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

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Message-ID: <CAJd=RBBSa2TuRDVGrY9JT9m3K68N1LWiZKyo3Y1mdQRo5TxBLQ@mail.gmail.com>
Date:	Fri, 15 Jun 2012 20:31:50 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	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>
Subject: Re: [resend][PATCH] mm, vmscan: fix do_try_to_free_pages() livelock

Hi Minchan and KOSAKI

On Fri, Jun 15, 2012 at 3:27 PM, Minchan Kim <minchan@...nel.org> wrote:
> 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.
>

Who left comment on unreclaimable there, and why?
		/*
		 * balance_pgdat() skips over all_unreclaimable after
		 * DEF_PRIORITY. Effectively, it considers them balanced so
		 * they must be considered balanced here as well if kswapd
		 * is to sleep
		 */

BTW, are you still using prefetch_prev_lru_page?

Good Weekend
Hillf
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