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

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Message-ID: <20190403080113.adj2m3szhhnvzu56@d104.suse.de>
Date:   Wed, 3 Apr 2019 10:01:16 +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 Tue, Apr 02, 2019 at 02:48:45PM +0200, Michal Hocko wrote:
> 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?

If you hot-add two memblocks, this means that either:

a) you hot-add a 256MB-memory-device (128MB per memblock)
b) you hot-add two 128MB-memory-device

Either way, hot-removing only works for memory-device as a whole, so
there is no problem.

Vmemmaps are created per hot-added operations, this means that
vmemmaps will be created for the hot-added range.
And since hot-add/hot-remove operations works with the same granularity,
there is no problem.

E.g:

# (qemu) object_add memory-backend-ram,id=ram0,size=128M
# (qemu) device_add pc-dimm,id=dimm0,memdev=ram0,node=1

# (qemu) object_add memory-backend-ram,id=ram1,size=512M
# (qemu) device_add pc-dimm,id=dimm1,memdev=ram1,node=1

# (qemu) object_add memory-backend-ram,id=ram2,size=1G
# (qemu) device_add pc-dimm,id=dimm2,memdev=ram2,node=1

These are three hot-add operations.
Each hot-add operation will create use vmemmaps to hold the memmap for
its hot-added sections.

-- 
Oscar Salvador
SUSE L3