linux-kernel - Re: [PATCH 0/4] Support dynamic (de)configuration of memory

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Message-ID: <0daf151e-d126-4e7b-adb6-5d6b745f4ad2@redhat.com>
Date: Tue, 7 Oct 2025 18:02:51 +0200
From: David Hildenbrand <david@...hat.com>
To: Sumanth Korikkar <sumanthk@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm <linux-mm@...ck.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 linux-s390 <linux-s390@...r.kernel.org>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>
Subject: Re: [PATCH 0/4] Support dynamic (de)configuration of memory

On 07.10.25 16:30, Sumanth Korikkar wrote:
> On Fri, Sep 26, 2025 at 03:15:23PM +0200, Sumanth Korikkar wrote:
>> Hi,
>>
>> Patchset provides a new interface for dynamic configuration and
>> deconfiguration of hotplug memory on s390, allowing with/without
>> memmap_on_memory support. It is a follow up on the discussion with David
>> when introducing memmap_on_memory support for s390 and support dynamic
>> (de)configuration of memory:
>> https://lore.kernel.org/all/ee492da8-74b4-4a97-8b24-73e07257f01d@redhat.com/
>> https://lore.kernel.org/all/20241202082732.3959803-1-sumanthk@linux.ibm.com/
>>
>> The original motivation for introducing memmap_on_memory on s390 was to
>> avoid using online memory to store struct pages metadata, particularly
>> for standby memory blocks. This became critical in cases where there was
>> an imbalance between standby and online memory, potentially leading to
>> boot failures due to insufficient memory for metadata allocation.
>>
>> To address this, memmap_on_memory was utilized on s390. However, in its
>> current form, it adds struct pages metadata at the start of each memory
>> block at the time of addition (only standby memory), and this
>> configuration is static. It cannot be changed at runtime  (When the user
>> needs continuous physical memory).
>>
>> Inorder to provide more flexibility to the user and overcome the above
>> limitation, add an option to dynamically configure and deconfigure
>> hotpluggable memory block with/without memmap_on_memory.
>>
>> With the new interface, s390 will not add all possible hotplug memory in
>> advance, like before, to make it visible in sysfs for online/offline
>> actions. Instead, before memory block can be set online, it has to be
>> configured via a new interface in /sys/firmware/memory/memoryX/config,
>> which makes s390 similar to others.  i.e. Adding of hotpluggable memory is
>> controlled by the user instead of adding it at boottime.
> 
> Hi David,
> 
> Looking forward to your feedback to proceed further.

Thanks for bumping it up in my inbox, will comment today :)

-- 
Cheers

David / dhildenb