Re: list corruption in deferred_split_scan()

Yang Shi
2019-07-17T00:12:55Z

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?

Thanks,
Yang

>
> Yang, can you please send the above change with signed-off and CC to
> stable as well?
>
> thanks,
> Shakeel