * TODO: maybe necessary to use big numbers in big irons.
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2f4fcb182883..536be7bdc98f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -338,6 +338,41 @@ static struct obj_cgroup *obj_cgroup_alloc(void)
 	return objcg;
 }
 
+static LIST_HEAD(reparent_ops_head);
+
+static void memcg_reparent_lock(struct mem_cgroup *memcg,
+				struct mem_cgroup *parent)
+{
+	struct memcg_reparent_ops *ops;
+
+	list_for_each_entry(ops, &reparent_ops_head, list)
+		ops->lock(memcg, parent);
+}
+
+static void memcg_reparent_unlock(struct mem_cgroup *memcg,
+				  struct mem_cgroup *parent)
+{
+	struct memcg_reparent_ops *ops;
+
+	list_for_each_entry(ops, &reparent_ops_head, list)
+		ops->unlock(memcg, parent);
+}
+
+static void memcg_do_reparent(struct mem_cgroup *memcg,
+			      struct mem_cgroup *parent)
+{
+	struct memcg_reparent_ops *ops;
+
+	list_for_each_entry(ops, &reparent_ops_head, list)
+		ops->reparent(memcg, parent);
+}
+
+void __init register_memcg_repatent(struct memcg_reparent_ops *ops)
+{
+	BUG_ON(!ops->lock || !ops->unlock || !ops->reparent);
+	list_add(&ops->list, &reparent_ops_head);
+}
+
 static void memcg_reparent_objcgs(struct mem_cgroup *memcg)
 {
 	struct obj_cgroup *objcg, *iter;
@@ -347,9 +382,13 @@ static void memcg_reparent_objcgs(struct mem_cgroup *memcg)
 	if (!parent)
 		parent = root_mem_cgroup;
 
+	local_irq_disable();
+
+	memcg_reparent_lock(memcg, parent);
+
 	objcg = rcu_replace_pointer(memcg->objcg, NULL, true);
 
-	spin_lock_irq(&css_set_lock);
+	spin_lock(&css_set_lock);
 
 	/* 1) Ready to reparent active objcg. */
 	list_add(&objcg->list, &memcg->objcg_list);
@@ -361,7 +400,13 @@ static void memcg_reparent_objcgs(struct mem_cgroup *memcg)
 	/* 3) Move already reparented objcgs to the parent's list */
 	list_splice(&memcg->objcg_list, &parent->objcg_list);
 
-	spin_unlock_irq(&css_set_lock);
+	spin_unlock(&css_set_lock);
+
+	memcg_do_reparent(memcg, parent);
+
+	memcg_reparent_unlock(memcg, parent);
+
+	local_irq_enable();
 
 	percpu_ref_kill(&objcg->refcnt);
 }
-- 
2.11.0