[PATCH 0/6] chmem/lsmem: dynamic (de)configuration of memory

[Sumanth Korikkar <sumanthk@linux.ibm.com>]

Hi all,

Patchset extends chmem and lsmem with support for dynamically configuring and
deconfiguring hotpluggable memory blocks on s390, including per-block
memmap-on-memory handling.

On s390, the memmap-on-memory feature was
introduced to ensure that the struct page array (metadata) for hotpluggable
standby memory can be allocated from the memory block itself. This allowed
hot-add operations even under memory pressure, especially in systems with an
imbalance between boot-time online memory and standby memory.

The original implementation had few limitations:
* All hotpluggable standby memory was added at boot, making blocks
  visible for online/offline operations earlier.
* The use of memmap-on-memory was global and static, decided at boot
  time. Either all standby blocks used it, or none of them did.
* memmap-on-memory choice could not be changed at runtime, limiting
  flexibility.

The s390 kernel (linux-next) ff18dcb19aab ("s390/sclp: Add support for
dynamic (de)configuration of memory") no longer pre-adds all standby
memory at boot. Instead, users must explicitly configure a block before
it can be used for online/offline actions.  At configuration time, users
can dynamically decide whether to use optional memmap-on-memory for each
memory block, where value of 1 allocates metadata (such as struct pages
array) from the hotplug memory itself, enabling hot-add operations even
under memory pressure. A value of 0 stores metadata in regular system
memory and enables continuous physical memory across memory blocks.

Kernel changes for dynamic (de)configuration of memory (available on
linux-next):
https://lore.kernel.org/all/20251010085147.2175918-1-sumanthk@linux.ibm.com/
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/log/drivers/s390/char/sclp_mem.c

The kernel sysfs interface provides configuration and memmap-on-memory toggling:
echo 1 > /sys/firmware/memory/memoryX/config – configure block
echo 0 > /sys/firmware/memory/memoryX/config – deconfigure block
echo 1 > /sys/firmware/memory/memoryX/memmap_on_memory – enable memmap-on-memory
echo 0 > /sys/firmware/memory/memoryX/memmap_on_memory – disable memmap-on-memory

Patchset teaches chmem and lsmem to make use of these interfaces, mirroring
existing online/offline semantics:

chmem -c 128M -m 1 : configure memory with memmap-on-memory
chmem -g 128M : deconfigure memory
chmem -e 128M : configure (if supported by architecture) and online memory
chmem -d 128M : offline and deconfigure memory (if supported by
architecture)
lsmem -o RANGE,SIZE,STATE,BLOCK,CONFIGURED,MEMMAP-ON-MEMORY

memmap-on-memory can only be toggled when a block is in a deconfigured state,
and is supported via the --configure option.

Thank you

Sumanth Korikkar (6):
  lsmem: display global memmap on memory parameter
  lsmem: add support to display dynamic (de)configuration of memory
  chmem: add support for dynamic (de)configuration of hotplug memory
  chmem: add chmem documentation for dynamic (de)configuration of memory
  lsmem: add doc for dynamic (de)configuration and memmap-on-memory
    support
  lsmem,chmem: add configure/deconfigure bash completion options

 bash-completion/chmem  |   3 +
 bash-completion/lsmem  |   2 +-
 sys-utils/chmem.8.adoc |  47 +++++-
 sys-utils/chmem.c      | 372 +++++++++++++++++++++++++++++++++++++----
 sys-utils/lsmem.1.adoc |  46 ++++-
 sys-utils/lsmem.c      | 144 ++++++++++++++--
 6 files changed, 564 insertions(+), 50 deletions(-)

-- 
2.48.1