Re: [PATCH V9 0/7] Add NUMA mempolicy support for KVM guest-memfd

[David Hildenbrand <david@redhat.com>]

On 13.07.25 19:43, Shivank Garg wrote:
> This series introduces NUMA-aware memory placement support for KVM guests
> with guest_memfd memory backends. It builds upon Fuad Tabba's work that
> enabled host-mapping for guest_memfd memory [1].
> 
> == Background ==
> KVM's guest-memfd memory backend currently lacks support for NUMA policy
> enforcement, causing guest memory allocations to be distributed across host
> nodes  according to kernel's default behavior, irrespective of any policy
> specified by the VMM. This limitation arises because conventional userspace
> NUMA control mechanisms like mbind(2) don't work since the memory isn't
> directly mapped to userspace when allocations occur.
> Fuad's work [1] provides the necessary mmap capability, and this series
> leverages it to enable mbind(2).
> 
> == Implementation ==
> 
> This series implements proper NUMA policy support for guest-memfd by:
> 
> 1. Adding mempolicy-aware allocation APIs to the filemap layer.
> 2. Introducing 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 NUMA
>     policy.
> 
> With these changes, VMMs can now control guest memory placement by mapping
> guest_memfd file descriptor and using mbind(2) to specify:
> - Policy modes: default, bind, interleave, or preferred
> - Host NUMA nodes: List of target nodes for memory allocation
> 
> These Policies affect only 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 by design).
> 
> == Upstream Plan ==
> Phased approach as per David's guest_memfd extension overview [2] and
> community calls [3]:
> 
> Phase 1 (this series):
> 1. Focuses on shared guest_memfd support (non-CoCo VMs).
> 2. Builds on Fuad's host-mapping work.

Just to clarify: this is based on Fuad's stage 1 and should probably still be
tagged "RFC" until stage-1 is finally upstream.

(I was hoping stage-1 would go upstream in 6.17, but I am not sure yet if that is
still feasible looking at the never-ending review)

I'm surprised to see that

commit cbe4134ea4bc493239786220bd69cb8a13493190
Author: Shivank Garg <shivankg@amd.com>
Date:   Fri Jun 20 07:03:30 2025 +0000

     fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass
     
was merged with the kvm export

	EXPORT_SYMBOL_GPL_FOR_MODULES(anon_inode_make_secure_inode, "kvm");

I thought I commented that this is something to done separately and not really
"fix" material.

Anyhow, good for this series, no need to touch that.

-- 
Cheers,

David / dhildenb