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

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Date:	Wed, 30 Jul 2008 14:16:01 +0200
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	Yasunori Goto <y-goto@...fujitsu.com>
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>,
	Mel Gorman <mel@....ul.ie>,
	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.

Thanks,
Gerald

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