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

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Message-ID: <20190402124845.GD28293@dhcp22.suse.cz>
Date:   Tue, 2 Apr 2019 14:48:45 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
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 Tue 02-04-19 10:28:15, Oscar Salvador wrote:
> 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.

So what is going to happen when you hotadd two memblocks. The first one
holds memmaps and then you want to hotremove (not just offline) it?
-- 
Michal Hocko
SUSE Labs