Re: [PATCH 0/4] Support dynamic (de)configuration of memory
From: David Hildenbrand
Date: Tue Oct 07 2025 - 12:11:21 EST
On 26.09.25 15:15, Sumanth Korikkar wrote:
Hi,
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@xxxxxxxxxx/
https://lore.kernel.org/all/20241202082732.3959803-1-sumanthk@xxxxxxxxxxxxx/
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.
This will cleanly add/remove the memory, including the directmap and other tracking data, so I like it.
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).
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?
* Configure memory
echo 1 > /sys/firmware/memory/memory16/config
The granularity here is also memory_block_size_bytes(), correct?
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
0x80000000-0x87ffffff 128M offline 16 yes yes
0x88000000-0xffffffff 1.9G offline 17-31 no yes
* Deconfigure memory
echo 0 > /sys/firmware/memory/memory16/config
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
* Enable memmap_on_memory and online it.
(Deconfigure first)
echo 0 > /sys/devices/system/memory/memory5/online
echo 0 > /sys/firmware/memory/memory5/config
lsmem -o RANGE,SIZE,STATE,BLOCK,CONFIGURED,MEMMAP_ON_MEMORY
RANGE SIZE STATE BLOCK CONFIGURED MEMMAP_ON_MEMORY
0x00000000-0x27ffffff 640M online 0-4 yes no
0x28000000-0x2fffffff 128M offline 5 no no
0x30000000-0x7fffffff 1.3G online 6-15 yes no
0x80000000-0xffffffff 2G offline 16-31 no yes
(Enable memmap_on_memory and online it)
echo 1 > /sys/firmware/memory/memory5/memmap_on_memory
echo 1 > /sys/firmware/memory/memory5/config
echo 1 > /sys/devices/system/memory/memory5/online
I guess the use for memmap_on_memory would now be limited to making hotplug more likely to succeed in OOM scenarios.
lsmem -o RANGE,SIZE,STATE,BLOCK,CONFIGURED,MEMMAP_ON_MEMORY
RANGE SIZE STATE BLOCK CONFIGURED MEMMAP_ON_MEMORY
0x00000000-0x27ffffff 640M online 0-4 yes no
0x28000000-0x2fffffff 128M online 5 yes yes
0x30000000-0x7fffffff 1.3G online 6-15 yes no
0x80000000-0xffffffff 2G offline 16-31 no yes
* Disable memmap_on_memory and online it.
(Deconfigure first)
echo 0 > /sys/devices/system/memory/memory5/online
echo 0 > /sys/firmware/memory/memory5/config
lsmem -o RANGE,SIZE,STATE,BLOCK,CONFIGURED,MEMMAP_ON_MEMORY
RANGE SIZE STATE BLOCK CONFIGURED MEMMAP_ON_MEMORY
0x00000000-0x27ffffff 640M online 0-4 yes no
0x28000000-0x2fffffff 128M offline 5 no yes
0x30000000-0x7fffffff 1.3G online 6-15 yes no
0x80000000-0xffffffff 2G offline 16-31 no yes
(Disable memmap_on_memory and online it)
echo 0 > /sys/firmware/memory/memory5/memmap_on_memory
echo 1 > /sys/firmware/memory/memory5/config
echo 1 > /sys/devices/system/memory/memory5/online
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
* Userspace changes:
lsmem/chmem tool is also changed to use the new interface. I will send
it to util-linux soon.
Patch 1 adds support for removal of boot-allocated memory blocks.
Patch 2 provides option to dynamically configure and deconfigure memory
with/without memmap_on_memory.
Patch 3 removes MHP_OFFLINE_INACCESSIBLE from s390. The mhp flag was
used to mark memory as not accessible until memory hotplug online phase
begins. However, with patch 2, it is no longer essential. Memory can be
brought to accessible state before adding memory, as the memory is added
during runttime now instead of boottime.
Nice.
Patch 4 removes the MEM_PREPARE_ONLINE/MEM_FINISH_OFFLINE notifiers. It
is no longer needed. Memory can be brought to accessible state before
adding memory now, with runtime (de)configuration of memory.
Nice.
--
Cheers
David / dhildenb