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Date:   Mon, 26 Nov 2018 00:00:07 +0100
From:   Pavel Machek <pavel@ucw.cz>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     'Kevin Easton' <kevin@guarana.org>,
        Daniel Colascione <dancol@google.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
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        "primiano@google.com" <primiano@google.com>,
        "joelaf@google.com" <joelaf@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Roman Gushchin <guro@fb.com>, Vlastimil Babka <vbabka@suse.cz>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Prashant Dhamdhere <pdhamdhe@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "Steven Rostedt (VMware)" <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Michal Hocko <mhocko@suse.com>,
        "MatthewWilcox@ip-172-31-15-78" 
        <MatthewWilcox@ip-172-31-15-78.karlin.mff.cuni.cz>
Subject: Re: [PATCH] Add /proc/pid_generation
Message-ID: <20181125230007.GC30242@amd>
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On Fri 2018-11-23 11:14:17, David Laight wrote:
> From: Kevin Easton
> > Sent: 22 November 2018 11:20
> >=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.
> >=20
> > I see downthread this patch has been withdrawn, but nonetheless I'm
> > still curious - does this actually solve the problem?
> >=20
> > 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:
>=20
> 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 reus=
ed.
> 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
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

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