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SubjectRe: [PATCH 3/3] mm, memcg: Ensure valid memcg from objcg within a RCU critical section
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On 10/1/21 4:24 PM, Shakeel Butt wrote:
> On Fri, Oct 1, 2021 at 12:10 PM Waiman Long <longman@redhat.com> wrote:
>> 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

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