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.  David Chinner says this shouldn't
have any performance impact on filesystems with default block sizes;
it will only affect filesystems with large block sizes.

Signed-off-by: Jeremy Fitzhardinge <redacted>
Cc: David Chinner <redacted>
Cc: Nick Piggin <redacted>
Cc: XFS masters <redacted>
Cc: Stable kernel <stable@kernel.org>
Cc: Morten =?utf-8?q?B=C3=B8geskov?= <redacted>
Cc: Mark Williamson <redacted>

---
 fs/xfs/linux-2.6/xfs_buf.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

===================================================================

--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c

@@ -186,6 +186,19 @@ free_address(
 	void		*addr)
 {
 	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)) {

--