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

[David Hildenbrand]

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 ..

But LGTM

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>


-- 
Thanks,

David / dhildenb