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

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Message-ID: <5e3b16ec-9ef9-483e-b97e-bff0c1915b19@redhat.com>
Date: Tue, 7 Oct 2025 21:35:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Sumanth Korikkar <sumanthk@...ux.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm <linux-mm@...ck.org>,
 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 19:56, Sumanth Korikkar wrote:
>>> 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.
>>
>> Before I dig into the details, will onlining/offling still trigger
>> hypervisor action, or does that now really happen when memory is
>> added/removed?
>>
>> That would be really nice, because it would remove the whole need for
>> "standby" memory, and having to treat hotplugged memory differently under
>> LPAR/z/VM than anywhere else (-> keep it offline).
> 
> With this approach, hypervisor actions are triggered only when memory is
> actually added or removed.
> 
> Online and offline operations are common code memory hotplug actions and
> the s390 memory notifier actions are none/minimal.

Very nice.

> 
>>> s390 kernel sysfs interface to configure/deconfigure memory with
>>> memmap_on_memory (with upcoming lsmem changes):
>>> * Initial memory layout:
>>> lsmem -o RANGE,SIZE,STATE,BLOCK,CONFIGURED,MEMMAP_ON_MEMORY
>>> RANGE                 SIZE   STATE BLOCK CONFIGURED MEMMAP_ON_MEMORY
>>> 0x00000000-0x7fffffff   2G  online 0-15  yes        no
>>> 0x80000000-0xffffffff   2G offline 16-31 no         yes
>>
>> Could we instead modify "STATE" to reflect that it is "not added" / "not
>> configured" / "disabled" etc?
>>
>> Like
>>
>> lsmem -o RANGE,SIZE,STATE,BLOCK,MEMMAP_ON_MEMORY
>> RANGE                 SIZE    STATE BLOCK
>> 0x00000000-0x7fffffff   2G   online 0-15
>> 0x80000000-0xffffffff   2G disabled 16-31
>>
>> Or is that an attempt to maintain backwards compatibility?
> 
> Mostly. Also, similar to lscpu output, where CPU status shows
> CONFIGURED/STATE column.

Care to share an example output? I only have a s390x VM with 2 CPUs and 
no way to configure/deconfigure.

> 
> Also, older scripts to get list of offline memory typically use:
> lsmem | grep offline
> 
> and
> 
> chmem -e <SIZE> would work as usual, where <SIZE> specifies amount of
> memory to set online.
> 
> chmem changes would look like:
> chmem -c 128M -m 1 : configure memory with memmap-on-memory enabled
> chmem -g 128M : deconfigure memory

I wonder if the above two are really required. I would expect most/all 
users to simply keep using -e / -d.

Sure, there might be some corner cases, but I would assume most people 
to not want to care about memmap-on-memory with the new model.

> chmem -e 128M : optionally configure (if supported by architecture) and
> 		always online memory
> chmem -d 128M : offline and optionally deconfigure memory (if supported
> 		by architecture)

-- 
Cheers

David / dhildenb