linux-kernel - Re: [PATCH] Add /proc/pid

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Message-ID: <20181125230007.GC30242@amd>
Date:   Mon, 26 Nov 2018 00:00:07 +0100
From:   Pavel Machek <pavel@....cz>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Kevin Easton' <kevin@...rana.org>,
        Daniel Colascione <dancol@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "timmurray@...gle.com" <timmurray@...gle.com>,
        "primiano@...gle.com" <primiano@...gle.com>,
        "joelaf@...gle.com" <joelaf@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
        "MatthewWilcox@...172-31-15-78" 
        <MatthewWilcox@...172-31-15-78.karlin.mff.cuni.cz>
Subject: Re: [PATCH] Add /proc/pid_generation

On Fri 2018-11-23 11:14:17, David Laight wrote:
> From: Kevin Easton
> > Sent: 22 November 2018 11:20
> > 
> > On Wed, Nov 21, 2018 at 12:14:44PM -0800, Daniel Colascione wrote:
> > > This change adds a per-pid-namespace 64-bit generation number,
> > > incremented on PID rollover, and exposes it via a new proc file
> > > /proc/pid_generation. By examining this file before and after /proc
> > > enumeration, user code can detect the potential reuse of a PID and
> > > restart the task enumeration process, repeating until it gets a
> > > coherent snapshot.
> > 
> > I see downthread this patch has been withdrawn, but nonetheless I'm
> > still curious - does this actually solve the problem?
> > 
> > It seems to me that a PID could be reused within a scan even if the
> > generation number remains the same at the beginning and end of a scan:
> 
> Why not allocate a 48bit generation number to each 16bit pid?
> Then you have a 64bit 'extended-pid' that can be assumed to never be reused.
> Provided enough interfaces are enhanced to support 'extended-pid' values
> you'll never get reused values.

For the record, I really like this proposal.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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