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

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Message-ID: <aOUj_RE8z9OiIIfN@li-2b55cdcc-350b-11b2-a85c-a78bff51fc11.ibm.com>
Date: Tue, 7 Oct 2025 16:30:21 +0200
From: Sumanth Korikkar <sumanthk@...ux.ibm.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, 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 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.

Thank you,
Sumanth