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

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Date:   Tue, 14 Nov 2023 19:27:35 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Sumanth Korikkar <sumanthk@...ux.ibm.com>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 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.

But certainly not on the online/offline path triggered by sysfs.

-- 
Cheers,

David / dhildenb