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

[Yasunori Goto <y-goto@jp.fujitsu.com>]

Hi.


> 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?

I believe you are right. Current hot-remove code is NOT perfect.
You may remove some sections, but may not other sections,
because there are some un-removable pages by some reasons
(not only MIGRATE_RESERVED).

I think MIGRATE_RESERVED pages should be move to MIGRATE_MOVABLE when 
those pages must be removed, and should recalculate MIGRATE_RESERVED pages.

Bye.

-- 
Yasunori Goto