[pve-devel] [RFC qemu-server] vm_resume: correctly honor $nocheck

Fabian Grünbichler f.gruenbichler at proxmox.com
Thu May 23 21:22:22 CEST 2019

for both vm_mon_cmd calls. under certain circumstances, the following
sequence of events can otherwise fail when live-migrating under load:

S...source node
T...target node

0: migration is complete, handover from S to T starts
1: S: logically move VM config file from S to T via rename()
2: S: rename returns, config file is (visibly) moved on S
3: S: trigger resume on T via mtunnel
4a: T: call vm_resume while config file move is not yet visible on T
4b: T: call vm_resume while config file move is already visible on T

4a instead of 4b means vm_mon_cmd will die in check_running unless
vm_mon_cmd_nocheck is used.

under heavy pmxcfs load and a slow cluster/corosync network, there can
be a few seconds of delay between 1 and 2, with a subsequent race ending in 4a
instead of 4b.

this issue was reported to occur on bulk migrations.

Signed-off-by: Fabian Grünbichler <f.gruenbichler at proxmox.com>
RFC mainly since I am not entirely sure whether we should also take a closer
look at whether there is a bug in pmxcfs somewhere here..

See the link below for user reports:

 PVE/QemuServer.pm | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
index c342add..1a22fb4 100644
--- a/PVE/QemuServer.pm
+++ b/PVE/QemuServer.pm
@@ -5766,8 +5766,8 @@ sub vm_resume {
     my ($vmid, $skiplock, $nocheck) = @_;
     PVE::QemuConfig->lock_config($vmid, sub {
-	my $res = vm_mon_cmd($vmid, 'query-status');
+	my $vm_mon_cmd = $nocheck ? \&vm_mon_cmd_nocheck : \&vm_mon_cmd;
+	my $res = $vm_mon_cmd->($vmid, 'query-status');
 	my $resume_cmd = 'cont';
 	if ($res->{status} && $res->{status} eq 'suspended') {
@@ -5780,12 +5780,9 @@ sub vm_resume {
 		if !($skiplock || PVE::QemuConfig->has_lock($conf, 'backup'));
-	    vm_mon_cmd($vmid, $resume_cmd);
-	} else {
-	    vm_mon_cmd_nocheck($vmid, $resume_cmd);
+	$vm_mon_cmd->($vmid, $resume_cmd);

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