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Subject: Re: [PATCH v2 1/2] pid: Replace pid bitmap implementation with IDR
 API
From: Rik van Riel <riel@surriel.com>
To: Gargi Sharma <gs051095@gmail.com>,
        Christoph Hellwig <hch@infradead.org>
Cc: linux-kernel@vger.kernel.org, Julia Lawall <julia.lawall@lip6.fr>,
        akpm@linux-foundation.org, mingo@kernel.org, pasha.tatashin@oracle.com,
        ktkhai@virtuozzo.com, Oleg Nesterov <oleg@redhat.com>
Date: Mon, 02 Oct 2017 09:14:34 -0400
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On Sun, 2017-10-01 at 16:05 +0530, Gargi Sharma wrote:
> On Sun, Oct 1, 2017 at 2:45 PM, Christoph Hellwig <hch@infradead.org>
> wrote:
> > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0task_active_pid_ns(current)->last_pid);
> > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0task_active_pid_ns(current)->idr.idr_next-1);
> >=20
> > I think we want a well documented helper for this pattern instead
> > of poking into the internals.
>=20
> idr_get_cursor() get can be used instead of idr.idr_next, so that we
> do not
> expose the internals.
> >=20
> > Also is last - 1 always the correct answer?=C2=A0=C2=A0Even with
> > idr_alloc_cyclic
> > we could wrap around, couldn't we?
>=20
> -1 will be incorrect when the pids wrap around. Should we go back to
> setting up last_pid as it was done before? Or should we use
> idr_get_cursor
> and determine if pid was rolled over and then perform necessary
> action?

Looking at it some more, it appears the value is only ever used
in /proc/loadavg.

Would anyone object to the code simply calling idr_get_cursor()
as is, and leaving out the -1?

Somehow I suspect nobody will care that the pid value in /proc/loadavg
reflects the next PID allocated, rather than the previous one.

Any objections?

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