LoongArch: Fix build errors due to backported TIMENS

Commit eb3710efff ("LoongArch: Add support
to clone a time namespace") backports the TIMENS support for LoongArch
(corresponding upstream commit aa5e65dc08)
but causes build errors:

  CC      arch/loongarch/kernel/vdso.o
arch/loongarch/kernel/vdso.c: In function ‘vvar_fault’:
arch/loongarch/kernel/vdso.c:54:36: error: implicit declaration of
function ‘find_timens_vvar_page’ [-Werror=implicit-function-declaration]
   54 |         struct page *timens_page = find_timens_vvar_page(vma);
      |                                    ^~~~~~~~~~~~~~~~~~~~~
arch/loongarch/kernel/vdso.c:54:36: warning: initialization of ‘struct
page *’ from ‘int’ makes pointer from integer without a cast
[-Wint-conversion]
arch/loongarch/kernel/vdso.c: In function ‘vdso_join_timens’:
arch/loongarch/kernel/vdso.c:143:25: error: implicit declaration of
function ‘zap_vma_pages’; did you mean ‘zap_vma_ptes’?
[-Werror=implicit-function-declaration]
  143 |                         zap_vma_pages(vma);
      |                         ^~~~~~~~~~~~~
      |                         zap_vma_ptes
cc1: some warnings being treated as errors

Because in 6.1.y we should define find_timens_vvar_page() by ourselves
and use zap_page_range() instead of zap_vma_pages(), so fix it.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/loongarch/kernel/vdso.c

@@ -40,6 +40,8 @@ static struct page *vdso_pages[] = { NULL };
 struct vdso_data *vdso_data = generic_vdso_data.data;
 struct vdso_pcpu_data *vdso_pdata = loongarch_vdso_data.vdata.pdata;
 
+static struct page *find_timens_vvar_page(struct vm_area_struct *vma);
+
 static int vdso_mremap(const struct vm_special_mapping *sm, struct vm_area_struct *new_vma)
 {
 	current->mm->context.vdso = (void *)(new_vma->vm_start);
@@ -139,13 +141,37 @@ int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
 
 	mmap_read_lock(mm);
 	for_each_vma(vmi, vma) {
+		unsigned long size = vma->vm_end - vma->vm_start;
+
 		if (vma_is_special_mapping(vma, &vdso_info.data_mapping))
-			zap_vma_pages(vma);
+			zap_page_range(vma, vma->vm_start, size);
 	}
 	mmap_read_unlock(mm);
 
 	return 0;
 }
+
+static struct page *find_timens_vvar_page(struct vm_area_struct *vma)
+{
+	if (likely(vma->vm_mm == current->mm))
+		return current->nsproxy->time_ns->vvar_page;
+
+	/*
+	 * VM_PFNMAP | VM_IO protect .fault() handler from being called
+	 * through interfaces like /proc/$pid/mem or
+	 * process_vm_{readv,writev}() as long as there's no .access()
+	 * in special_mapping_vmops.
+	 * For more details check_vma_flags() and __access_remote_vm()
+	 */
+	WARN(1, "vvar_page accessed remotely");
+
+	return NULL;
+}
+#else
+static struct page *find_timens_vvar_page(struct vm_area_struct *vma)
+{
+	return NULL;
+}
 #endif
 
 static unsigned long vdso_base(void)