Re: [BUG] Early OOM and kernel NULL pointer dereference in 4.19.69

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 9/3/19 10:41 AM, Michal Hocko wrote:
> On Mon 02-09-19 21:34:29, Thomas Lindroth wrote:
>> On 9/2/19 9:16 AM, Michal Hocko wrote:
>>> On Sun 01-09-19 22:43:05, Thomas Lindroth wrote:
>>>> After upgrading to the 4.19 series I've started getting problems with
>>>> early OOM.
>>>
>>> What is the kenrel you have updated from? Would it be possible to try
>>> the current Linus' tree?
>>
>> I did some more testing and it turns out this is not a regression after all.
>>
>> I followed up on my hunch and monitored memory.kmem.max_usage_in_bytes while
>> running cgexec -g memory:12G bash -c 'find / -xdev -type f -print0 | \
>>         xargs -0 -n 1 -P 8 stat > /dev/null'
>>
>> Just as memory.kmem.max_usage_in_bytes = memory.kmem.limit_in_bytes the OOM
>> killer kicked in and killed my X server.
>>
>> Using the find|stat approach it was easy to test the problem in a testing VM.
>> I was able to reproduce the problem in all these kernels:
>>   4.9.0
>>   4.14.0
>>   4.14.115
>>   4.19.0
>>   5.2.11
>>
>> 5.3-rc6 didn't build in the VM. The build environment is too old probably.
>>
>> I was curious why I initially couldn't reproduce the problem in 4.14 by
>> building chromium. I was again able to successfully build chromium using
>> 4.14.115. Turns out memory.kmem.max_usage_in_bytes was 1015689216 after
>> building and my limit is set to 1073741824. I guess some unrelated change in
>> memory management raised that slightly for 4.19 triggering the problem.
>>
>> If you want to reproduce for yourself here are the steps:
>> 1. build any kernel above 4.9 using something like my .config
>> 2. setup a v1 memory cgroup with memory.kmem.limit_in_bytes lower than
>>    memory.limit_in_bytes. I used 100M in my testing VM.
>> 3. Run "find / -xdev -type f -print0 | xargs -0 -n 1 -P 8 stat > /dev/null"
>>    in the cgroup.
>> 4. Assuming there is enough inodes on the rootfs the global OOM killer
>>    should kick in when memory.kmem.max_usage_in_bytes =
>>    memory.kmem.limit_in_bytes and kill something outside the cgroup.
> 
> This is certainly a bug. Is this still an OOM triggered from
> pagefault_out_of_memory? Since 4.19 (29ef680ae7c21) the memcg charge
> path should invoke the memcg oom killer directly from the charge path.
> If that doesn't happen then the failing charge is either GFP_NOFS or a
> large allocation.
> 
> The former has been fixed just recently by http://lkml.kernel.org/r/cbe54ed1-b6ba-a056-8899-2dc42526371d@i-love.sakura.ne.jp
> and I suspect this is a fix you are looking for. Although it is curious
> that you can see a global oom even before because the charge path would
> mark an oom situation even for NOFS context and it should trigger the
> memcg oom killer on the way out from the page fault path. So essentially
> the same call trace except the oom killer should be constrained to the
> memcg context.
> 
> Could you try the above patch please?
> 

It won't help. We hitting ->kmem limit here, not the ->memory or ->memsw, so try_charge() is successful and
only __memcg_kmem_charge_memcg() fails to charge ->kmem and returns -ENOMEM.

Limiting kmem just never worked and it doesn't work now. AFAIK this feature hasn't been finished because 
there was no clear purpose/use case found. I remember that there was some discussion on lsfmm about this https://lwn.net/Articles/636331/
but I don't remember the discussion itself.