Re: [PATCH v2] kthread: dynamically allocate memory to store kthread's full name

[David Hildenbrand]

On 25.11.21 15:46, Petr Mladek wrote:
> On Thu 2021-11-25 10:36:49, David Hildenbrand wrote:
>> On 20.11.21 12:28, Yafang Shao wrote:
>>> When I was implementing a new per-cpu kthread cfs_migration, I found the
>>> comm of it "cfs_migration/%u" is truncated due to the limitation of
>>> TASK_COMM_LEN. For example, the comm of the percpu thread on CPU10~19 are
>>> all with the same name "cfs_migration/1", which will confuse the user. This
>>> issue is not critical, because we can get the corresponding CPU from the
>>> task's Cpus_allowed. But for kthreads correspoinding to other hardware
>>> devices, it is not easy to get the detailed device info from task comm,
>>> for example,
>>>
>>>     jbd2/nvme0n1p2-
>>>     xfs-reclaim/sdf
>>>
>>> Currently there are so many truncated kthreads:
>>>
>>>     rcu_tasks_kthre
>>>     rcu_tasks_rude_
>>>     rcu_tasks_trace
>>>     poll_mpt3sas0_s
>>>     ext4-rsv-conver
>>>     xfs-reclaim/sd{a, b, c, ...}
>>>     xfs-blockgc/sd{a, b, c, ...}
>>>     xfs-inodegc/sd{a, b, c, ...}
>>>     audit_send_repl
>>>     ecryptfs-kthrea
>>>     vfio-irqfd-clea
>>>     jbd2/nvme0n1p2-
>>>     ...
>>>
>>> We can shorten these names to work around this problem, but it may be
>>> not applied to all of the truncated kthreads. Take 'jbd2/nvme0n1p2-' for
>>> example, it is a nice name, and it is not a good idea to shorten it.
>>>
>>> One possible way to fix this issue is extending the task comm size, but
>>> as task->comm is used in lots of places, that may cause some potential
>>> buffer overflows. Another more conservative approach is introducing a new
>>> pointer to store kthread's full name if it is truncated, which won't
>>> introduce too much overhead as it is in the non-critical path. Finally we
>>> make a dicision to use the second approach. See also the discussions in
>>> this thread:
>>> https://lore.kernel.org/lkml/20211101060419.4682-1-laoar.shao@xxxxxxxxx/
>>>
>>> After this change, the full name of these truncated kthreads will be
>>> displayed via /proc/[pid]/comm:
>>>
>>>     rcu_tasks_kthread
>>>     rcu_tasks_rude_kthread
>>>     rcu_tasks_trace_kthread
>>>     poll_mpt3sas0_statu
>>>     ext4-rsv-conversion
>>>     xfs-reclaim/sdf1
>>>     xfs-blockgc/sdf1
>>>     xfs-inodegc/sdf1
>>>     audit_send_reply
>>>     ecryptfs-kthread
>>>     vfio-irqfd-cleanup
>>>     jbd2/nvme0n1p2-8
>>
>> I do wonder if that could break some user space that assumes these names
>> have maximum length ..
> 
> There is high chance that we will be on the safe side. Workqueue
> kthreads already provided longer names. They are even dynamic
> because the currently handled workqueue name is part of the name,
> see wq_worker_comm().

Great, thanks!


-- 
Thanks,

David / dhildenb