From: Nikita Kalyazin <kalyazin@amazon.com>

userfaultfd missing mode allows populating guest memory with the content
supplied by userspace on demand.

Extend guest_memfd implementation of vm_uffd_ops to support MISSING
mode.

Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 virt/kvm/guest_memfd.c | 60 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 59 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 087e7632bf70..14cca057fc0e 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -431,6 +431,14 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
 			ret = VM_FAULT_UFFD_MINOR;
 			goto out_folio;
 		}
+
+		/*
+		 * Check if userfaultfd is registered in missing mode. If so,
+		 * check if a folio exists in the page cache. If not, return
+		 * VM_FAULT_UFFD_MISSING to trigger the userfaultfd handler.
+		 */
+		if (userfaultfd_missing(vmf->vma) && IS_ERR_OR_NULL(folio))
+			return VM_FAULT_UFFD_MISSING;
 	}
 
 	/* folio not in the pagecache, try to allocate */
@@ -507,9 +515,59 @@ static bool kvm_gmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_fla
 	return true;
 }
 
+static struct folio *kvm_gmem_folio_alloc(struct vm_area_struct *vma,
+					  unsigned long addr)
+{
+	struct inode *inode = file_inode(vma->vm_file);
+	pgoff_t pgoff = linear_page_index(vma, addr);
+	struct mempolicy *mpol;
+	struct folio *folio;
+	gfp_t gfp;
+
+	if (unlikely(pgoff >= (i_size_read(inode) >> PAGE_SHIFT)))
+		return NULL;
+
+	gfp = mapping_gfp_mask(inode->i_mapping);
+	mpol = mpol_shared_policy_lookup(&GMEM_I(inode)->policy, pgoff);
+	mpol = mpol ?: get_task_policy(current);
+	folio = folio_alloc_mpol(gfp, 0, mpol, pgoff, numa_node_id());
+	mpol_cond_put(mpol);
+
+	return folio;
+}
+
+static int kvm_gmem_filemap_add(struct folio *folio,
+				struct vm_area_struct *vma,
+				unsigned long addr)
+{
+	struct inode *inode = file_inode(vma->vm_file);
+	struct address_space *mapping = inode->i_mapping;
+	pgoff_t pgoff = linear_page_index(vma, addr);
+	int err;
+
+	__folio_set_locked(folio);
+	err = filemap_add_folio(mapping, folio, pgoff, GFP_KERNEL);
+	if (err) {
+		folio_unlock(folio);
+		return err;
+	}
+
+	return 0;
+}
+
+static void kvm_gmem_filemap_remove(struct folio *folio,
+				    struct vm_area_struct *vma)
+{
+	filemap_remove_folio(folio);
+	folio_unlock(folio);
+}
+
 static const struct vm_uffd_ops kvm_gmem_uffd_ops = {
-	.can_userfault = kvm_gmem_can_userfault,
+	.can_userfault     = kvm_gmem_can_userfault,
 	.get_folio_noalloc = kvm_gmem_get_folio_noalloc,
+	.alloc_folio       = kvm_gmem_folio_alloc,
+	.filemap_add       = kvm_gmem_filemap_add,
+	.filemap_remove    = kvm_gmem_filemap_remove,
 };
 #endif /* CONFIG_USERFAULTFD */
 
-- 
2.51.0