linux-kernel - Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from direct reclaim path completely

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Date:	Thu, 24 Mar 2011 15:32:51 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-mm <linux-mm@...ck.org>,
	Andrey Vagin <avagin@...nvz.org>,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from direct
 reclaim path completely

On Thu, Mar 24, 2011 at 3:16 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
> Hi
>
>> Thanks for your effort, Kosaki.
>> But I still doubt this patch is good.
>>
>> This patch makes early oom killing in hibernation as it skip
>> all_unreclaimable check.
>> Normally,  hibernation needs many memory so page_reclaim pressure
>> would be big in small memory system. So I don't like early give up.
>
> Wait. When occur big pressure? hibernation reclaim pressure
> (sc->nr_to_recliam) depend on physical memory size. therefore
> a pressure seems to don't depend on the size.

It depends on physical memory size and /sys/power/image_size.
If you want to tune image size bigger, reclaim pressure would be big.

>
>
>> Do you think my patch has a problem? Personally, I think it's very
>> simple and clear. :)
>
> To be honest, I dislike following parts. It's madness on madness.
>
>        static bool zone_reclaimable(struct zone *zone)
>        {
>                if (zone->all_unreclaimable)
>                        return false;
>
>                return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
>        }
>
>
> The function require a reviewer know
>
>  o pages_scanned and all_unreclaimable are racy

Yes. That part should be written down of comment.

>  o at hibernation, zone->all_unreclaimable can be false negative,
>   but can't be false positive.

The comment of all_unreclaimable already does explain it well, I think.

>
> And, a function comment of all_unreclaimable() says
>
>         /*
>          * As hibernation is going on, kswapd is freezed so that it can't mark
>          * the zone into all_unreclaimable. It can't handle OOM during hibernation.
>          * So let's check zone's unreclaimable in direct reclaim as well as kswapd.
>          */
>
> But, now it is no longer copy of kswapd algorithm.

The comment don't say it should be a copy of kswapd.

>
> If you strongly prefer this idea even if you hear above explanation,
> please consider to add much and much comments. I can't say
> current your patch is enough readable/reviewable.

My patch isn't a formal patch for merge but just a concept to show.
If you agree the idea, of course, I will add more concrete comment
when I send formal patch.

Before, I would like to get a your agreement. :)
If you solve my concern(early give up in hibernation) in your patch, I
don't insist on my patch, either.

Thanks for the comment, Kosaki.

>
> Thanks.
>
>
>



-- 
Kind regards,
Minchan Kim
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