memory hotplug: hot-remove fails on lowest chunk in ZONE_MOVABLE

[Gerald Schaefer <gerald.schaefer@de.ibm.com>]

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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