From: Chris Lalancette <clalance@redhat.com>
Subject: [RHEL5.1 PATCH]: Xen: FIx privcmd to remove nopage handler
Date: Fri, 24 Aug 2007 13:05:29 -0400
Bugzilla: 249409
Message-Id: <46CF0FD9.8000905@redhat.com>
Changelog: [xen] Fix privcmd to remove nopage handler


All,
     Attached is a simple patch to set the nopage handler in the Xen privcmd
stuff to NULL.  As Rik pointed out, when your vma has VM_PFNMAP, you should
*not* define a nopage handler, because handle_pte_fault() calls do_no_page iff
your vma has a nopage handler defined.  If you do have VM_PFNMAP on your vma,
and you also have a nopage handler, it leads to this BUG_ON:

Kernel BUG at mm/memory.c:2290
invalid opcode: 0000 [1] SMP

By setting the nopage handler to NULL, we never even call do_no_page(), so we do
not hit this BUG_ON anymore.

Before this patch, doing an HVM live migrate from an x86_64 box to another
x86_64 box would crash the target box fairly reliably.  After the patch, doing
the same live migrate did not crash the target box.  Note that there is another
bug in the userland portion of the code that still causes the live migrate to
have problems; that will be posted as a separate patch.

I tested this in the following ways:
1)  x86_64 live-migrate, off-line migrate, save/restore of PV and HVM guests;
all succeeded
2)  i686 live-migrate, off-line migrate, save/restore of PV and HVM guests; all
succeeded
3)  The tester program from BZ 253479; before the patch, it would crash the box
you ran it on; after the patch, the box would no longer crash.

This fixes BZ 249409.  Please ACK.

Chris Lalancette

--- linux-2.6.18.noarch/drivers/xen/privcmd/privcmd.c.orig	2007-08-22 13:38:09.000000000 -0400
+++ linux-2.6.18.noarch/drivers/xen/privcmd/privcmd.c	2007-08-22 13:38:28.000000000 -0400
@@ -217,15 +217,8 @@ static int privcmd_ioctl(struct inode *i
 }
 
 #ifndef HAVE_ARCH_PRIVCMD_MMAP
-static struct page *privcmd_nopage(struct vm_area_struct *vma,
-				unsigned long address,
-				int *type)
-{
-	return NOPAGE_SIGBUS;
-}
-
 static struct vm_operations_struct privcmd_vm_ops = {
-	.nopage = privcmd_nopage
+	.nopage = NULL
 };
 
 static int privcmd_mmap(struct file * file, struct vm_area_struct * vma)