Re: [Xen-users] Xen / Linux 4.3.3

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Patrick Velder writes ("[Xen-users] Xen / Linux 4.3.3"):
> I just tried to boot kernel 4.3.3 on my xen host. xl list does not work 
> (freezes), but the domUs are up. Reboot is impossible, and it's not 
> possible to kill the hanging xl processes.

There is a configuration change in 4.3.x which revealed a latent bug,
which causes xenstored to crash:
 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=810472


In more detail, under Xen, without
  9c17d965 "xen/gntdev: Grant maps should not be subject to NUMA balancing"
Linux might attempt to do memory migration for NUMA balancing on pages
that are mapped in from other Xen domains.

AIUI in 4.3.y SCHED_NUMA became enabled by default, and Matthew Vernon
reports in
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=810472
that Debian's linux-image-4.3.0-0.bpo.1-amd64 is unreliable as Xen dom0.
We investigated and found that xenstored was getting SIGBUS accessing
a client domain's shared ring page.


Ian.


>From 9c17d96500f78d7ecdb71ca6942830158bc75a2b Mon Sep 17 00:00:00 2001
From: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Date: Tue, 10 Nov 2015 15:10:33 -0500
Subject: [PATCH] xen/gntdev: Grant maps should not be subject to NUMA
 balancing

Doing so will cause the grant to be unmapped and then, during
fault handling, the fault to be mistakenly treated as NUMA hint
fault.

In addition, even if those maps could partcipate in NUMA
balancing, it wouldn't provide any benefit since we are unable
to determine physical page's node (even if/when VNUMA is
implemented).

Marking grant maps' VMAs as VM_IO will exclude them from being
part of NUMA balancing.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 drivers/xen/gntdev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 2ea0b3b..1be5dd0 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -804,7 +804,7 @@ static int gntdev_mmap(struct file *flip, struct 
vm_area_struct *vma)
 
        vma->vm_ops = &gntdev_vmops;
 
-       vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
+       vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP | VM_IO;
 
        if (use_ptemod)
                vma->vm_flags |= VM_DONTCOPY;
-- 
1.7.10.4


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