KVM: guest_memfd: extract __kvm_gmem_get_pfn()

In preparation for adding a function that walks a set of pages
provided by userspace and populates them in a guest_memfd,
add a version of kvm_gmem_get_pfn() that has a "bool prepare"
argument and passes it down to kvm_gmem_get_folio().

Populating guest memory has to call repeatedly __kvm_gmem_get_pfn()
on the same file, so make the new function take struct file*.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/guest_memfd.c

@@ -528,33 +528,29 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
 	fput(file);
 }
 
-int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
-		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
+static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
+		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prepare)
 {
 	pgoff_t index = gfn - slot->base_gfn + slot->gmem.pgoff;
-	struct kvm_gmem *gmem;
+	struct kvm_gmem *gmem = file->private_data;
 	struct folio *folio;
 	struct page *page;
-	struct file *file;
 	int r;
 
-	file = kvm_gmem_get_file(slot);
-	if (!file)
+	if (file != slot->gmem.file) {
+		WARN_ON_ONCE(slot->gmem.file);
 		return -EFAULT;
+	}
 
 	gmem = file->private_data;
-
 	if (xa_load(&gmem->bindings, index) != slot) {
 		WARN_ON_ONCE(xa_load(&gmem->bindings, index));
-		r = -EIO;
-		goto out_fput;
+		return -EIO;
 	}
 
-	folio = kvm_gmem_get_folio(file_inode(file), index, true);
-	if (IS_ERR(folio)) {
-		r = PTR_ERR(folio);
-		goto out_fput;
-	}
+	folio = kvm_gmem_get_folio(file_inode(file), index, prepare);
+	if (IS_ERR(folio))
+		return PTR_ERR(folio);
 
 	if (folio_test_hwpoison(folio)) {
 		r = -EHWPOISON;
@@ -571,9 +567,21 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 
 out_unlock:
 	folio_unlock(folio);
-out_fput:
-	fput(file);
 
 	return r;
 }
+
+int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
+		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
+{
+	struct file *file = kvm_gmem_get_file(slot);
+	int r;
+
+	if (!file)
+		return -EFAULT;
+
+	r = __kvm_gmem_get_pfn(file, slot, gfn, pfn, max_order, true);
+	fput(file);
+	return r;
+}
 EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);