[090/192] mm: memcontrol: simplify the logic of objcg pinning memcg

Commit Message

Andrew Morton June 29, 2021, 2:38 a.m. UTC

From: Muchun Song <[e-mail]>
Subject: mm: memcontrol: simplify the logic of objcg pinning memcg

The obj_cgroup_release() and memcg_reparent_objcgs() are serialized by the
css_set_lock.  We do not need to care about objcg->memcg being released in
the process of obj_cgroup_release().  So there is no need to pin memcg
before releasing objcg.  Remove those pinning logic to simplfy the code.

There are only two places that modifies the objcg->memcg.  One is the
initialization to objcg->memcg in the memcg_online_kmem(), another is
objcgs reparenting in the memcg_reparent_objcgs().  It is also impossible
for the two to run in parallel.  So xchg() is unnecessary and it is enough
to use WRITE_ONCE().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Muchun Song <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Reviewed-by: Shakeel Butt <[e-mail]>
Acked-by: Roman Gushchin <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Vladimir Davydov <[e-mail]>
Cc: Xiongchun Duan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/memcontrol.c |   20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

Patch

--- a/mm/memcontrol.c~mm-memcontrol-simplify-the-logic-of-objcg-pinning-memcg
+++ a/mm/memcontrol.c
@@ -261,7 +261,6 @@  static void obj_cgroup_uncharge_pages(st
 static void obj_cgroup_release(struct percpu_ref *ref)
 {
 	struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
-	struct mem_cgroup *memcg;
 	unsigned int nr_bytes;
 	unsigned int nr_pages;
 	unsigned long flags;
@@ -291,11 +290,9 @@  static void obj_cgroup_release(struct pe
 	nr_pages = nr_bytes >> PAGE_SHIFT;
 
 	spin_lock_irqsave(&css_set_lock, flags);
-	memcg = obj_cgroup_memcg(objcg);
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
 	list_del(&objcg->list);
-	mem_cgroup_put(memcg);
 	spin_unlock_irqrestore(&css_set_lock, flags);
 
 	percpu_ref_exit(ref);
@@ -330,17 +327,12 @@  static void memcg_reparent_objcgs(struct
 
 	spin_lock_irq(&css_set_lock);
 
-	/* Move active objcg to the parent's list */
-	xchg(&objcg->memcg, parent);
-	css_get(&parent->css);
-	list_add(&objcg->list, &parent->objcg_list);
-
-	/* Move already reparented objcgs to the parent's list */
-	list_for_each_entry(iter, &memcg->objcg_list, list) {
-		css_get(&parent->css);
-		xchg(&iter->memcg, parent);
-		css_put(&memcg->css);
-	}
+	/* 1) Ready to reparent active objcg. */
+	list_add(&objcg->list, &memcg->objcg_list);
+	/* 2) Reparent active objcg and already reparented objcgs to parent. */
+	list_for_each_entry(iter, &memcg->objcg_list, list)
+		WRITE_ONCE(iter->memcg, parent);
+	/* 3) Move already reparented objcgs to the parent's list */
 	list_splice(&memcg->objcg_list, &parent->objcg_list);
 
 	spin_unlock_irq(&css_set_lock);