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

[Garg, Shivank]

On Wed, 2026-08-05 at 06:40 +0000, Shivank Garg wrote:
> 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/ambEdSPjerZIVN0b@192.168.1.215/
> [3] https://sashiko.dev/#/patchset/20260611-shivank-gmem-migrate-v1-0-2d266bfc6f95%40amd.com
> [4] https://lore.kernel.org/all/20260630-shivank-batch-migrate-offload-v6-1-da95d7e8b8a2@xxxxxxx
> 
> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
> ---
> 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/20260728-shivank-gmem-migrate-v2-0-269ac1f84e2b@xxxxxxx
> 
> 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/20260611-shivank-gmem-migrate-v1-0-2d266bfc6f95@xxxxxxx
> 
> ---
> 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
> 
> 

Should I spin the independent patches out of this series for next cycle?
Patch 1 is Sashiko reported fix and patches 5-8 are selftest fixes and
cleanups. Migration patches can wait until its setting design discussion
is settled.

Thanks,
Shivank