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Date: Mon, 12 Sep 2016 09:16:51 +0200
From: Michal Hocko <mhocko@suse.cz>
Subject: Re: [PATCH] mm: Don't emit warning from pagefault_out_of_memory()
Message-ID: <20160912071651.GB14524@dhcp22.suse.cz>
References: <1473442120-7246-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
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To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, David Rientjes <rientjes@google.com>, Johannes Weiner <hannes@cmpxchg.org>

On Sat 10-09-16 02:28:40, Tetsuo Handa wrote:
> Commit c32b3cbe0d067a9c ("oom, PM: make OOM detection in the freezer path
> raceless") inserted a WARN_ON() into pagefault_out_of_memory() in order
> to warn when we raced with disabling the OOM killer. But emitting same
> backtrace forever after the OOM killer/reaper are disabled is pointless
> because the system is already OOM livelocked.

How that would that be forever? Pagefaults are not GFP_NOFAIL and the
killed task would just enter the exit path. Sure we will see one warning
per each g-u-p after that point but the above text seems to be
misleading to me. So can you just drop the last sentence?

> Now, patch "oom, suspend: fix oom_killer_disable vs. pm suspend properly"
> introduced a timeout for oom_killer_disable(). Even if we raced with
> disabling the OOM killer and the system is OOM livelocked, the OOM killer
> will be enabled eventually (in 20 seconds by default) and the OOM livelock
> will be solved. Therefore, we no longer need to warn when we raced with
> disabling the OOM killer.

That being said I guess the warning is really no longer needed as you
say. So I am not against the patch. But the changelog wording seems
misleading to me.

> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/oom_kill.c | 12 +-----------
>  1 file changed, 1 insertion(+), 11 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 0034baf..f284e92 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -1069,16 +1069,6 @@ void pagefault_out_of_memory(void)
>  
>  	if (!mutex_trylock(&oom_lock))
>  		return;
> -
> -	if (!out_of_memory(&oc)) {
> -		/*
> -		 * There shouldn't be any user tasks runnable while the
> -		 * OOM killer is disabled, so the current task has to
> -		 * be a racing OOM victim for which oom_killer_disable()
> -		 * is waiting for.
> -		 */
> -		WARN_ON(test_thread_flag(TIF_MEMDIE));
> -	}
> -
> +	out_of_memory(&oc);
>  	mutex_unlock(&oom_lock);
>  }
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs

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