linux-kernel - Re: [PATCH 1/4] vmscan: all_unreclaimable() use zone->all

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Message-Id: <20110412100417.43F2.A69D9226@jp.fujitsu.com>
Date:	Tue, 12 Apr 2011 10:04:15 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	kosaki.motohiro@...fujitsu.com, Andrey Vagin <avagin@...nvz.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	David Rientjes <rientjes@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/4] vmscan: all_unreclaimable() use zone->all_unreclaimable as a name

Hi

> > zone->all_unreclaimable and zone->pages_scanned are neigher atomic
> > variables nor protected by lock. Therefore zones can become a state
> > of zone->page_scanned=0 and zone->all_unreclaimable=1. In this case,
> > current all_unreclaimable() return false even though
> > zone->all_unreclaimabe=1.
> > 
> > Is this ignorable minor issue? No. Unfortunatelly, x86 has very
> > small dma zone and it become zone->all_unreclamble=1 easily. and
> > if it become all_unreclaimable=1, it never restore all_unreclaimable=0.
> > Why? if all_unreclaimable=1, vmscan only try DEF_PRIORITY reclaim and
> > a-few-lru-pages>>DEF_PRIORITY always makes 0. that mean no page scan
> > at all!
> > 
> > Eventually, oom-killer never works on such systems. That said, we
> > can't use zone->pages_scanned for this purpose. This patch restore
> > all_unreclaimable() use zone->all_unreclaimable as old. and in addition,
> > to add oom_killer_disabled check to avoid reintroduce the issue of
> > commit d1908362.
> 
> The above is a nice analysis of the bug and how it came to be
> introduced.  But we don't actually have a bug description!  What was
> the observeable problem which got fixed?

The above says "Eventually, oom-killer never works". Is this no enough?
The above says
  1) current logic have a race
  2) x86 increase a chance of the race by dma zone
  3) if race is happen, oom killer don't work

> 
> Such a description will help people understand the importance of the
> patch and will help people (eg, distros) who are looking at a user's
> bug report and wondering whether your patch will fix it.
> 



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