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SubjectRe: [PATCH] Mark timer_stats as incompatible with multiple pid namespaces

* Eric W. Biederman <ebiederm@xmission.com> wrote:

> > What the heck??? Please solve this properly instead of hiding it.
> > /proc/timer_stats is damn useful and it's a must-have for powertop
> > to work.
>
> Hmm. Perhaps the dependency conflict should go in the other direction
> then.
>
> My goal is to document the issue while a proper fix is being written.
> I have known about this for all of about 1 day now. It was added
> since last time I went through the kernel and made a thorough sweep of
> pid users.
>
> What the proper fix is isn't even obvious at this point. Possibly it
> is making /proc/timer_stats disappear in child pid namespaces.
> Which we don't currently have the infrastructure fore.
>
> Possibly it is reworking the stats collection so we store a struct pid
> * instead of a pid_t value. So we would know if the reader of the
> value can even see processes you have collected stats for.

the problem is, this interface stores historic PIDs too - i.e. PIDs of
tasks that might have exited already.

the proper way would be to also store the namespace address, and to
filter out non-matching entries. (If leakage of this data across
namespace creation is of any concern then flush out existing namespace
data when a namespace is destroyed)

Ingo


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