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

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Message-Id: <20110324151701.CC7F.A69D9226@jp.fujitsu.com>
Date:	Thu, 24 Mar 2011 15:16:25 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com, 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

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.


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

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. 

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.

Thanks.


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