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Message-ID: <20251112192936.2574429-1-gourry@gourry.net>
Date: Wed, 12 Nov 2025 14:29:16 -0500
From: Gregory Price <gourry@...rry.net>
To: linux-mm@...ck.org
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Subject: [RFC LPC2026 PATCH v2 00/11] Specific Purpose Memory NUMA Nodes
This is a code RFC for discussion related to
"Mempolicy is dead, long live memory policy!"
https://lpc.events/event/19/contributions/2143/
base-commit: 24172e0d79900908cf5ebf366600616d29c9b417
(version notes at end)
At LSF 2026, I plan to discuss:
- Why? (In short: shunting to DAX is a failed pattern for users)
- Other designs I considered (mempolicy, cpusets, zone_device)
- Why mempolicy.c and cpusets as-is are insufficient
- SPM types seeking this form of interface (Accelerator, Compression)
- Platform extensions that would be nice to see (SPM-only Bits)
Open Questions
- Single SPM nodemask, or multiple based on features?
- Apply SPM/SysRAM bit on-boot only or at-hotplug?
- Allocate extra "possible" NUMA nodes for flexbility?
- Should SPM Nodes be zone-restricted? (MOVABLE only?)
- How to handle things like reclaim and compaction on these nodes.
With this set, we aim to enable allocation of "special purpose memory"
with the page allocator (mm/page_alloc.c) without exposing the same
memory as "System RAM". Unless a non-userland component, and does so
with the GFP_SPM_NODE flag, memory on these nodes cannot be allocated.
This isolation mechanism is a requirement for memory policies which
depend on certain sets of memory never being used outside special
interfaces (such as a specific mm/component or driver).
We present an example of using this mechanism within ZSWAP, as-if
a "compressed memory node" was present. How to describe the features
of memory present on nodes is left up to comment here and at LPC '26.
Userspace-driven allocations are restricted by the sysram_nodes mask,
nothing in userspace can explicitly request memory from SPM nodes.
Instead, the intent is to create new components which understand memory
features and register those nodes with those components. This abstracts
the hardware complexity away from userland while also not requiring new
memory innovations to carry entirely new allocators.
The ZSwap example demonstrates this with the `mt_spm_nodemask`. This
hack treats all spm nodes as-if they are compressed memory nodes, and
we bypass the software compression logic in zswap in favor of simply
copying memory directly to the allocated page. In a real design
There are 4 major changes in this set:
1) Introducing mt_sysram_nodelist in mm/memory-tiers.c which denotes
the set of nodes which are eligible for use as normal system ram
Some existing users now pass mt_sysram_nodelist into the page
allocator instead of NULL, but passing a NULL pointer in will simply
have it replaced by mt_sysram_nodelist anyway. Should a fully NULL
pointer still make it to the page allocator, without GFP_SPM_NODE
SPM node zones will simply be skipped.
mt_sysram_nodelist is always guaranteed to contain the N_MEMORY nodes
present during __init, but if empty the use of mt_sysram_nodes()
will return a NULL to preserve current behavior.
2) The addition of `cpuset.mems.sysram` which restricts allocations to
`mt_sysram_nodes` unless GFP_SPM_NODE is used.
SPM Nodes are still allowed in cpuset.mems.allowed and effective.
This is done to allow separate control over sysram and SPM node sets
by cgroups while maintaining the existing hierarchical rules.
current cpuset configuration
cpuset.mems_allowed
|.mems_effective < (mems_allowed ∩ parent.mems_effective)
|->tasks.mems_allowed < cpuset.mems_effective
new cpuset configuration
cpuset.mems_allowed
|.mems_effective < (mems_allowed ∩ parent.mems_effective)
|.sysram_nodes < (mems_effective ∩ default_sys_nodemask)
|->task.sysram_nodes < cpuset.sysram_nodes
This means mems_allowed still restricts all node usage in any given
task context, which is the existing behavior.
3) Addition of MHP_SPM_NODE flag to instruct memory_hotplug.c that the
capacity being added should mark the node as an SPM Node.
A node is either SysRAM or SPM - never both. Attempting to add
incompatible memory to a node results in hotplug failure.
DAX and CXL are made aware of the bit and have `spm_node` bits added
to their relevant subsystems.
4) Adding GFP_SPM_NODE - which allows page_alloc.c to request memory
from the provided node or nodemask. It changes the behavior of
the cpuset mems_allowed and mt_node_allowed() checks.
v1->v2:
- naming improvements
default_node -> sysram_node
protected -> spm (Specific Purpose Memory)
- add missing constify patch
- add patch to update callers of __cpuset_zone_allowed
- add additional logic to the mm sysram_nodes patch
- fix bot build issues (ifdef config builds)
- fix out-of-tree driver build issues (function renames)
- change compressed_nodelist to spm_nodelist
- add latch mechanism for sysram/spm nodes (Dan Williams)
this drops some extra memory-hotplug logic which is nice
v1: https://lore.kernel.org/linux-mm/20251107224956.477056-1-gourry@gourry.net/
Gregory Price (11):
mm: constify oom_control, scan_control, and alloc_context nodemask
mm: change callers of __cpuset_zone_allowed to cpuset_zone_allowed
gfp: Add GFP_SPM_NODE for Specific Purpose Memory (SPM) allocations
memory-tiers: Introduce SysRAM and Specific Purpose Memory Nodes
mm: restrict slub, oom, compaction, and page_alloc to sysram by
default
mm,cpusets: rename task->mems_allowed to task->sysram_nodes
cpuset: introduce cpuset.mems.sysram
mm/memory_hotplug: add MHP_SPM_NODE flag
drivers/dax: add spm_node bit to dev_dax
drivers/cxl: add spm_node bit to cxl region
[HACK] mm/zswap: compressed ram integration example
drivers/cxl/core/region.c | 30 ++++++
drivers/cxl/cxl.h | 2 +
drivers/dax/bus.c | 39 ++++++++
drivers/dax/bus.h | 1 +
drivers/dax/cxl.c | 1 +
drivers/dax/dax-private.h | 1 +
drivers/dax/kmem.c | 2 +
fs/proc/array.c | 2 +-
include/linux/cpuset.h | 62 +++++++------
include/linux/gfp_types.h | 5 +
include/linux/memory-tiers.h | 47 ++++++++++
include/linux/memory_hotplug.h | 10 ++
include/linux/mempolicy.h | 2 +-
include/linux/mm.h | 4 +-
include/linux/mmzone.h | 6 +-
include/linux/oom.h | 2 +-
include/linux/sched.h | 6 +-
include/linux/swap.h | 2 +-
init/init_task.c | 2 +-
kernel/cgroup/cpuset-internal.h | 8 ++
kernel/cgroup/cpuset-v1.c | 7 ++
kernel/cgroup/cpuset.c | 158 ++++++++++++++++++++------------
kernel/fork.c | 2 +-
kernel/sched/fair.c | 4 +-
mm/compaction.c | 10 +-
mm/hugetlb.c | 8 +-
mm/internal.h | 2 +-
mm/memcontrol.c | 3 +-
mm/memory-tiers.c | 66 ++++++++++++-
mm/memory_hotplug.c | 7 ++
mm/mempolicy.c | 34 +++----
mm/migrate.c | 4 +-
mm/mmzone.c | 5 +-
mm/oom_kill.c | 11 ++-
mm/page_alloc.c | 57 +++++++-----
mm/show_mem.c | 11 ++-
mm/slub.c | 15 ++-
mm/vmscan.c | 6 +-
mm/zswap.c | 66 ++++++++++++-
39 files changed, 532 insertions(+), 178 deletions(-)
--
2.51.1
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