xen/privcmd: create address space to allow writable mmaps

These are necessary to allow writeable mmap of the privcmd node to
succeed without being marked read-only for writenotify purposes. Which
in turn is necessary to allow mappings of foreign guest pages

[ Impact: bugfix: allow writable mappings ]

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/xenfs/super.c

@@ -12,6 +12,8 @@
 #include <linux/module.h>
 #include <linux/fs.h>
 #include <linux/magic.h>
+#include <linux/mm.h>
+#include <linux/backing-dev.h>
 
 #include <xen/xen.h>
 
@@ -22,12 +24,30 @@
 MODULE_DESCRIPTION("Xen filesystem");
 MODULE_LICENSE("GPL");
 
+static int xenfs_set_page_dirty(struct page *page)
+{
+	if (!PageDirty(page))
+		SetPageDirty(page);
+	return 0;
+}
+
+static const struct address_space_operations xenfs_aops = {
+	.set_page_dirty = xenfs_set_page_dirty,
+};
+
+static struct backing_dev_info xenfs_backing_dev_info = {
+	.ra_pages	= 0,	/* No readahead */
+	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK,
+};
+
 static struct inode *xenfs_make_inode(struct super_block *sb, int mode)
 {
 	struct inode *ret = new_inode(sb);
 
 	if (ret) {
 		ret->i_mode = mode;
+		ret->i_mapping->a_ops = &xenfs_aops;
+		ret->i_mapping->backing_dev_info = &xenfs_backing_dev_info;
 		ret->i_uid = ret->i_gid = 0;
 		ret->i_blocks = 0;
 		ret->i_atime = ret->i_mtime = ret->i_ctime = CURRENT_TIME;
@@ -119,11 +139,25 @@ static struct file_system_type xenfs_type = {
 
 static int __init xenfs_init(void)
 {
-	if (xen_domain())
-		return register_filesystem(&xenfs_type);
+	int err;
+	if (!xen_domain()) {
+		printk(KERN_INFO "xenfs: not registering filesystem on non-xen platform\n");
+		return 0;
+	}
 
-	printk(KERN_INFO "XENFS: not registering filesystem on non-xen platform\n");
-	return 0;
+	err = register_filesystem(&xenfs_type);
+	if (err) {
+		printk(KERN_ERR "xenfs: Unable to register filesystem!\n");
+		goto out;
+	}
+
+	err = bdi_init(&xenfs_backing_dev_info);
+	if (err)
+		unregister_filesystem(&xenfs_type);
+
+ out:
+
+	return err;
 }
 
 static void __exit xenfs_exit(void)