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Date:   Thu, 16 Nov 2023 20:03:54 +0100
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Subject: Re: [PATCH 4/8] mm/memory_hotplug: introduce MEM_PHYS_ONLINE/OFFLINE
 memory notifiers
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To:     Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc:     linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        linux-s390 <linux-s390@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20231114180238.1522782-1-sumanthk@linux.ibm.com>
 <20231114180238.1522782-5-sumanthk@linux.ibm.com>
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From:   David Hildenbrand <david@redhat.com>
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On 15.11.23 15:23, Sumanth Korikkar wrote:
> 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?

See my reply to Gerald.

In an ideal world, there would not be any new callbacks, we would get 
rid of them, and just let the architecture properly hotplug memory to 
the system when requested by the user.

-- 
Cheers,

David / dhildenb