Re: [libvirt] Quantifying libvirt errors in launching the libguestfs appliance

On 01/13/2016 05:18 AM, Richard W.M. Jones wrote: > As people may know, we frequently encounter errors caused by libvirt > when running the libguestfs appliance. > > I wanted to find out exactly how frequently these happen and classify > the errors, so I ran the 'virt-df' tool overnight 1700 times. This > tool runs several parallel qemu:///session libvirt connections both > creating a short-lived appliance guest. > > Note that I have added Cole's patch to fix https://bugzilla.redhat.com/1271183 > "XML-RPC error : Cannot write data: Transport endpoint is not connected" > > Results: > > The test failed 538 times (32% of the time), which is pretty dismal. > To be fair, virt-df is aggressive about how it launches parallel > libvirt connections. Most other virt-* tools use only a single > libvirt connection and are consequently more reliable. > > Of the failures, 518 (96%) were of the form: > > process exited while connecting to monitor: qemu: could not load kernel '/home/rjones/d/libguestfs/tmp/.guestfs-1000/appliance.d/kernel': Permission denied > > which is https://bugzilla.redhat.com/921135 or maybe > https://bugzilla.redhat.com/1269975. It's not clear to me if these > bugs have different causes, but if they do then potentially we're > seeing a mix of both since my test has no way to distinguish them. > I just experimented with this, I think it's the issue I suggested at: https://bugzilla.redhat.com/show_bug.cgi?id=1269975#c4 I created two VMs, kernel1 and kernel2, just booting off a kernel in $HOME/session-kernel/vmlinuz. Then I added this patch: diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c index f083f3f..5d9f0fa 100644 --- a/src/qemu/qemu_process.c +++ b/src/qemu/qemu_process.c @@ -4901,6 +4901,13 @@ qemuProcessLaunch(virConnectPtr conn, incoming ? incoming->path : NULL) < 0) goto cleanup; + if (STREQ(vm->def->name, "kernel1")) { + for (int z = 0; z < 30; z++) { + printf("kernel1: sleeping %d of 30\n", z + 1); + sleep(1); + } + } + /* Security manager labeled all devices, therefore * if any operation from now on fails, we need to ask the caller to * restore labels. Which is right after selinux labels are set on VM startup. This is then easy to reproduce with: virsh start kernel1 (sleeps) virsh start kernel2 && virsh destroy kernel2 The shared vmlinuz is reset to user_home_t after kernel2 is shut down, so kernel1 fails to start after the patch's timeout When we detect similar issues with <disk> devices, like when the media already has the expected label, we encode 'relabel=no' in the disk XML, which tells libvirt not to run restorecon on the disks path when the VM is shutdown. However kernel/initrd XML doesn't have support for this XML, so it won't work there. Adding that could be one fix. But I think there's longer term plans for this type of issue by using ACLs, or virtlockd or something, Michal had patches but I don't know the specifics. Unfortunately even hardlinks share selinux labels so I don't think there's any workaround on the libguestfs side short of using a separate copy of the appliance kernel for each VM