linux-kernel - Re: [PATCH 4/8] mm/memory_hotplug: introduce MEM_PHYS

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Date:   Wed, 15 Nov 2023 15:23:18 +0100
From:   Sumanth Korikkar <sumanthk@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] mm/memory_hotplug: introduce MEM_PHYS_ONLINE/OFFLINE
 memory notifiers

On Tue, Nov 14, 2023 at 07:27:35PM +0100, David Hildenbrand wrote:
> On 14.11.23 19:02, Sumanth Korikkar wrote:
> > Add new memory notifiers to mimic the dynamic ACPI event triggered logic
> > for memory hotplug on platforms that do not generate such events. This
> > will be used to implement "memmap on memory" feature for s390 in a later
> > patch.
> > 
> > Platforms such as x86 can support physical memory hotplug via ACPI. When
> > there is physical memory hotplug, ACPI event leads to the memory
> > addition with the following callchain:
> > acpi_memory_device_add()
> >    -> acpi_memory_enable_device()
> >       -> __add_memory()
> > 
> > After this, the hotplugged memory is physically accessible, and altmap
> > support prepared, before the "memmap on memory" initialization in
> > memory_block_online() is called.
> > 
> > On s390, memory hotplug works in a different way. The available hotplug
> > memory has to be defined upfront in the hypervisor, but it is made
> > physically accessible only when the user sets it online via sysfs,
> > currently in the MEM_GOING_ONLINE notifier. This requires calling
> > add_memory() during early memory detection, in order to get the sysfs
> > representation, but we cannot use "memmap on memory" altmap support at
> > this stage, w/o having it physically accessible.
> > 
> > Since no ACPI or similar events are generated, there is no way to set up
> > altmap support, or even make the memory physically accessible at all,
> > before the "memmap on memory" initialization in memory_block_online().
> > 
> > The new MEM_PHYS_ONLINE notifier allows to work around this, by
> > providing a hook to make the memory physically accessible, and also call
> > __add_pages() with altmap support, early in memory_block_online().
> > Similarly, the MEM_PHYS_OFFLINE notifier allows to make the memory
> > inaccessible and call __remove_pages(), at the end of
> > memory_block_offline().
> > 
> > Calling __add/remove_pages() requires mem_hotplug_lock, so move
> > mem_hotplug_begin/done() to include the new notifiers.
> > 
> > All architectures ignore unknown memory notifiers, so this patch should
> > not introduce any functional changes.
> 
> Sorry to say, no. No hacks please, and this is a hack for memory that has
> already been added to the system.
> 
> If you want memory without an altmap to suddenly not have an altmap anymore,
> then look into removing and readding that memory, or some way to convert
> offline memory.

Sorry, I couldnt get the context. Could you please give me more details?

Thanks