From: Christopher Lalancette <clalance@redhat.com>
Date: Thu, 21 Jan 2010 19:59:54 -0500
Subject: [x86] xen: invalidate dom0 pages before starting guest
Message-id: <4B58B23A.2060901@redhat.com>
Patchwork-id: 22696
O-Subject: [RHEL5.5 Xen PATCH]: Invalidate dom0 page-cache before starting a
	guest
Bugzilla: 466681
RH-Acked-by: Rik van Riel <riel@redhat.com>
RH-Acked-by: Andrew Jones <drjones@redhat.com>
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

All,
     We've had a long standing issue with certain Xen setups not properly
seeing updated guest information.  The sequence of events is:

1)  Start up a guest with pygrub
2)  Edit /boot/grub/grub.conf inside the guest
3)  Shutdown the guest
4)  Start up a guest with pygrub

In certain circumstances, the changes made in 3) were not being reflected
in 4).  After quite a bit of discussion with Michal Novotny, hch, Ric Wheeler,
and Eric Sandeen (thanks!), we determined that the problem is in the way
that Xen blkback submits data to the block layer.  In particular it submits
bio's directly to the block layer instead of going through the normal
mechanisms, which means it doesn't properly invalidate the page cache.
     The solution is fairly simple; when we first start up a blkback kernel
thread to handle I/O, we flush the in-memory cache for that device before
doing anything else.  This ensures that the data that pygrub (on the dom0)
sees is consistent with what the guest wrote.
     I couldn't reproduce the problem myself, so my testing was just basic
smoke testing to make sure things worked.  One of the reporters on the BZ
downloaded my test kernel and confirmed that this patch fixes his problem.
     This was committed to upstream linux-2.6.18-xen.hg as c/s 986, and
should resolve BZ 466681.  Please review and ACK.

Signed-off-by: Jarod Wilson <jarod@redhat.com>

diff --git a/drivers/xen/blkback/xenbus.c b/drivers/xen/blkback/xenbus.c
index 49aa93f..4da2651 100644
--- a/drivers/xen/blkback/xenbus.c
+++ b/drivers/xen/blkback/xenbus.c
@@ -87,6 +87,13 @@ static void update_blkif_status(blkif_t *blkif)
 		return;
 	}
 
+	err = filemap_write_and_wait(blkif->vbd.bdev->bd_inode->i_mapping);
+	if (err) {
+		xenbus_dev_error(blkif->be->dev, err, "block flush");
+		return;
+	}
+	invalidate_inode_pages2(blkif->vbd.bdev->bd_inode->i_mapping);
+
 	blkif->xenblkd = kthread_run(blkif_schedule, blkif, name);
 	if (IS_ERR(blkif->xenblkd)) {
 		err = PTR_ERR(blkif->xenblkd);