[RFC PATCH v2 05/18] mm: memcontrol: simplify the logic of objcg pinning memcg

[Muchun Song]

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().

Signed-off-by: Muchun Song <songmuc...@bytedance.com>
---
 mm/memcontrol.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1f807448233e..90c1ac58c64c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -261,7 +261,6 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup 
*objcg,
 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 percpu_ref *ref)
        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 +327,14 @@ static void memcg_reparent_objcgs(struct mem_cgroup 
*memcg,
 
        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);
+       /* 1) Ready to reparent active objcg. */
+       list_add(&objcg->list, &memcg->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);
-       }
+       /* 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);
-- 
2.11.0