linux-kernel - Re: [PATCH] memory_hotplug: fix the panic when memory end is not on the section boundary

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Message-Id: <20180912162856.697038a8@thinkpad>
Date:   Wed, 12 Sep 2018 16:28:56 +0200
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Pasha Tatashin <Pavel.Tatashin@...rosoft.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        "zaslonko@...ux.ibm.com" <zaslonko@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        "osalvador@...e.de" <osalvador@...e.de>
Subject: Re: [PATCH] memory_hotplug: fix the panic when memory end is not on
 the section boundary

On Mon, 10 Sep 2018 15:26:55 +0000
Pasha Tatashin <Pavel.Tatashin@...rosoft.com> wrote:

> 
> I agree memoryblock is a hack, it fails to do both things it was
> designed to do:
> 
> 1. On bare metal you cannot free a physical dimm of memory using
> memoryblock granularity because memory devices do not equal to physical
> dimms. Thus, if for some reason a particular dimm must be
> remove/replaced, memoryblock does not help us.
> 
> 2. On machines with hypervisors it fails to provide an adequate
> granularity to add/remove memory.
> 
> We should define a new user interface where memory can be added/removed
> at a finer granularity: sparse section size, but without a memory
> devices for each section. We should also provide an optional access to
> legacy interface where memory devices are exported but each is of
> section size.
> 
> So, when legacy interface is enabled, current way would work:
> 
> echo offline > /sys/devices/system/memory/memoryXXX/state
> 
> And new interface would allow us to do something like this:
> 
> echo offline 256M > /sys/devices/system/node/nodeXXX/memory
> 
> With optional start address for offline memory.
> echo offline [start_pa] size > /sys/devices/system/node/nodeXXX/memory
> start_pa and size must be section size aligned (128M).
> 
> It would probably be a good discussion for the next MM Summit how to
> solve the current memory hotplug interface limitations.

Please keep lsmem/chmem from util-linux in mind, when changing the
memory hotplug user interface.