linux-kernel - Re: [resend][PATCH] mm, vmscan: fix do_try_to_free

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Date:	Fri, 15 Jun 2012 16:27:08 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, Nick Piggin <npiggin@...il.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [resend][PATCH] mm, vmscan: fix do_try_to_free_pages() livelock

On 06/15/2012 01:10 AM, KOSAKI Motohiro wrote:

> On Thu, Jun 14, 2012 at 10:57 AM, Minchan Kim <minchan@...nel.org> wrote:
>> Hi KOSAKI,
>>
>> Sorry for late response.
>> Let me ask a question about description.
>>
>> On Thu, Jun 14, 2012 at 04:13:12AM -0400, kosaki.motohiro@...il.com wrote:
>>> From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
>>>
>>> Currently, do_try_to_free_pages() can enter livelock. Because of,
>>> now vmscan has two conflicted policies.
>>>
>>> 1) kswapd sleep when it couldn't reclaim any page when reaching
>>>    priority 0. This is because to avoid kswapd() infinite
>>>    loop. That said, kswapd assume direct reclaim makes enough
>>>    free pages to use either regular page reclaim or oom-killer.
>>>    This logic makes kswapd -> direct-reclaim dependency.
>>> 2) direct reclaim continue to reclaim without oom-killer until
>>>    kswapd turn on zone->all_unreclaimble. This is because
>>>    to avoid too early oom-kill.
>>>    This logic makes direct-reclaim -> kswapd dependency.
>>>
>>> In worst case, direct-reclaim may continue to page reclaim forever
>>> when kswapd sleeps forever.
>>
>> I have tried imagined scenario you mentioned above with code level but
>> unfortunately I got failed.
>> If kswapd can't meet high watermark on order-0, it doesn't sleep if I don't miss something.
> 
> pgdat_balanced() doesn't recognized zone. Therefore kswapd may sleep
> if node has multiple zones. Hm ok, I realized my descriptions was
> slightly misleading. priority 0 is not needed. bakance_pddat() calls
> pgdat_balanced()
> every priority. Most easy case is, movable zone has a lot of free pages and
> normal zone has no reclaimable page.
> 
> btw, current pgdat_balanced() logic seems not correct. kswapd should
> sleep only if every zones have much free pages than high water mark
> _and_ 25% of present pages in node are free.
> 


Sorry. I can't understand your point.
Current kswapd doesn't sleep if relevant zones don't have free pages above high watermark.
It seems I am missing your point.
Please anybody correct me.

-- 
Kind regards,
Minchan Kim
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