Re: [libvirt] [PATCH 1/2] vircgroupv2: Fix VM startup when legacy cgroups are defined

On Fedora 31, starting a 'mock' build alters /proc/$pid/cgroup, probably due to usage of systemd-nspawn. Before: $ cat /proc/self/cgroup 0::/user.slice/user-1000.slice/... After: $ cat /proc/self/cgroup 1:name=systemd:/ 0::/user.slice/user-1000.slice/... The cgroupv2 code mishandles that first line in the second case, which causes VM startup to fail with: Unable to read from '/sys/fs/cgroup/machine/cgroup.controllers': No such file or directory The kernel docs[1] say that the cgroupv2 path will always start with '0::', which in the code here controllers="". Only set the v2 placement path when we see that cgroup file entry. [1] https://www.kernel.org/doc/html/v5.3/admin-guide/cgroup-v2.html#processes https://bugzilla.redhat.com/show_bug.cgi?id=1751120 Signed-off-by: Cole Robinson <crobinso(a)redhat.com&gt; --- src/util/vircgroupv2.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c index 0663c67190..0cb20e4896 100644 --- a/src/util/vircgroupv2.c +++ b/src/util/vircgroupv2.c @@ -194,12 +194,16 @@ virCgroupV2DetectMounts(virCgroupPtr group, static int virCgroupV2DetectPlacement(virCgroupPtr group, const char *path, - const char *controllers ATTRIBUTE_UNUSED, + const char *controllers, const char *selfpath) { if (group->unified.placement) return 0; + /* controllers="" indicates the cgroupv2 controller path */ + if (STRNEQ_NULLABLE(controllers, "")) + return 0;