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

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Message-ID: <CAHGf_=qcA5OfuNgk0BiwyshcLftNWoPfOO_VW9H6xQTX2tAbuA@mail.gmail.com>
Date:	Thu, 14 Jun 2012 12:10:47 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Minchan Kim <minchan@...nel.org>
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 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.



> So if kswapd sleeps, it means we already have enough order-0 free pages.
> Hmm, could you describe scenario you found in detail with code level?
>
> Anyway, as I look at your patch, I can't find any problem.
> I just want to understand scenario you mentioned completely in my head.
> Maybe It can help making description clear.
>
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