Re: [PATCH cgroup/for-6.18-fixes] cgroup: Skip showing PID 0 in cgroup.procs and cgroup.threads

[Tejun Heo <tj@kernel.org>]

On Fri, Nov 07, 2025 at 03:34:43AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Fri, Nov 07, 2025 at 10:57:54AM +0100, Michal Koutný wrote:
> > On Thu, Nov 06, 2025 at 12:07:45PM -1000, Tejun Heo <tj@kernel.org> wrote:
> > > css_task_iter_next() pins and returns a task, but the task can do whatever
> > > between that and cgroup_procs_show() being called, including dying and
> > > losing its PID. When that happens, task_pid_vnr() returns 0.
> > 
> > task_pid_vnr() would return 0 also when the process is not from reader's
> > pidns (IMO more common than the transitional effect).
> 
> Hmm... haven't thought about that.
> 
> > > Showing "0" in cgroup.procs or cgroup.threads is confusing and can lead to
> > > surprising outcomes. For example, if a user tries to kill PID 0, it kills
> > > all processes in the current process group.
> > 
> > It's still info about present processes.
> > 
> > > 
> > > Skip entries with PID 0 by returning SEQ_SKIP.
> > 
> > It's likely OK to skip for these exiting tasks but with the external pidns tasks
> > in mind, reading cgroup.procs now may give false impression of an empty
> > cgroup.
> > 
> > Where does the 0 from of the exiting come from? (Could it be
> > distinguished from foreign pidns?)
> 
> Yeah, I think it can be distinguished. We just need to check whether the
> task has pid attached at all after getting 0 return from task_pid_vnr().

Let me drop this patch for now.

Thanks.

-- 
tejun