Re: [PATCH 3/3] mm, memcg: Ensure valid memcg from objcg within a RCU critical section

On 10/1/21 4:24 PM, Shakeel Butt wrote:

On Fri, Oct 1, 2021 at 12:10 PM Waiman Long [off-list ref] wrote:

quoted

To ensure that a to-be-offlined memcg fetched from objcg remains
valid (has non-zero reference count) within a RCU critical section,
a synchronize_rcu() call is inserted at the end of memcg_offline_kmem().

With that change, we no longer need to use css_tryget()
in get_mem_cgroup_from_objcg() as the final css_put() in
css_killed_work_fn() would not have been called yet.

The obj_cgroup_uncharge_pages() function is simplifed to perform
the whole uncharge operation within a RCU critical section saving a
css_get()/css_put() pair.

Signed-off-by: Waiman Long <longman@redhat.com>
---
  mm/memcontrol.c | 16 +++++++++++-----
  1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8177f253a127..1dbb37d96e49 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2769,10 +2769,8 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
         struct mem_cgroup *memcg;

         rcu_read_lock();
-retry:
         memcg = obj_cgroup_memcg(objcg);
-       if (unlikely(!css_tryget(&memcg->css)))
-               goto retry;
+       css_get(&memcg->css);
         rcu_read_unlock();

         return memcg;

@@ -2947,13 +2945,14 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
  {
         struct mem_cgroup *memcg;

-       memcg = get_mem_cgroup_from_objcg(objcg);
+       rcu_read_lock();
+       memcg = obj_cgroup_memcg(objcg);

         if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
                 page_counter_uncharge(&memcg->kmem, nr_pages);
         refill_stock(memcg, nr_pages);

-       css_put(&memcg->css);
+       rcu_read_unlock();
  }

  /*

@@ -3672,6 +3671,13 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
         memcg_drain_all_list_lrus(kmemcg_id, parent);

         memcg_free_cache_id(kmemcg_id);
+
+       /*
+        * To ensure that a to-be-offlined memcg fetched from objcg remains
+        * valid within a RCU critical section, we need to wait here until
+        * the a grace period has elapsed.
+        */
+       synchronize_rcu();

This is called with cgroup_mutex held from css_offline path and
synchronize_rcu() can be very expensive on a busy system, so, this
will indirectly impact all the code paths which take cgroup_mutex.

Yes, you are right. Just don't consider this patch for the time being. I will need to find a way to work around that.

Thanks,
Longman