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

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Message-ID: <20190403081232.GB15605@dhcp22.suse.cz>
Date:   Wed, 3 Apr 2019 10:12:32 +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 Wed 03-04-19 10:01:16, Oscar Salvador wrote:
> 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.

What does prevent calling somebody arch_add_memory for a range spanning
multiple memblocks from a driver directly. In other words aren't you
making  assumptions about a future usage based on the qemu usecase?

-- 
Michal Hocko
SUSE Labs