linux-kernel - Re: memory hotplug: hot-remove fails on lowest chunk in ZONE

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Message-Id: <20080731134956.2A3B.E1E9C6FF@jp.fujitsu.com>
Date:	Thu, 31 Jul 2008 14:16:10 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	Mel Gorman <mel@....ul.ie>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Andy Whitcroft <apw@...dowen.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: memory hotplug: hot-remove fails on lowest chunk in ZONE_MOVABLE

> On Wed, 2008-07-30 at 12:16 +0900, Yasunori Goto wrote:
> > Well, I didn't mean changing pages_min value. There may be side effect as
> > you are saying.
> > I meant if some pages were MIGRATE_RESERVE attribute when hot-remove are
> > -executing-, their attribute should be changed.
> > 
> > For example, how is like following dummy code?  Is it impossible?
> > (Not only here, some places will have to be modified..)
> 
> Right, this should be possible. I was somewhat wandering from the subject,
> because I noticed that there may be a bigger problem with MIGRATE_RESERVE
> pages in ZONE_MOVABLE, and that we may not want to have them in the first
> place.
> 
> The more memory we add to ZONE_MOVABLE, the less reserved pages will
> remain to the other zones. In setup_per_zone_pages_min(), min_free_kbytes
> will be redistributed to a zone where the kernel cannot make any use of
> it, effectively reducing the available min_free_kbytes. This just doesn't
> sound right. I believe that a similar situation is the reason why highmem
> pages are skipped in the calculation and I think that we need that for
> ZONE_MOVABLE too. Any thoughts on that problem?
> 
> Setting pages_min to 0 for ZONE_MOVABLE, while not capping pages_low
> and pages_high, could be an option. I don't have a sufficient memory
> managment overview to tell if that has negative side effects, maybe
> someone with a deeper insight could comment on that.

At least, pages_min should not be 0. It is used as watermark when
memory shortage situation. If it is 0, kernel will misunderstand
shortage situation. Certainly, pages_min value may be not appropriate value
for ZONE_MOVABLE. But it is not memory-hotplug issue.

True your question is why ZONE_MOVABLE has MIGRATE_RESREVE pages, right?
However, I think it is intended for emergency pool of memory shortage situation
for ZONE_MOVABLE via fallback[]. If not, these MIGRATE_RESERVE pages are not made
originally.
It is why I wrote previous mail.

Mel Gormal-san knows around here very well. He may explain its detail more.

Bye.

-- 
Yasunori Goto 


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