On 4/10/2025 7:10 PM, 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 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@redhat.com Signed-off-by: Shivank Garg shivankg@amd.com
virt/kvm/guest_memfd.c | 75 ++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 73 insertions(+), 2 deletions(-)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 0ccbb152483a..233d3fd5781c 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -4,6 +4,7 @@ #include <linux/backing-dev.h> #include <linux/falloc.h> #include <linux/kvm_host.h> +#include <linux/mempolicy.h> #include <linux/pseudo_fs.h> #include <linux/pagemap.h> #include <linux/anon_inodes.h> @@ -19,6 +20,7 @@ struct kvm_gmem { }; struct kvm_gmem_inode_info {
- struct shared_policy policy; struct inode vfs_inode;
};
What are the pros and cons that you see of storing struct shared_policy in a containing struct kvm_gmem_inode_info, as opposed to storing it in inode->i_private?
I've just been using inode->i_private for sharability and hugetlb metadata and didn't consider this option.
Could one reason be that struct shared_policy is a requirement for all inodes (not a CONFIG flag) but sharability and hugetlb metadata are both configurable, possibly at runtime?
This makes sense.
I considered using i_private but opted for the kvm_gmem_inode_info container approach finally. I think it's more extensible if we need to add fields later for new features and it's more maintainable when the struct grows more complex with time. It follows the established patterns in other filesystems (shmem, ext4, etc.) which have proven maintainable over time - so I don't have to worry about it. Also, since you're already using i_private for flags, I'd have needed to create a wrapper struct anyway to contain both policy and flags.
@@ -27,6 +29,9 @@ static inline struct kvm_gmem_inode_info *KVM_GMEM_I(struct inode *inode) return container_of(inode, struct kvm_gmem_inode_info, vfs_inode); } +static struct mempolicy *kvm_gmem_get_pgoff_policy(struct kvm_gmem_inode_info *info,
pgoff_t index);/**
- folio_file_pfn - like folio_file_page, but return a pfn.
- @folio: The folio which contains this index.
@@ -113,7 +118,24 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot, static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index) { /* TODO: Support huge pages. */
- return filemap_grab_folio(inode->i_mapping, index);
- struct mempolicy *policy;
- struct folio *folio;
- /*
* Fast-path: See if folio is already present in mapping to avoid* policy_lookup.*/- folio = __filemap_get_folio(inode->i_mapping, index,
FGP_LOCK | FGP_ACCESSED, 0);- if (!IS_ERR(folio))
return folio;- policy = kvm_gmem_get_pgoff_policy(KVM_GMEM_I(inode), index);
- folio = filemap_grab_folio_mpol(inode->i_mapping, index, policy,
NO_INTERLEAVE_INDEX);- mpol_cond_put(policy);
- return folio;
} static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start, @@ -336,12 +358,14 @@ static struct inode *kvm_gmem_alloc_inode(struct super_block *sb) if (!info) return NULL;
- mpol_shared_policy_init(&info->policy, NULL);
- return &info->vfs_inode;
} static void kvm_gmem_destroy_inode(struct inode *inode) {
- mpol_free_shared_policy(&KVM_GMEM_I(inode)->policy);
} static void kvm_gmem_free_inode(struct inode *inode) @@ -384,7 +408,54 @@ static void kvm_gmem_init_mount(void) kvm_gmem_mnt->mnt_flags |= MNT_NOEXEC; } +#ifdef CONFIG_NUMA +static int kvm_gmem_set_policy(struct vm_area_struct *vma, struct mempolicy *mpol) +{
- struct inode *inode = file_inode(vma->vm_file);
- return mpol_set_shared_policy(&KVM_GMEM_I(inode)->policy, vma, mpol);
+}
+static struct mempolicy *kvm_gmem_get_policy(struct vm_area_struct *vma,
unsigned long addr, pgoff_t *pgoff)+{
- struct inode *inode = file_inode(vma->vm_file);
- *pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT);
- return mpol_shared_policy_lookup(&KVM_GMEM_I(inode)->policy, *pgoff);
+}
+static struct mempolicy *kvm_gmem_get_pgoff_policy(struct kvm_gmem_inode_info *info,
pgoff_t index)+{
- struct mempolicy *mpol;
- mpol = mpol_shared_policy_lookup(&info->policy, index);
- return mpol ? mpol : get_task_policy(current);
+} +#else +static struct mempolicy *kvm_gmem_get_pgoff_policy(struct kvm_gmem_inode_info *info,
pgoff_t index)+{
- return NULL;
+} +#endif /* CONFIG_NUMA */
+static const struct vm_operations_struct kvm_gmem_vm_ops = { +#ifdef CONFIG_NUMA
- .get_policy = kvm_gmem_get_policy,
- .set_policy = kvm_gmem_set_policy,
+#endif +};
+static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma) +{
- vma->vm_ops = &kvm_gmem_vm_ops;
- return 0;
+}
static struct file_operations kvm_gmem_fops = {
- .mmap = kvm_gmem_mmap, .open = generic_file_open, .release = kvm_gmem_release, .fallocate = kvm_gmem_fallocate,
-- 2.34.1