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

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Message-Id: <1216745719.4871.8.camel@localhost.localdomain>
Date:	Tue, 22 Jul 2008 18:55:19 +0200
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-mm@...ck.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Andy Whitcroft <apw@...dowen.org>
Subject: memory hotplug: hot-remove fails on lowest chunk in ZONE_MOVABLE

I've been testing memory hotplug on s390, on a system that starts w/o
memory in ZONE_MOVABLE at first, and then some memory chunks will be
added to ZONE_MOVABLE via memory hot-add. Now I observe the following
problem:

Memory hot-remove of the lowest memory chunk in ZONE_MOVABLE will fail
because of some reserved pages at the beginning of each zone
(MIGRATE_RESERVED).

During memory hot-add, setup_per_zone_pages_min() will be called from
online_pages() to redistribute/recalculate the reserved page blocks.
This will mark some page blocks at the beginning of each zone as
MIGRATE_RESERVE. Now, the memory chunk containing these blocks cannot
be set offline again, because only MIGRATE_MOVABLE pages can be isolated
(offline_pages -> start_isolate_page_range).

So you cannot remove all the memory chunks that have been added via
memory hotplug. I'm not sure if I am missing something here, or if this
really is a bug. Any thoughts?

Thanks,
Gerald

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