linux-kernel - Re: [PATCH] mm: offlining memory may block forever

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Message-ID: <4FE27AE8.2080906@kernel.org>
Date:	Thu, 21 Jun 2012 10:37:44 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Aaditya Kumar <aaditya.kumar.30@...il.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	stable@...nel.org, kosaki.motohiro@...fujitsu.com,
	gregkh@...uxfoundation.org, Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>, tim.bird@...sony.com,
	frank.rowand@...sony.com, takuzo.ohara@...sony.com,
	kan.iibuchi@...sony.com, aaditya.kumar@...sony.com
Subject: Re: [PATCH] mm: offlining memory may block forever

On 06/21/2012 01:23 AM, Aaditya Kumar wrote:

> Offlining memory may block forever, waiting for kswapd() to wake up because
> kswapd() does not check the event kthread->should_stop before sleeping.

> 

> The proper pattern, from Documentation/memory-barriers.txt, is:
>    ---  waker  ---
>    event_indicated = 1;
>    wake_up_process(event_daemon);
> 
>    ---  sleeper  ---
>    for (;;) {
>       set_current_state(TASK_UNINTERRUPTIBLE);
>       if (event_indicated)
>          break;
>       schedule();
>    }
> 
>    set_current_state() may be wrapped by:
>       prepare_to_wait();
> 
> In the kswapd() case, event_indicated is kthread->should_stop.
> ---  offlining memory (waker)  ---
>    kswapd_stop()
>       kthread_stop()
>          kthread->should_stop = 1
>          wake_up_process()
>          wait_for_completion()
> 
> 
> ---  kswapd_try_to_sleep (sleeper)  ---
>    kswapd_try_to_sleep()
>       prepare_to_wait()
>            .
>            .
>       schedule()
>            .
>            .
>       finish_wait()
> 
>    The schedule() needs to be protected by a test of kthread->should_stop,
>    which is wrapped by kthread_should_stop().
> 
> Reproducer:
>    Do heavy file I/O in background.
>    Do a memory offline/online in a tight loop
> 
> 
> Signed-off-by: Aaditya Kumar <aaditya.kumar@...sony.com>

Reviewed-by: Minchan Kim <minchan@...nel.org>

Nitpick: We can remove kthread_should_stop check earlier in kswapd_try_to_sleep.
         But it's no biggie. And I hope you change patch title 
	 
	 Title : Fix loss of kswapd wakeup in kswapd_stop
	 Description: Offlining memory may block forever because blah, blah, blah.

-- 
Kind regards,
Minchan Kim
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