[XFS] eagerly remove vmap mappings to avoid upsetting Xen

XFS leaves stray mappings around when it vmaps memory to make it virtually
contigious. This upsets Xen if one of those pages is being recycled into a
pagetable, since it finds an extra writable mapping of the page.

This patch solves the problem in a brute force way, by making XFS always
eagerly unmap its mappings.

SGI-PV: 971902
SGI-Modid: xfs-linux-melb:xfs-kern:29886a
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>

fs/xfs/linux-2.6/xfs_buf.c

@@ -187,6 +187,19 @@ free_address(
 {
 	a_list_t	*aentry;
 
+#ifdef CONFIG_XEN
+	/*
+	 * Xen needs to be able to make sure it can get an exclusive
+	 * RO mapping of pages it wants to turn into a pagetable.  If
+	 * a newly allocated page is also still being vmap()ed by xfs,
+	 * it will cause pagetable construction to fail.  This is a
+	 * quick workaround to always eagerly unmap pages so that Xen
+	 * is happy.
+	 */
+	vunmap(addr);
+	return;
+#endif
+
 	aentry = kmalloc(sizeof(a_list_t), GFP_NOWAIT);
 	if (likely(aentry)) {
 		spin_lock(&as_lock);