Subject: Re: memory hotplug: hot-remove fails on lowest chunk in
	ZONE_MOVABLE
From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
To: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, schwidefsky@de.ibm.com,
       heiko.carstens@de.ibm.com,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Dave Hansen <haveblue@us.ibm.com>, Andy Whitcroft <apw@shadowen.org>,
       Christoph Lameter <cl@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>, Peter Zijlstra <peterz@infradead.org>,
       Mel Gorman <mel@csn.ul.ie>, Andrew Morton <akpm@linux-foundation.org>
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	 <20080730110444.27DE.E1E9C6FF@jp.fujitsu.com>
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Date: Wed, 30 Jul 2008 14:16:01 +0200
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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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