linux-kernel - Re: [PATCH 0/4] mm,memory_hotplug: allocate memmap from hotadded memory

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Date:   Tue, 2 Apr 2019 10:28:15 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 0/4] mm,memory_hotplug: allocate memmap from hotadded
 memory

On Mon, Apr 01, 2019 at 01:53:06PM +0200, Michal Hocko wrote:
> On Mon 01-04-19 09:59:36, Oscar Salvador wrote:
> > On Fri, Mar 29, 2019 at 02:42:43PM +0100, Michal Hocko wrote:
> > > Having a larger contiguous area is definitely nice to have but you also
> > > have to consider the other side of the thing. If we have a movable
> > > memblock with unmovable memory then we are breaking the movable
> > > property. So there should be some flexibility for caller to tell whether
> > > to allocate on per device or per memblock. Or we need something to move
> > > memmaps during the hotremove.
> > 
> > By movable memblock you mean a memblock whose pages can be migrated over when
> > this memblock is offlined, right?
> 
> I am mostly thinking about movable_node kernel parameter which makes
> newly hotpluged memory go into ZONE_MOVABLE and people do use that to
> make sure such a memory can be later hotremoved.

Uhm, I might be missing your point, but hot-added memory that makes use of
vmemmap pages can be hot-removed as any other memory.

Vmemmap pages do not account as unmovable memory, they just stick around
until all sections they referred to have been removed, and then, we proceed
with removing them.
So, to put it in another way: vmemmap pages are left in the system until the
whole memory device (DIMM, virt mem-device or whatever) is completely
hot-removed.

-- 
Oscar Salvador
SUSE L3