Hello Shakeel,
Shakeel Butt <shakeelb@xxxxxxxxxx> writes:
>>
>> V3: Handle common case where use_hierarchy=1 and update description.
>>
>> mm/memcontrol.c | 7 +++++--
>> 1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 6877c765b8d0..34b8c4a66853 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -291,7 +291,7 @@ static void obj_cgroup_release(struct percpu_ref *ref)
>>
>> spin_lock_irqsave(&css_set_lock, flags);
>> memcg = obj_cgroup_memcg(objcg);
>> - if (nr_pages)
>> + if (nr_pages && (!mem_cgroup_is_root(memcg) || memcg->use_hierarchy))
>
> If we have non-root memcg with use_hierarchy as 0 and this objcg was
> reparented then this __memcg_kmem_uncharge() can potentially underflow
> the page counter and give the same warning.
Yes, although the kernel considers such a config to be broken, and
prints a warning to the log, it does allow it.
>
> We never set root_mem_cgroup->objcg, so, no need to check for root
I don't think that is relevant as we get the memcg from objcg->memcg
which is set during reparenting. I suppose however, we can determine if
the objcg was reparented by inspecting memcg->objcg.
> here. I think checking just memcg->use_hierarchy should be sufficient.
If we just check use_hierarchy then objects directly charged to the
memcg where use_hierarchy=0 will not be uncharged. However, maybe it is
better to check if it was reparented and if use_hierarchy=0.
>
>> __memcg_kmem_uncharge(memcg, nr_pages);
>> list_del(&objcg->list);
>> mem_cgroup_put(memcg);
>> @@ -3100,6 +3100,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
>> static void drain_obj_stock(struct memcg_stock_pcp *stock)
>> {
>> struct obj_cgroup *old = stock->cached_objcg;
>> + struct mem_cgroup *memcg;
>>
>> if (!old)
>> return;
>> @@ -3110,7 +3111,9 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
>>
>> if (nr_pages) {
>> rcu_read_lock();
>> - __memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages);
>> + memcg = obj_cgroup_memcg(old);
>> + if (!mem_cgroup_is_root(memcg) || memcg->use_hierarchy)
>> + __memcg_kmem_uncharge(memcg, nr_pages);
>> rcu_read_unlock();
>> }
>>
>> --
>> 2.28.0
>>
--
Thank you,
Richard.
