[PATCH] Fix generic_file_readonly_mmap()

We cannot clear VM_MAYWRITE in there - it turns writeable MAP_PRIVATE
mappings into readonly ones.

So change it back to the 2.4 form - disallow a writeable MAP_SHARED mapping
against filesystems which do not implement ->writepage().

mm/filemap.c

@@ -1306,11 +1306,13 @@ int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
 	return 0;
 }
 
+/*
+ * This is for filesystems which do not implement ->writepage.
+ */
 int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE))
+	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
 		return -EINVAL;
-	vma->vm_flags &= ~VM_MAYWRITE;
 	return generic_file_mmap(file, vma);
 }
 #else