Re: list corruption in deferred_split_scan()

[Yang Shi <yang.shi@linux.alibaba.com>]

On 7/17/19 10:02 AM, Shakeel Butt wrote:
> On Tue, Jul 16, 2019 at 5:12 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>
>>
>> On 7/16/19 4:36 PM, Shakeel Butt wrote:
>>> Adding related people.
>>>
>>> The thread starts at:
>>> http://lkml.kernel.org/r/1562795006.8510.19.camel@lca.pw
>>>
>>> On Mon, Jul 15, 2019 at 8:01 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>>>
>>>> On 7/15/19 6:36 PM, Qian Cai wrote:
>>>>>> On Jul 15, 2019, at 8:22 PM, Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 7/15/19 2:23 PM, Qian Cai wrote:
>>>>>>> On Fri, 2019-07-12 at 12:12 -0700, Yang Shi wrote:
>>>>>>>>> Another possible lead is that without reverting the those commits below,
>>>>>>>>> kdump
>>>>>>>>> kernel would always also crash in shrink_slab_memcg() at this line,
>>>>>>>>>
>>>>>>>>> map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map, true);
>>>>>>>> This looks a little bit weird. It seems nodeinfo[nid] is NULL? I didn't
>>>>>>>> think of where nodeinfo was freed but memcg was still online. Maybe a
>>>>>>>> check is needed:
>>>>>>> Actually, "memcg" is NULL.
>>>>>> It sounds weird. shrink_slab() is called in mem_cgroup_iter which does pin the memcg. So, the memcg should not go away.
>>>>> Well, the commit “mm: shrinker: make shrinker not depend on memcg kmem” changed this line in shrink_slab_memcg(),
>>>>>
>>>>> -     if (!memcg_kmem_enabled() || !mem_cgroup_online(memcg))
>>>>> +     if (!mem_cgroup_online(memcg))
>>>>>                 return 0;
>>>>>
>>>>> Since the kdump kernel has the parameter “cgroup_disable=memory”, shrink_slab_memcg() will no longer be able to handle NULL memcg from mem_cgroup_iter() as,
>>>>>
>>>>> if (mem_cgroup_disabled())
>>>>>         return NULL;
>>>> Aha, yes. memcg_kmem_enabled() implicitly checks !mem_cgroup_disabled().
>>>> Thanks for figuring this out. I think we need add mem_cgroup_dsiabled()
>>>> check before calling shrink_slab_memcg() as below:
>>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index a0301ed..2f03c61 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -701,7 +701,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int
>>>> nid,
>>>>            unsigned long ret, freed = 0;
>>>>            struct shrinker *shrinker;
>>>>
>>>> -       if (!mem_cgroup_is_root(memcg))
>>>> +       if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
>>>>                    return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>>>
>>>>            if (!down_read_trylock(&shrinker_rwsem))
>>>>
>>> We were seeing unneeded oom-kills on kernels with
>>> "cgroup_disabled=memory" and Yang's patch series basically expose the
>>> bug to crash. I think the commit aeed1d325d42 ("mm/vmscan.c:
>>> generalize shrink_slab() calls in shrink_node()") missed the case for
>>> "cgroup_disabled=memory". However I am surprised that root_mem_cgroup
>>> is allocated even for "cgroup_disabled=memory" and it seems like
>>> css_alloc() is called even before checking if the corresponding
>>> controller is disabled.
>> I'm surprised too. A quick test with drgn shows root memcg is definitely
>> allocated:
>>
>>   >>> prog['root_mem_cgroup']
>> *(struct mem_cgroup *)0xffff8902cf058000 = {
>> [snip]
>>
>> But, isn't this a bug?
> It can be treated as a bug as this is not expected but we can discuss
> and take care of it later. I think we need your patch urgently as
> memory reclaim and /proc/sys/vm/drop_caches is broken for
> "cgroup_disabled=memory" kernel. So, please send your patch asap.

Sure. I'm going to post the patch soon.

>
> thanks,
> Shakeel