linux-kernel - Re: [PATCH v2 1/2] pid: Replace pid bitmap implementation with IDR API

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Date:   Mon, 02 Oct 2017 09:14:34 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Gargi Sharma <gs051095@...il.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Julia Lawall <julia.lawall@...6.fr>,
        akpm@...ux-foundation.org, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2 1/2] pid: Replace pid bitmap implementation with IDR
 API

On Sun, 2017-10-01 at 16:05 +0530, Gargi Sharma wrote:
> On Sun, Oct 1, 2017 at 2:45 PM, Christoph Hellwig <hch@...radead.org>
> wrote:
> > > -             task_active_pid_ns(current)->last_pid);
> > > +             task_active_pid_ns(current)->idr.idr_next-1);
> > 
> > I think we want a well documented helper for this pattern instead
> > of poking into the internals.
> 
> idr_get_cursor() get can be used instead of idr.idr_next, so that we
> do not
> expose the internals.
> > 
> > Also is last - 1 always the correct answer?  Even with
> > idr_alloc_cyclic
> > we could wrap around, couldn't we?
> 
> -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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