Re: WARNING in try_charge

[Tetsuo Handa]

On 2018/08/10 0:07, Michal Hocko wrote:
> On Thu 09-08-18 22:57:43, Tetsuo Handa wrote:
>> >From b1f38168f14397c7af9c122cd8207663d96e02ec Mon Sep 17 00:00:00 2001
>> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>> Date: Thu, 9 Aug 2018 22:49:40 +0900
>> Subject: [PATCH] mm, oom: task_will_free_mem(current) should retry until
>>  memory reserve fails
>>
>> Commit 696453e66630ad45 ("mm, oom: task_will_free_mem should skip
>> oom_reaped tasks") changed to select next OOM victim as soon as
>> MMF_OOM_SKIP is set. But we don't need to select next OOM victim as
>> long as ALLOC_OOM allocation can succeed. And syzbot is hitting WARN(1)
>> caused by this race window [1].
> 
> It is not because the syzbot was exercising a completely different code
> path (memcg charge rather than the page allocator).

I know syzbot is hitting memcg charge path.

> 
>> Since memcg OOM case uses forced charge if current thread is killed,
>> out_of_memory() can return true without selecting next OOM victim.
>> Therefore, this patch changes task_will_free_mem(current) to ignore
>> MMF_OOM_SKIP unless ALLOC_OOM allocation failed.
> 
> And the patch is simply wrong for memcg.
> 

Why? I think I should have done

-+	page = __alloc_pages_may_oom(gfp_mask, order, alloc_flags == ALLOC_OOM
-+				     || (gfp_mask & __GFP_NOMEMALLOC), ac,
-+				     &did_some_progress);
++	page = __alloc_pages_may_oom(gfp_mask, order, alloc_flags == ALLOC_OOM,
++				     ac, &did_some_progress);

because nobody will use __GFP_NOMEMALLOC | __GFP_NOFAIL. But for memcg charge
path, task_will_free_mem(current, false) == true and out_of_memory() will return
true, which avoids unnecessary OOM killing.

Of course, this patch cannot avoid unnecessary OOM killing if out_of_memory()
is called by not yet killed process. But to mitigate it, what can we do other
than defer setting MMF_OOM_SKIP using a timeout based mechanism? Making
the OOM reaper unconditionally reclaim all memory is not a valid answer.