KVM's guest-memfd memory backend currently lacks support for NUMA policy enforcement, causing guest memory allocations to be distributed arbitrarily across host NUMA nodes regardless of the policy specified by the VMM. This occurs because conventional userspace NUMA control mechanisms like mbind() are ineffective with guest-memfd, as the memory isn't directly mapped to userspace when allocations occur.
This patch-series adds NUMA-aware memory placement for guest_memfd backed KVM guests. Based on community feedback, the approach has evolved as follows:
- v1,v2: Extended the KVM_CREATE_GUEST_MEMFD IOCTL to pass mempolicy. - v3: Introduced fbind() syscall for VMM memory-placement configuration. - v4-v6: Current approach using shared_policy support and vm_ops (based on suggestions from David[1] and guest_memfd biweekly upstream calls[2][4]). - v7: Use inodes to store NUMA policy instead of file[5].
== Implementation ==
This series implements proper NUMA policy support for guest-memfd by:
1. Adding mempolicy-aware allocation APIs to the filemap layer. 2. Add custom inodes (via a dedicated slab-allocated inode cache, kvm_gmem_inode_info) to store NUMA policy and metadata for guest memory. 3. Implementing get/set_policy vm_ops in guest_memfd to support shared policy.
With these changes, VMMs can now control guest memory placement by specifying: - Policy modes: default, bind, interleave, or preferred - Host NUMA nodes: List of target nodes for memory allocation
Policies only affect future allocations and do not migrate existing memory. This matches mbind(2)'s default behavior which affects only new allocations unless overridden with MPOL_MF_MOVE/MPOL_MF_MOVE_ALL flags (Not supported for guest_memfd as it is unmovable).
This series builds on the existing guest-memfd support in KVM and provides a clean integration path for NUMA-aware memory management in confidential computing environments. The work is primarily focused on supporting SEV-SNP requirements, though the benefits extend to any VMM using the guest-memfd backend that needs control over guest memory placement.
== Example usage with QEMU (requires patched QEMU from [3]) ==
Snippet of the QEMU changes[3] needed to support this feature:
/* Create and map guest-memfd region */ new_block->guest_memfd = kvm_create_guest_memfd( new_block->max_length, 0, errp); ... void *ptr_memfd = mmap(NULL, new_block->max_length, PROT_READ | PROT_WRITE, MAP_SHARED, new_block->guest_memfd, 0); ... /* Apply NUMA policy */ int ret = mbind(ptr_memfd, new_block->max_length, backend->policy, backend->host_nodes, maxnode+1, 0); ...
QEMU Command to run SEV-SNP guest with interleaved memory across nodes 0 and 1 of the host:
$ qemu-system-x86_64 \ -enable-kvm \ ... -machine memory-encryption=sev0,vmport=off \ -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1 \ -numa node,nodeid=0,memdev=ram0,cpus=0-15 \ -object memory-backend-memfd,id=ram0,host-nodes=0-1,policy=interleave,size=1024M,share=true,prealloc=false
== Experiment and Analysis ==
SEV-SNP enabled host, AMD Zen 3, 2 socket 2 NUMA node system NUMA for Policy Guest Node 0: policy=interleave, host-node=0-1
Test: Allocate and touch 50GB inside guest on node=0.
* Generic Kernel (without NUMA supported guest-memfd): Node 0 Node 1 Total Before running Test: MemUsed 9981.60 3312.00 13293.60 After running Test: MemUsed 61451.72 3201.62 64653.34
Arbitrary allocations: all ~50GB allocated on node 0.
* With NUMA supported guest-memfd: Node 0 Node 1 Total Before running Test: MemUsed 5003.88 3963.07 8966.94 After running Test: MemUsed 30607.55 29670.00 60277.55
Balanced memory distribution: Equal increase (~25GB) on both nodes.
== Conclusion ==
Adding the NUMA-aware memory management to guest_memfd will make a lot of sense. Improving performance of memory-intensive and locality-sensitive workloads with fine-grained control over guest memory allocations, as pointed out in the analysis.
Please review and provide feedback!
Thanks, Shivank
[1] https://lore.kernel.org/all/6fbef654-36e2-4be5-906e-2a648a845278@redhat.com [2] https://lore.kernel.org/all/6f2bfac2-d9e7-4e4a-9298-7accded16b4f@redhat.com [3] https://github.com/shivankgarg98/qemu/tree/guest_memfd_mbind_NUMA [4] https://lore.kernel.org/all/2b77e055-98ac-43a1-a7ad-9f9065d7f38f@amd.com [5] https://lore.kernel.org/all/diqzbjumm167.fsf@ackerleytng-ctop.c.googlers.com
== Earlier postings and changelogs ==
v7 (current): - Add fixes suggested by Vlastimil and Ackerley. - Store NUMA policy in custom inode struct instead of file.
v6: - https://lore.kernel.org/all/20250226082549.6034-1-shivankg@amd.com - Rebase to linux mainline - Drop RFC tag - Add selftests to ensure NUMA support for guest_memfd works correctly.
v5: - https://lore.kernel.org/all/20250219101559.414878-1-shivankg@amd.com - Fix documentation and style issues. - Use EXPORT_SYMBOL_GPL - Split preparatory change in separate patch
v4: - https://lore.kernel.org/all/20250210063227.41125-1-shivankg@amd.com - Dropped fbind() approach in favor of shared policy support.
v3: - https://lore.kernel.org/all/20241105164549.154700-1-shivankg@amd.com - Introduce fbind() syscall and drop the IOCTL-based approach.
v2: - https://lore.kernel.org/all/20240919094438.10987-1-shivankg@amd.com - Add fixes suggested by Matthew Wilcox.
v1: - https://lore.kernel.org/all/20240916165743.201087-1-shivankg@amd.com - Proposed IOCTL based approach to pass NUMA mempolicy.
Ackerley Tng (1): KVM: guest_memfd: Make guest mem use guest mem inodes instead of anonymous inodes
Shivank Garg (6): mm/mempolicy: Export memory policy symbols security: Export security_inode_init_security_anon for KVM guest_memfd KVM: Add kvm_gmem_exit() cleanup function KVM: guest_memfd: Add slab-allocated inode cache KVM: guest_memfd: Enforce NUMA mempolicy using shared policy KVM: guest_memfd: selftests: Add tests for mmap and NUMA policy support
Shivansh Dhiman (1): mm/filemap: Add mempolicy support to the filemap layer
include/linux/pagemap.h | 41 +++ include/uapi/linux/magic.h | 1 + mm/filemap.c | 27 +- mm/mempolicy.c | 6 + security/security.c | 1 + .../testing/selftests/kvm/guest_memfd_test.c | 86 +++++- virt/kvm/guest_memfd.c | 261 ++++++++++++++++-- virt/kvm/kvm_main.c | 2 + virt/kvm/kvm_mm.h | 6 + 9 files changed, 402 insertions(+), 29 deletions(-)