On 01/13/2016 06:45 PM, Cole Robinson wrote:
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
Whoops, should have checked my libvirt mail first, you guys already came to this conclusion elsewhere in the thread :) - Cole