linux-kernel - Re: [PATCH V9 6/7] KVM: guest_memfd: Enforce NUMA mempolicy using shared policy

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Message-ID: <651d0f59-990d-460e-829b-be5cb1ee7fe7@suse.cz>
Date: Mon, 21 Jul 2025 15:30:19 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Shivank Garg <shivankg@....com>, seanjc@...gle.com, david@...hat.com,
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Subject: Re: [PATCH V9 6/7] KVM: guest_memfd: Enforce NUMA mempolicy using
 shared policy

On 7/13/25 19:43, Shivank Garg wrote:
> Previously, guest-memfd allocations followed local NUMA node id in absence
> of process mempolicy, resulting in arbitrary memory allocation.
> Moreover, mbind() couldn't be used  by the VMM as guest memory wasn't
> mapped into userspace when allocation occurred.
> 
> Enable NUMA policy support by implementing vm_ops for guest-memfd mmap
> operation. This allows the VMM to map the memory and use mbind() to set the
> desired NUMA policy. The policy is stored in the inode structure via
> kvm_gmem_inode_info, as memory policy is a property of the memory (struct
> inode) itself. The policy is then retrieved via mpol_shared_policy_lookup()
> and passed to filemap_grab_folio_mpol() to ensure that allocations follow
> the specified memory policy.
> 
> This enables the VMM to control guest memory NUMA placement by calling
> mbind() on the mapped memory regions, providing fine-grained control over
> guest memory allocation across NUMA nodes.
> 
> The policy change only affect future allocations and does 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, which are not supported for guest_memfd as it is unmovable.
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Shivank Garg <shivankg@....com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>