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Date: Mon, 6 Jun 2016 14:13:40 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] oom_reaper: avoid pointless atomic_inc_not_zero usage.
Message-Id: <20160606141340.86c96c1d3dc29823438313d9@linux-foundation.org>
In-Reply-To: <1465024759-8074-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
References: <1465024759-8074-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
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Sender: owner-linux-mm@kvack.org
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To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: linux-mm@kvack.org, Michal Hocko <mhocko@suse.com>, Arnd Bergmann <arnd@arndb.de>

On Sat,  4 Jun 2016 16:19:19 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> Since commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper
> managed to unmap the address space") changed to use find_lock_task_mm()
> for finding a mm_struct to reap, it is guaranteed that mm->mm_users > 0
> because find_lock_task_mm() returns a task_struct with ->mm != NULL.
> Therefore, we can safely use atomic_inc().
> 
> ...
>
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -474,13 +474,8 @@ static bool __oom_reap_task(struct task_struct *tsk)
>  	p = find_lock_task_mm(tsk);
>  	if (!p)
>  		goto unlock_oom;
> -
>  	mm = p->mm;
> -	if (!atomic_inc_not_zero(&mm->mm_users)) {
> -		task_unlock(p);
> -		goto unlock_oom;
> -	}
> -
> +	atomic_inc(&mm->mm_users);
>  	task_unlock(p);
>  
>  	if (!down_read_trylock(&mm->mmap_sem)) {

In an off-list email (please don't do that!) you asked me to replace
mmoom_reaper-dont-call-mmput_async-without-atomic_inc_not_zero.patch
with this above patch.

But the
mmoom_reaper-dont-call-mmput_async-without-atomic_inc_not_zero.patch
changelog is pretty crappy:

: Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task")
: reduced frequency of needlessly selecting next OOM victim, but was
: calling mmput_async() when atomic_inc_not_zero() failed.

because it doesn't explain that the patch potentially fixes a kernel
crash.

And the changelog for this above patch is similarly crappy - it fails
to described the end-user visible effects of the bug which is being
fixed.  Please *always* do this.  Always always always.

Please send me a complete changelog for this patch, thanks.

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