Re: WARNING in try_charge

On 2018/08/10 0:07, Michal Hocko wrote:

On Thu 09-08-18 22:57:43, Tetsuo Handa wrote:

quoted

quoted

From b1f38168f14397c7af9c122cd8207663d96e02ec Mon Sep 17 00:00:00 2001

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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.

quoted

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.