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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aOX_L1_2S30XhLRA@li-2b55cdcc-350b-11b2-a85c-a78bff51fc11.ibm.com>
Date: Wed, 8 Oct 2025 08:05:35 +0200
From: Sumanth Korikkar <sumanthk@...ux.ibm.com>
To: David Hildenbrand <david@...hat.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

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

lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2 ONLINE CONFIGURED POLARIZATION ADDRESS
  0    0      0    0      0    0 0:0:0         yes yes        vert-medium  0
  1    0      0    0      0    0 1:1:1         yes yes        vert-medium  1
  2    0      0    0      0    1 2:2:2         yes yes        vert-low     2
  3    0      0    0      0    1 3:3:3         yes yes        vert-low     3
  
# chcpu -d 2-3
CPU 2 disabled
CPU 3 disabled
# chcpu -g 2
CPU 2 deconfigured
# chcpu -c 2
CPU 2 configured
# chcpu -e 2-3
CPU 2 enabled
CPU 3 enabled

> > 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.

I believe this remains very beneficial for customers in the following
scenario:

1) Initial memory layout:
4 GB configured online
512 GB standby

If memory_hotplug.memmap_on_memory=Y is set in the kernel command line:
Suppose user requires more memory and onlines 256 GB. With memmap-on-memory
enabled, this likely succeeds by default.

Later, the user needs 256 GB of contiguous physical memory across memory
blocks. Then, the user can still configure those memory blocks with
memmap-on-memory disabled and online it.

2) If the administrator forgets to configure
memory_hotplug.memmap_on_memory=Y, the following steps can be taken:
Rescue from OOM situations: configure with memmap-on-memory enabled, online it.

Thank you,
Sumanth