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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/21/26 14:34, Garg, Shivank wrote:
> 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@amd.com
>>
>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>> ---
>> 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@amd.com
>>
>> 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@amd.com
>>
>> ---
>> 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?

I'd say, yes :)

-- 
Cheers,

David