Re: [PATCH v6 4/5] KVM: guest_memfd: Enforce NUMA mempolicy using shared policy

[Vlastimil Babka <vbabka@suse.cz>]

On 2/28/25 18:25, Ackerley Tng wrote:
> Shivank Garg <shivankg@amd.com> writes:
> 
>> 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 since memory wasn't mapped to userspace
>> in the VMM.
>>
>> 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 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@redhat.com>
>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>> ---
>>  virt/kvm/guest_memfd.c | 76 +++++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 75 insertions(+), 1 deletion(-)
>>
>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
>> index f18176976ae3..b3a8819117a0 100644
>> --- a/virt/kvm/guest_memfd.c
>> +++ b/virt/kvm/guest_memfd.c
>> @@ -2,6 +2,7 @@
>>  #include <linux/backing-dev.h>
>>  #include <linux/falloc.h>
>>  #include <linux/kvm_host.h>
>> +#include <linux/mempolicy.h>
>>  #include <linux/pagemap.h>
>>  #include <linux/anon_inodes.h>
>>
>> @@ -11,8 +12,12 @@ struct kvm_gmem {
>>  	struct kvm *kvm;
>>  	struct xarray bindings;
>>  	struct list_head entry;
>> +	struct shared_policy policy;
>>  };
>>
> 
> struct shared_policy should be stored on the inode rather than the file,
> since the memory policy is a property of the memory (struct inode),
> rather than a property of how the memory is used for a given VM (struct
> file).

That makes sense. AFAICS shmem also uses inodes to store policy.

> When the shared_policy is stored on the inode, intra-host migration [1]
> will work correctly, since the while the inode will be transferred from
> one VM (struct kvm) to another, the file (a VM's view/bindings of the
> memory) will be recreated for the new VM.
> 
> I'm thinking of having a patch like this [2] to introduce inodes.

shmem has it easier by already having inodes

> With this, we shouldn't need to pass file pointers instead of inode
> pointers.

Any downsides, besides more work needed? Or is it feasible to do it using
files now and convert to inodes later?

Feels like something that must have been discussed already, but I don't
recall specifics.