Hi Christoph There is a regression problem which is introduced by 84d7d462b16d (blk-cgroup: pin the gendisk in struct blkcg_gq). The test commands below can reproduce this: mdadm -CR /dev/md0 -l10 -n4 /dev/nvme[0-3]n1 --assume-clean mdadm --stop /dev/md0 mdadm -CR /dev/md0 -l10 -n4 /dev/nvme[0-3]n1 --assume-clean mdadm: Fail to create md0 when using /sys/module/md_mod/parameters/new_array, fallback to creation via node mdadm: unexpected failure opening /dev/md0 The reason is that the gendisk kobj can't be released, so md_free_disk can't be called. It looks like the patch mentioned above doesn't have problem. Before this patch, it didn't add the reference to the kobj of the gendisk. So all things work well. Now it adds the reference to the kobj of the gendisk in blkg_alloc, but it can't be decremented in blkg_release. After adding some debug logs, it only adds the reference of blkg->refcnt, but it doesn't call blkcg_rstat_flush which calls percpu_ref_put. So the patch 84d7d462b16d only exposes the reference problem in block cgroup. Best Regards Xiao
