[RFC v3 0/3] move_phys_pages syscall - migrate page contents given

[Gregory Price]

v3:
- pull forward to v6.8
- style and small fixups recommended by jcameron
- update syscall number (will do all archs when RFC tag drops)
- update for new folio code
- added OCP link to device-tracked address hotness proposal
- kept void* over __u64 simply because it integrates cleanly with
  existing migration code. If there's strong opinions, I can refactor.

This patch set is a proposal for a syscall analogous to move_pages,
that migrates pages between NUMA nodes using physical addressing.

The intent is to better enable user-land system-wide memory tiering
as CXL devices begin to provide memory resources on the PCIe bus.

For example, user-land software which is making decisions based on
data sources which expose physical address information no longer
must convert that information to virtual addressing to act upon it
(see background for info on how physical addresses are acquired).

The syscall requires CAP_SYS_ADMIN, since physical address source
information is typically protected by the same (or CAP_SYS_NICE).

This patch set broken into 3 patches:
  1) refactor of existing migration code for code reuse
  2) The sys_move_phys_pages system call.
  3) ktest of the syscall

The sys_move_phys_pages system call validates the page may be
migrated by checking migratable-status of each vma mapping the page,
and the intersection of cpuset policies each vma's task.


Background:

Userspace job schedulers, memory managers, and tiering software
solutions depend on page migration syscalls to reallocate resources
across NUMA nodes. Currently, these calls enable movement of memory
associated with a specific PID. Moves can be requested in coarse,
process-sized strokes (as with migrate_pages), and on specific virtual
pages (via move_pages).

However, a number of profiling mechanisms provide system-wide information
that would benefit from a physical-addressing version move_pages.

There are presently at least 4 ways userland can acquire physical
address information for use with this interface, and 1 hardware offload
mechanism being proposed by opencompute.

1) /proc/pid/pagemap: can be used to do page table translations.
     This is only really useful for testing, and the ktest was
     written using this functionality.

2) X86:  IBS (AMD) and PEBS (Intel) can be configured to return physical
     and/or vitual address information.

3) zoneinfo:  /proc/zoneinfo exposes the start PFN of zones

4) /sys/kernel/mm/page_idle:  A way to query whether a PFN is idle.
   So long as the page size is known, this can be used to identify
   system-wide idle pages that could be migrated to lower tiers.

   https://docs.kernel.org/admin-guide/mm/idle_page_tracking.html

5) CXL Offloaded Hotness Monitoring (Proposed): a CXL memory device
   may provide hot/cold information about its memory. For example,
   it may report the hottest device addresses (0-based) or a physical
   address (if it has access to decoders for convert bases).

   DPA can be cheaply converted to HPA by combining it with data
   exposed by /sys/bus/cxl/ information (region address bases).

See: https://www.opencompute.org/documents/ocp-cms-hotness-tracking-requirements-white-paper-pdf-1


Information from these sources facilitates systemwide resource management,
but with the limitations of migrate_pages and move_pages applying to
individual tasks, their outputs must be converted back to virtual addresses
and re-associated with specific PIDs.

Doing this reverse-translation outside of the kernel requires considerable
space and compute, and it will have to be performed again by the existing
system calls.  Much of this work can be avoided if the pages can be
migrated directly with physical memory addressing.

Gregory Price (3):
  mm/migrate: refactor add_page_for_migration for code re-use
  mm/migrate: Create move_phys_pages syscall
  ktest: sys_move_phys_pages ktest

 arch/x86/entry/syscalls/syscall_32.tbl  |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl  |   1 +
 include/linux/syscalls.h                |   5 +
 include/uapi/asm-generic/unistd.h       |   8 +-
 kernel/sys_ni.c                         |   1 +
 mm/migrate.c                            | 288 ++++++++++++++++++++----
 tools/include/uapi/asm-generic/unistd.h |   8 +-
 tools/testing/selftests/mm/migration.c  |  99 ++++++++
 8 files changed, 370 insertions(+), 41 deletions(-)

-- 
2.39.1