[PATCH v3 0/9] KVM: guest_memfd: folio migration for non-confidential VMs

guest_memfd folios are currently always marked unmovable, so the kernel cannot
perform memory compaction, offlining, etc. This is unavoidable for
confidential VMs (SEV-SNP, TDX), since memory is encrypted and copying it
needs firmware assistance. However, for non-confidential VMs (like
Firecracker), we can migrate the folios.

This series enables folio migration for non-confidential guest_memfd and
also lays the groundwork for migrating confidential guest_memfd later.
Once firmware-assisted copying support is available, those VMs can be
made movable, the confidential folio content can be copied separately,
and the destination folio marked with FOLIO_CONTENT_COPIED[4] so
__migrate_folio() skips the host-side folio_mc_copy().

Testing
-------
Host: 7.2-rc6+(c21bb419386) + this, AMD EPYC ZEN 3, 2 NUMA nodes

- KVM selftest: allocate folios on node 0, migrate them to node 1 and
  back and verify resulting NUMA node and the folio contents at each
  step.

- Firecracker [1]: booted a microVM backed by guest_memfd. While the
  guest was running, forced host-side migration of its folios via
  migratepages(8) and explicit move_pages(2) of guest_memfd
  pages. Verify with /proc/firecracker_pid/numa_maps.

Notes
-----
- Sashiko pointed out a pre-existing ABBA deadlock between
  kvm_gmem_error_folio() and truncation. It's being addressed separately
  by Hao Zhang. [2][3]

[1] https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding
    In builder.rs, add GUEST_MEMFD_FLAG_MIGRATABLE to bit-2 and pass it instead
    of GUEST_MEMFD_FLAG_NO_DIRECT_MAP to vm.create_guest_memfd().
[2] https://lore.kernel.org/all/[email protected]/
[3] https://sashiko.dev/#/patchset/20260611-shivank-gmem-migrate-v1-0-2d266bfc6f95%40amd.com
[4] https://lore.kernel.org/all/[email protected]

Signed-off-by: Shivank Garg <[email protected]>
---
Changes in v3:
- Fix unbalanced mmu_invalidate_in_progress count unbinding dying guest_memfd. (Sashiko)
- Fix maxnode handling in xapic_ipi_test selftest.
- Add GUEST_MEMFD_FLAG_MIGRATABLE documentation
- Replace open-coded sizeof() * 8 calculation with BITS_PER_TYPE()
- Add get_numa_mem_nodes() and use  MPOL_F_MEMS_ALLOWED for allowed NUMA ndoes
  instead of hardcoded NUMA node IDs. (Sashiko)
- Extend migration selftest to verify rejection without MIGRATABLE flag and
  move repeated checks into common helpers.
- Drop RFC tag.
- Link to v2: https://lore.kernel.org/r/[email protected]

Changes in v2:
- Make folio migration opt-in through GUEST_MEMFD_FLAG_MIGRATABLE,
  preserving unmovable behavior if userspace don't explictly ask. (Alexandru, David, Sean)
- Add kvm_arch_supports_gmem_migration() so arch can control whether
  GUEST_MEMFD_FLAG_MIGRATABLE is advertised.
- Allocate movable folios with GFP_HIGHUSER_MOVABLE. (David)
- Keep guest_memfd unevictable. (David, Sashiko, Sean)
- Split migrate_folio() implementation and enablement as separate patches.
- Update selftest with new flag.
- Link to v1: https://lore.kernel.org/r/[email protected]

---
Shivank Garg (9):
      KVM: guest_memfd: take the invalidate lock when unbinding a dying file
      mm: split AS_UNMOVABLE back out of AS_INACCESSIBLE
      KVM: guest_memfd: implement folio migration for non-confidential VMs
      KVM: guest_memfd: add GUEST_MEMFD_FLAG_MIGRATABLE
      KVM: selftests: fix maxnode arguments in xapic_ipi_test
      KVM: selftests: use BITS_PER_TYPE() for NUMA masks
      KVM: selftests: add get_numa_mem_nodes()
      KVM: selftests: use allowed NUMA nodes in guest_memfd_test
      KVM: selftests: exercise guest_memfd folio migration

 Documentation/virt/kvm/api.rst                   |   3 +
 arch/x86/kvm/x86.c                               |   9 ++
 include/linux/kvm_host.h                         |   4 +
 include/linux/pagemap.h                          |  24 ++-
 include/uapi/linux/kvm.h                         |   1 +
 mm/compaction.c                                  |  12 +-
 mm/migrate.c                                     |   2 +-
 tools/include/uapi/linux/kvm.h                   |   1 +
 tools/testing/selftests/kvm/guest_memfd_test.c   | 193 +++++++++++++++++++----
 tools/testing/selftests/kvm/include/numaif.h     |  52 +-----
 tools/testing/selftests/kvm/x86/xapic_ipi_test.c |  14 +-
 virt/kvm/guest_memfd.c                           |  76 +++++++--
 12 files changed, 284 insertions(+), 107 deletions(-)
---
base-commit: c21bb4193868a8de71fc4693fa741e195fdf5d86
change-id: 20260611-shivank-gmem-migrate-8c1c519b30a6

Best regards,
-- 
Shivank Garg <[email protected]>