On 25.11.21 15:46, Petr Mladek wrote:

On Thu 2021-11-25 10:36:49, David Hildenbrand wrote:

quoted

On 20.11.21 12:28, Yafang Shao wrote:

quoted

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@gmail.com/ (local)

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