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Subject: Re: [QUESTION]cgroup mount return -EBUSY
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From: Lu Jialin <lujialin4@huawei.com>
To: Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>, Johannes
 Weiner <hannes@cmpxchg.org>
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ping

On 2024/4/26 12:55, Lu Jialin wrote:
> Hello
> 
> I encountered a problem when I try to unmount the subsystem right after
> all it's subcgroups are removed, the cgroup_root would remain. Mounting
> this subsystem to another cgroup_root would return -EBUSY.
> 
> The problem could be reproduced with the following script.
> 
> test.sh:
> 
> mkdir /tmp/test1
> mount -t cgroup -o pids pids /tmp/test1
> mkdir /tmp/test1/test
> rmdir /tmp/test1/test
> umount /tmp/test1
> mkdir /tmp/test
> mount -t cgroup -o pids,cpu none /tmp/test
> 
> test.sh should return this.
> mount: mounting none on /tmp/test failed: Device or resource busy.
> 
> It seems that when unmounting /tmp/test1, the original cgroup_root for
> this PID is not released.
> /test # cat /proc/cgroups
> 
> #subsys_name    hierarchy       num_cgroups     enabled
> 
> cpuset  0       1       1
> 
> cpu     0       1       1
> 
> cpuacct 0       1       1
> 
> blkio   0       1       1
> 
> devices 0       1       1
> 
> freezer 0       1       1
> 
> net_cls 0       1       1
> 
> perf_event      0       1       1
> 
> net_prio        0       1       1
> 
> hugetlb 0       1       1
> 
> pids    1       1       1
> 
> rdma    0       1       1
> 
> misc    0       1       1
> 
> debug   0       1       1
>